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Title: Ultrafast Dynamics in Warm Dense Matter Materials and Halide Perovskite.
Creator: Li, Dong, Cao, Jianming, Yang, Wei, Bonesteel, N. E., Chiorescu, Irinel, Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: The dissertation presents the recent development of the third-generation femtosecond electron diffractometer in Professor Jim Cao's group. Two techniques, femtosecond electron shadow imaging and deflectometry (FESID) and femtosecond electron diffraction (FED), were developed and applied to study ultrafast dynamics in laser-induced warm dense matter and quantum dots in real time. FESID provides both a global view and local prospect of the transient electric field, associated with laser-induced...
Show moreThe dissertation presents the recent development of the third-generation femtosecond electron diffractometer in Professor Jim Cao's group. Two techniques, femtosecond electron shadow imaging and deflectometry (FESID) and femtosecond electron diffraction (FED), were developed and applied to study ultrafast dynamics in laser-induced warm dense matter and quantum dots in real time. FESID provides both a global view and local prospect of the transient electric field, associated with laser-induced electron emission. The research activities cover two main objects: dynamics of ejected electron expansion from warm dense nanofilms and hyperthermal electron transport mechanisms in warm dense nanofilms. With FED, we measure laser-induced ultrafast structural dynamics of halide perovskite CsPbBr3 in real time. In the first project, we conduct ultrafast electron shadow imaging and deflection measurements of the laser-produced warm dense copper nanofilm. The results show that a significant number of electrons is ejected from the nanofilm, forming electron clouds of hundreds of microns on both sides of the pumped film. Furthermore, even for a thin 30-nm copper film, we find that the electron clouds develop asymmetry between the pumped front side and the rear side at the pump fluence of 4.5 J/cm2. The possible mechanisms leading to this ejected charge asymmetry and its implication are discussed. Next, we report a systematic study of the ejected charge dynamics surrounding laser produced 30-nm warm dense gold films using single-shot femtosecond electron shadow imaging and deflectometry. The results reveal a two-step dynamical process of the ejected electrons under the high pump fluence conditions: an initial emission and accumulation of a large number of electrons near the pumped surface region followed by the formation of hemispherical clouds of electrons on both sides of the film, which escape into the vacuum at a nearly isotropic and constant velocity with an unusually high kinetic energy of more than 300 eV. We also develop a model of the escaping charge distribution that not only reproduces the main features of the observed charge expansion dynamics but also allows us to extract the number of ejected electrons remaining in the cloud. In the second project, we investigate hyperthermal electron transport by single-shot measurements of warm dense gold and aluminum nanofilms using ultrafast electron shadow imaging and deflectometry. The results show a clear fluence limit of 0.26 J/cm2 and 0.83 J/cm2 for ballistic transport of nonthermal electrons for both two metals, respectively. This nonuniform heating is attributed to diffusive electrons. The last project, we have measured the ultrafast structural dynamics in halide perovskite CsPbBr3 in real time with Femtosecond electron diffraction. We observed CsPbBr3 experience significant ultrafast impulsive heating. This heating causes the CsPbBr3 to undergo an orthorhombic-to-cubic phase transition observable through FED. The photo induced phase transition occurs on the timescale of 1.1 ± 0.3 ps at fluences of 2.5 mJ/cm2.
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Date Issued: 2019
Identifier: 2019_Summer_Li_fsu_0071E_15420
Format: Thesis

Title: Magnetic Ordering and Magnetotransport at Molecular and Nano Scales.
Creator: Simmons, Danielle Theresa, Xiong, Peng, Shatruk, Mykhailo, Bonesteel, N. E., Baumbach, Ryan E., Piekarewicz, Jorge, Florida State University, College of Arts and Sciences,...
Show moreSimmons, Danielle Theresa, Xiong, Peng, Shatruk, Mykhailo, Bonesteel, N. E., Baumbach, Ryan E., Piekarewicz, Jorge, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: The understanding of magnetic materials has become crucial to both fundamental physics and technological advancement. Particularly, the interplay between magnetic and electronic properties has given rise to such novel physics as high temperature superconductivity and colossal magnetoresistance. Some magnetic materials potentially hold the key to the realization of new nanoscale memory and logic devices. Specifically, spintronics and molecular electronics are two fields drawing increasing...
Show moreThe understanding of magnetic materials has become crucial to both fundamental physics and technological advancement. Particularly, the interplay between magnetic and electronic properties has given rise to such novel physics as high temperature superconductivity and colossal magnetoresistance. Some magnetic materials potentially hold the key to the realization of new nanoscale memory and logic devices. Specifically, spintronics and molecular electronics are two fields drawing increasing attention due to their potential to address the ever-increasing need for continued device miniaturization. This dissertation focuses on probing magnetic ordering and magnetotransport at molecular and nano scales utilizing electronic measurement techniques in order to gain further understanding of these complex phenomena. The first project of this dissertation deals with the effect of electronic phase separation (EPS), which is believed to be at the root of several emergent correlated electron phenomena. The goal of this research is to gain further insight into the complex interplay between the magnetic and electronic interactions in the ferromagnetic semimetal EuB6 under applied hydrostatic pressure. Previous studies under ambient pressure have uncovered a remarkable manifestation of EPS in the nonlinear Hall effect of EuB6. The magnetotransport measurements under pressure that we performed revealed an increase in carrier concentration as well as an increase in the critical magnetization needed to instigate the percolation of the phase-separated ferromagnetic entities (magnetic polarons). Also discovered by these measurements was an intermediate state between the paramagnetic insulating phase and the ferromagnetic metallic phase, thereby indicating that the electronic phase separation is even more complicated than previously predicted. Previous work had shown a lattice constriction concomitant with the formation and percolation of magnetic polarons, suggesting that magnetostriction might provide a direct probe of their formation. These results inspired us to measure the magnetostriction of EuB6 under applied pressure in the phase-separated regions. Not only did our measurements show a reduction in the constriction necessary for polaron formation, but also showed lattice expansion above and below the polaron formation temperatures. While hybrid organic-electronic devices hold much promise in a variety of applications, there are several hurdles to overcome before they can be fully integrated. One such family of materials, known as spin-crossover molecules, have high-spin and low-spin states that can be activated thermally and/or through photo excitation. Since each molecule has an independent spin state, devices built from these materials would not rely on long-range magnetic order. Additionally, advancements in molecular nano-patterning and self-assembly make these molecules attractive for bottom-up device integration. On the other hand, the tiny magnetic signals from the change of spin state of a small volume of molecules necessitate more sensitive devices for measuring nano patterns of monolayers of the molecules. In the second project of the dissertation, we demonstrate the feasibility of magnetic measurements of monolayers of spin-crossover molecules. Using a high-sensitivity Hall magnetometry technique, we showed measurements of the light induced excited spin state trapping effect in Fe(ptz)6(BF4)2. These experiments provide clear guidelines for improving the magnetic moment sensitivity by using semiconductor heterostructures free of photoconductivity.
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Date Issued: 2019
Identifier: 2019_Spring_Simmons_fsu_0071E_13940
Format: Thesis

Title: Tuning Intertwined Energy Scales in f-Electron Systems by Chemical Substitution.
Creator: Lai, You, Baumbach, Ryan E., Xiong, Peng, Shatruk, Mykhailo, Graf, David, Schlottmann, Pedro U., Riley, Mark A., Florida State University, College of Arts and Sciences,...
Show moreLai, You, Baumbach, Ryan E., Xiong, Peng, Shatruk, Mykhailo, Graf, David, Schlottmann, Pedro U., Riley, Mark A., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Materials that contain f-electron elements often exhibit complex phase diagrams with different phenomena including nematic electronic states, charge and spin instabilities, the breakdown of Fermi liquid behavior, and unconventional superconductivity. This diversity of behavior is related to the fine balance between several factors including the magnetic exchange, Kondo effect, crystal electric field splitting and strong spin-orbit coupling. As a result, many such systems exhibit intertwined...
Show moreMaterials that contain f-electron elements often exhibit complex phase diagrams with different phenomena including nematic electronic states, charge and spin instabilities, the breakdown of Fermi liquid behavior, and unconventional superconductivity. This diversity of behavior is related to the fine balance between several factors including the magnetic exchange, Kondo effect, crystal electric field splitting and strong spin-orbit coupling. As a result, many such systems exhibit intertwined order parameters that are controllable through pressure, magnetic field, and chemical substitution. Here, we report results from chemical substitution studies in three distinct Kondo lattice systems. In each case, we are able to suppress an ordered state towards zero temperature at a possible quantum phase transition and study the resulting behavior. For CeCu$_2$Si$_2$, Si $\rightarrow$ P chemical substitution compresses the unit cell volume while adding $s/p$ electrons. This encourages complex magnetism and drives the system away from a quantum critical point. These results are understood by considering that the electronic hybridization between the f- and conduction electrons in this system is controlled by nearly independent parameters of unit cell volume and s; p; d shell filling, which drive the system's behavior along different axes. For CePd$_2$P$_2$, Pd $\rightarrow$ Ni substitution suppresses the ferromagnetism towards a disordered ferromagnetic QCP at $x_{\rm{cr}}$ = 0.7, where the breakdown of Fermi liquid behavior is observed. We also find that for CePd$_2$P$_2$, a pressure of $P_{\rm{c}}$ = 12 GPa would likely be sufficient to access a quantum phase transition. These results provide a useful experimental testbed for the Belitz-Kirkpatrick-Vojta (BKV) theory. For UCr$_2$Si$_2$, Cr $\rightarrow$ Ru substitution results in filling of the $d$-shell without significantly changing the unit cell volume. This suppresses the antiferromagnetic order $T_{\rm{N}}$ ($T_{\rm{N}}$ $\approx$ 24 K for UCr$_2$Si$_2$) and the structural phase transition $T_{\rm{S}}$ ($T_{\rm{S}}$ $\approx$ 200 K for UCr$_2$Si$_2$) that are seen in the parent compound. $T_{\rm{N}}$ approaches zero temperature near $x_{\rm{c, N}}$ = 0.08 while $T_{\rm{S}}$ reaches a minimum value near $x_{\rm{c, S}}$ = 0.16, after which the structural phase transition disappears for larger $x$. Near this concentration there is evidence for the breakdown of Fermi liquid behavior in the transport and heat capacity measurements, suggesting that this may be a model system for studying a lattice instability at zero temperature, its relationship to a nearby antiferromagnetic quantum critical point, and the resulting impact on electronic properties and lattice modes in a strongly correlated electron metal.
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Date Issued: 2019
Identifier: 2019_Spring_Lai_fsu_0071E_15055
Format: Thesis

Title: Topological Quantum Phase Transitions and Quench Dynamics.
Creator: Liou, Shiuan-Fan, Yang, Kun, Siegrist, Theo, Bonesteel, N. E., Balicas, Luis, Engel, Lloyd W., Xiong, Peng, Florida State University, College of Arts and Sciences, Department of...
Show moreLiou, Shiuan-Fan, Yang, Kun, Siegrist, Theo, Bonesteel, N. E., Balicas, Luis, Engel, Lloyd W., Xiong, Peng, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Topological phases of matter and the phase transitions between them have been the focus of much recent theoretical and experimental interest. In this thesis, we firstly study a type of topological phase transitions between quantum Hall states driven by pairing interactions through Feshbach resonances. Although quantum Hall effects were first discovered in electronic condensed matter systems, this type of phase transitions was proposed to ultra cold atom field for their ability of the control...
Show moreTopological phases of matter and the phase transitions between them have been the focus of much recent theoretical and experimental interest. In this thesis, we firstly study a type of topological phase transitions between quantum Hall states driven by pairing interactions through Feshbach resonances. Although quantum Hall effects were first discovered in electronic condensed matter systems, this type of phase transitions was proposed to ultra cold atom field for their ability of the control of interactions. In addition, we also investigate the quench dynamics of topological phase transitions based on specifically Haldane model and checkerboard model in the second part. In the study of topological phase transitions between fermionic integer quantum Hall (FIQH) and bosonic fractional quantum Hall (BFQH) phases, we first provide a general picture of this kind of quantum Hall phase transitions. Subsequently, we use exact diagonalization to study the quantum phases and phase transitions when a single species of fermionic atoms at Landau level filling factor vf = 1 in a rotating trap interact through a p-wave Feshbach resonance. We show that under weak pairing interaction, the system undergoes a second order quantum phase transition from vf = 1 fermionic integer quantum Hall (FIQH) state at positive detuning, to vb = ¼ bosonic fractional quantum Hall (BFQH) state at negative detuning. However, when the pairing interaction increases, a new phase between them emerges, corresponding to a fraction of fermionic atoms stay in a coherent superposition of bosonic molecule state and an unbound pair. The phase transition from FIQH phase to the new phase is of second order and that from the new phase to BFQH phase is of first order. Furthermore, we investigate the quantum phases and phase transition in a system made of two species of fermionic atoms that interact with each other via s-wave Feshbach resonance, and are subject to rotation or a synthetic gauge field that puts the fermions at Landau level filling factor vf = 2. We show that the system undergoes a continuous quantum phase transition from a vf = 2 fermionic integer quantum Hall state formed by atoms, to a vf = ½ bosonic fractional quantum Hall state formed by bosonic diatomic molecules. In the disk geometry we use, these two different topological phases are distinguished by their different gapless edge excitation spectra, and the quantum phase transition between them is signaled by the closing of the energy gap in the bulk. Comparisons will be made with field theoretical predictions, and the case of p-wave pairing. In the second part of this thesis, we study the dynamics of systems quenched through topological quantum phase transitions and investigate the behavior of the bulk and edge excitations with various quench rates. Specifically, we consider the Haldane model and checkerboard model in slow quench processes with distinct band-touching structures leading to topology changes. The generation of bulk excitations is found to obey the power-law relation Kibble-Zurek and Landau-Zener theories predict. However, an anti-Kibble-Zurek behavior is observed in the edge excitations. The mechanism of excitation generation on edge states is revealed, which explains the anti-Kibble-Zurek behavior.
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Date Issued: 2019
Identifier: 2019_Spring_Liou_fsu_0071E_15071
Format: Thesis

Title: Search for New and Unusual Strangeonia States Using γp → pφη with GlueX at Thomas Jefferson National Accelerator Facility.
Creator: Cannon, Bradford Emerson, Eugenio, Paul Michael, Aldrovandi, Ettore, Capstick, Simon, Wahl, Horst, Crede, Volker, Ostrovidov, Alexander, Florida State University, College of...
Show moreCannon, Bradford Emerson, Eugenio, Paul Michael, Aldrovandi, Ettore, Capstick, Simon, Wahl, Horst, Crede, Volker, Ostrovidov, Alexander, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: We perform an analysis dedicated to the search for new and unusual strangeonium states produced in the reaction γp → pφη. The data used for this analysis was recorded during the Spring 2017 physics run for Hall D of Thomas Jefferson National Accelerator Facility, where the GlueX experiment is located. The GlueX experiment uses a linearly polarized coherent bremsstrahlung beam of up to 12 GeV in energy. This photon beam interacts with a stationary liquid hydrogen target located inside the...
Show moreWe perform an analysis dedicated to the search for new and unusual strangeonium states produced in the reaction γp → pφη. The data used for this analysis was recorded during the Spring 2017 physics run for Hall D of Thomas Jefferson National Accelerator Facility, where the GlueX experiment is located. The GlueX experiment uses a linearly polarized coherent bremsstrahlung beam of up to 12 GeV in energy. This photon beam interacts with a stationary liquid hydrogen target located inside the GlueX detector. The subsequent photoproduction will provide final states ideal for studying both exotic and non-exotic ss ̄ mesons. After all cuts, a total of four different selection methods were used to study the φη parent state. Three of these methods used an event by event probabilistic weighting method in order to separate signal from background, and the fourth method was simply an elliptical subtraction which did not utilize probabilistic weighting. After comparing the φη invariant mass spectra for all selection methods, two structures were consistently observed. One of the structures was found to have a mass of (m = 1.657 ± 0.008)GeV/c^2 and a width of (σ = 0.190 ± 0.024)GeV/^2 ; and the second structure was found to have a mass of (m = 1.879 ± 0.004)GeV/c^2 and a width of (σ = 0.042 ± 0.014)GeV/c^2 .
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Date Issued: 2019
Identifier: 2019_Spring_Cannon_fsu_0071E_15063
Format: Thesis

Title: Backbending, Seniority and Pauli Blocking of Pairing Correlations at High Rotational Frequencies in Rapidly Rotating Nuclei: A Systematic Analysis of Er, Yb, Hf and W Isotopes and Nuclear Structure Studies of ¹⁷⁹, ¹⁸⁰W, ¹⁶⁰, ¹⁶¹Gd and ¹⁵⁵Sm.
Creator: Villafana, Kalisa A. (Kalisa Aneika), Riley, Mark A., Plewa, Tomasz, Bonesteel, N. E., Piekarewicz, Jorge, Florida State University, College of Arts and Sciences, Department of...
Show moreVillafana, Kalisa A. (Kalisa Aneika), Riley, Mark A., Plewa, Tomasz, Bonesteel, N. E., Piekarewicz, Jorge, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: The goal of nuclear structure experiments is to understand how properties of nuclei evolve as a function of key observables such as proton and neutron numbers, deformation, angular momentum and excitation energy, to name a few. In order to investigate how the nuclear structure evolves with these parameters, γ-ray spectroscopy can be utilized, which requires efficient γ-ray detection systems. This thesis details γ-ray spectroscopy to investigate the nuclear structure in a series of rare-earth...
Show moreThe goal of nuclear structure experiments is to understand how properties of nuclei evolve as a function of key observables such as proton and neutron numbers, deformation, angular momentum and excitation energy, to name a few. In order to investigate how the nuclear structure evolves with these parameters, γ-ray spectroscopy can be utilized, which requires efficient γ-ray detection systems. This thesis details γ-ray spectroscopy to investigate the nuclear structure in a series of rare-earth nuclei using state of the art γ-ray detector systems. In the first part of this thesis, high-spin states in ¹⁷⁹, ¹⁸⁰W (Z=74) produced via fusion-evaporation reactions carried out at Florida State University's John D. Fox Laboratory are discussed. The reaction used to produce excited states in these nuclei was a 14C beam on an enriched ¹⁷⁰Er target, and the 5n and 4n evaporation channels were studied to investigate ¹⁷⁹, ¹⁸⁰W respectively. The emitted γ-rays were detected using three Compton-suppressed clover detectors and seven single element Compton-suppressed high-purity germanium detectors. In this experiment, 852 million γ-γ coincidences and 82 million γ-γ-γ coincidences at 75 MeV beam energy were collected. Additionally, at a beam energy of 68 MeV, 119 million γ-γ coincidences and 9.6million γ-γ-γ coincidences. The primary purpose of this experiment was to add to a systematic investigation of band crossing frequencies in heavy tungsten nuclei in order to observe the effect of quasiparticle seniority and high rotational frequencies on pairing correlations. Additionally, due in part to results obtained from the first part of this analysis, new systematic data in the A ≈ 160 − 180 region is also discussed, with an emphasis on the role that pair-blocking effects play during the rotation of the nucleus. This systematic investigation builds upon the classic findings of Garrett et al. [1] who investigated systematically the critical band crossing frequencies resulting from the rotational alignment of the first pair of i₁₃/₂ neutrons (AB) in rare-earth nuclei. In that study, evidence was found for an odd-even neutron number dependence attributed to changes in the strength of neutron pairing correlations. The present work carries out a similar investigation at higher rotational frequencies for the second pair of aligning i₁₃/₂ neutrons (BC), advancing the work started by Scott Miller, formerly of the Riley group [2]. Again, a systematic difference in band crossing frequencies is observed between odd-N and even-N Er, Yb, Hf, and W nuclei, but in the BC case, it is opposite to the AB neutron-number dependence. These results are discussed in terms of a reduction of neutron pairing correlations at high rotational frequencies and of the effects of Pauli blocking on the pairing field by higher-seniority configurations. Also playing a significant role are the changes in deformation with proton and neutron number, the changes of location of single-particle orbitals as a function of quadrupole deformation, and the position of the Fermi surface with regard to the various Ω (projection of total angular momentum I onto the symmetry axis) components of the neutron i₁₃/₂ shell. The second part of this thesis discusses in detail the nuclear structure of ¹⁶⁰Gd and highlights some new band structures in ¹⁵⁵Sm and ¹⁶¹Gd. Two reactions were carried out to produce a multitude of neutron-rich isotopes performed at the Argonne Tandem Linear Accelerator System (ATLAS) at Argonne National Laboratory (ANL). Firstly, a ¹⁶⁰Gd beam of energy at 1000 MeV was impinged on a ¹⁵⁴Sm target and then in a second experiment on a ¹⁶⁴Dy target. The goal of the deep-inelastic collisions was to provide a mechanism to reach a number of neutron-rich isotopes, in particular those from the mid-shell region in rare-earth nuclei. Although many neutron-rich nuclei were produced, they were not populated strongly enough to see new results. However, a byproduct of the reactions was the strong Coulomb excitation of the 160 Gd beam. Many excited states in ¹⁶⁰Gd were produced, and as a result, a spectroscopic analysis of 160Gd was carried out, and will be discussed in detail in this thesis. Additionally, new γ-ray transitions in other isotopes such as ¹⁵⁵Sm and ¹⁶¹Gd were also produced and will be discussed. The Gammasphere detector array was used to detect γ-rays from the excited nuclei, because of its sensitivity to cleanly delineate the vast number of multi-nucleon transfer reaction channels. As a result of both analysis, many new decay transitions and new energy levels were observed in the aforementioned nuclei. Whenever possible, the intensities, angular correlations, spins, parities, and rotational behaviors of these newly discovered states were analyzed.
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Date Issued: 2019
Identifier: 2019_Summer_Villafana_fsu_0071E_15455
Format: Thesis

Title: Beyond the Standard Model of Particles: Effective Field Theories and Baryogenesis.
Creator: Yunesi, Arash, Agashe, Amod S. (Amod Sadanand), Reina, Laura, Huffenberger, Kevin M., Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: Two big questions in physics beyond the Standard Model of particles are nature of Dark Matter and a theory of Quantum Gravity. In this work, topics related to both of these improtant questions are presented. First, we introduce an effective theory for soft and collinear limits of gravitational scatterings. It is a well-known fact that amplitudes including gravitons are inherently difficult to calculate. Our effective theory in its target phase space, substantially simplifies calculations of...
Show moreTwo big questions in physics beyond the Standard Model of particles are nature of Dark Matter and a theory of Quantum Gravity. In this work, topics related to both of these improtant questions are presented. First, we introduce an effective theory for soft and collinear limits of gravitational scatterings. It is a well-known fact that amplitudes including gravitons are inherently difficult to calculate. Our effective theory in its target phase space, substantially simplifies calculations of scattering amplitudes including gravtions. Our step by step procedure gives all the relevant operators at leading and next to leading powers for any full theory that couples to gravitons. In addition, the soft graviton theorem and decoupling of collinear gravitons at the leading power are manifest from the outset in the effective symmetries of the theory. At the next-to-leading power, certain simple structures of amplitudes, which are completely obscure in Feynman diagrams of the full theory, are also revealed. We will also discuss how ambiguity in choice of light-cone coordinates introduces fundamental redundancies in Soft Collinear Effective Theory (SCET). SCET Lagrangian should be invariant under these transformations of coordinates, and the constraints from these transformations further reduce calculations needed for a scattering process. Second, thermal freeze-out of WIMPs can provide a unified origin of dark matter and baryon abundances in our universe. We show that this mechanism exhibits rich collider phenomenology. The collider signatures we point out can be tested at the current and future experiments at the LHC, even if the WIMPs are not charged under Standard Model and higgs interactions. In particular, the simplest such implementation can already offer a very clean signal of a TeV-scale resonance that decays to diphotons with a cross section that can easily be within the reach of the current and near-future LHC runs in the region of parameter space that leads to a successful baryogenesis. Other characteristic signatures include the production of multi-bottom and/or multi-top quarks, promptly or displaced. An even more exotic possibility is the production of two separate sets of isolated emerging jets connected by a charged track, which may require new dedicated studies. Finally, di-nucleon decay can also provide a powerful probe of the mechanism.
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Date Issued: 2019
Identifier: 2019_Summer_Yunesi_fsu_0071E_15396
Format: Thesis

Title: Quench Protection of Bi2Sr2CaCu2O8+X High Temperature Superconducting Magnets.
Creator: Davis, Daniel S. (Daniel Scott), Larbalestier, D. (David), Chiorescu, Irinel, Trociewitz, Ulf P (Ulf Peter Trociewitz), Owens, Joseph F. (Joseph Francis), Riley, Mark A.,...
Show moreDavis, Daniel S. (Daniel Scott), Larbalestier, D. (David), Chiorescu, Irinel, Trociewitz, Ulf P (Ulf Peter Trociewitz), Owens, Joseph F. (Joseph Francis), Riley, Mark A., Latturner, Susan, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: High temperature superconductors (HTS) allow for the construction of magnets generating fields of more than 30 T, enabling the investigation of new phenomena in condensed matter science and high energy physics. High field magnets store large amounts of energy that must be dissipated if the field collapses, but HTS materials require large quantities of heat for transitioning to the normal state. This makes HTS conductors very stable against fluctuations, yet difficult to protect from extreme...
Show moreHigh temperature superconductors (HTS) allow for the construction of magnets generating fields of more than 30 T, enabling the investigation of new phenomena in condensed matter science and high energy physics. High field magnets store large amounts of energy that must be dissipated if the field collapses, but HTS materials require large quantities of heat for transitioning to the normal state. This makes HTS conductors very stable against fluctuations, yet difficult to protect from extreme temperature rise in the case that something generates a propagating normal or resistive zone in the magnet, i.e. quenches the superconductor. As HTS conductor is much more expensive than normal conductors or low temperature superconductors (LTS), protecting even prototype research coils and especially large-scale user solenoids and accelerator dipoles is paramount. Recently, a quench protection system relying on interfilament coupling currents within superconductors has been developed with LTS magnet systems. Coupling-loss induced quench (CLIQ) protection attempts to safely distribute the stored energy of a superconducting magnet over a larger volume by quickly bringing a significant fraction into the normal state by introducing oscillating currents into sections of the magnet generating heat due to the rapidly varying magnetic field. As HTS have larger energy margins to the normal state, in addition to different AC loss characteristics and conductor geometries, experiments and simulations are underway to evaluate and optimize AC loss induced quench for each conductor. Addressing the pressing need for reliable high field HTS magnets, presented here are the results for implementing these systems in magnets made from Bi2Sr2CaCu2O8+x (Bi-2212), which is the practical HTS most similar to LTS in both single strand and cable designs. Recent advances in the current carrying capacity of Bi-2212 (>1000 A mm^-2 at 5 T) due to improved starting powder and over-pressure heat treatment make this HTS appealing for large-scale magnet projects. Long sample conductor property measurements are underway to investigate if Bi-2212 has consistent electrical and mechanical properties along its length. Collaboration between the National High Magnetic Field Laboratory (NHMFL) and Lawrence Berkeley National Laboratory (LBL) has allowed for testing on world-record sub-scale accelerator dipoles and test coils for a program working towards generating more than 1 GHz NMR spectra and fields in excess of 30 T.
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Date Issued: 2019
Identifier: 2019_Summer_Davis_fsu_0071E_15413
Format: Thesis

Title: Photoresponse and Charge Transport in Halide Perovskites.
Creator: Wang, Xi, Gao, Hanwei, Hellstrom, Eric, Bonesteel, N. E., Piekarewicz, Jorge, Xiong, Peng, Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: Halide Perovskites have recently risen as a new class of optoelectronic materials. Remarkable optical and electrical properties have led to the demonstration of various perovskite-based devices such as solar cells1, LEDs2-5, photodetectors6 and lasers7,8. Particularly, perovskite solar cells have reached >24% of the energy conversion efficiency and outperformed most of the single-junction thin film solar cells available on the market1. Unfortunately, most of the perovskite-based devices...
Show moreHalide Perovskites have recently risen as a new class of optoelectronic materials. Remarkable optical and electrical properties have led to the demonstration of various perovskite-based devices such as solar cells1, LEDs2-5, photodetectors6 and lasers7,8. Particularly, perovskite solar cells have reached >24% of the energy conversion efficiency and outperformed most of the single-junction thin film solar cells available on the market1. Unfortunately, most of the perovskite-based devices remained more-or-less unstable due to a series of unusual behaviors such as current-voltage hysteresis9 and photo-induced phase segregation10,11. Studies about the underlying mechanisms are in demand. In this dissertation, I focused on studying the charge transport and photoresponse of halide perovskites to reveal the mechanisms related to material stability, particularly under electrical and optical stimuli. The changes of halide perovskite materials in a device under electrical operation were studied by using a microscopic tool, scanning photocurrent microscopy. The results showed the dynamic nature of the doping concentration in the hybrid perovskite CH3NH3PbI3, as a function of the external biasing voltages. Further studies on the synthesis methods showed such a dynamic process could be attributed to electric field-assisted ion migration mainly through defect sites. The partial suppression of ion migration was observed in materials processed at higher temperature. Except the electric-field triggered instability of the internal potential distribution, while under illumination, a different type of stability, the phase stability in mixed-halide perovskites attracted a lot of attention. Phase separation in mixed-halide perovskites under illumination was a tough problem, which directly related to the degradation of desired device performance. In this dissertation, the correlation between the phase stability and morphology was discovered. A model based on thermodynamics was developed to explain such a correlation. Based on the thermodynamic model, the composite materials CsPbX3/Cs4PbX6 with guest-host structures were created with the phase separation problem successfully solved. Furthermore, the composites are sustainably functionalized even under extreme conditions, i.e., under extremely intense illumination, making the composited useful for devices required to work in extreme conditions. The optical and electrical properties of CsPbX3/Cs4PbX6 composites were further investigated for the application of such composites to functional devices. Surprisingly, the presence of the photoluminescence inactive Cs4PbBr6 can significantly enhance the light emitting efficiency of CsPbBr3 in the composites. The unique negative thermal quenching observed near the liquid nitrogen temperature indicates that a type of shallow states generated at the CsPbBr3/Cs4PbBr6 interfaces is responsible for the enhancement of photoluminescence. Finally, light emitting diodes based on CsPbBr3/Cs4PbBr6 composites are demonstrated. Both quantum efficiency and emission brightness are improved significantly compared with similar devices constructed using pure CsPbBr3. The unfavorable charge transport property of host matrix Cs4PbBr6 could be circumvented by optimizing the ratio between the host and the guest components and the total thickness of the composite thin films. The inorganic composition of the emitting layer also leads to improved device stability under the condition of continuous operation. The studies in this dissertation indicated great potentials of composite materials with optimized designed properties. Depends on the application purposes, more matrix materials with the combination of halide perovskites need to be explored. The future plan will more directed to the investigations of fundamental photophysics and charge transport in a large collection of compositing combinations.
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Date Issued: 2019
Identifier: 2019_Summer_Wang_fsu_0071E_15292
Format: Thesis

Title: De Haas-Van Alphen Measurements in Topological Metals and Semimetals.
Creator: Chen, Kuan-Wen, Baumbach, Ryan E., Balicas, Luis, Greene, Laura H., Shatruk, Mykhailo, Manousakis, Efstratios, Graf, David E. (David Earl), Collins, David C. (David Christopher)...
Show moreChen, Kuan-Wen, Baumbach, Ryan E., Balicas, Luis, Greene, Laura H., Shatruk, Mykhailo, Manousakis, Efstratios, Graf, David E. (David Earl), Collins, David C. (David Christopher), Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: This thesis studied the electronic structure at the Fermi level and the topological character of topological semimetals via torque magnetometry. Torque magnetometry measures the anisotropic magnetization of the sample in a tilted magnetic field. In our measurements, the magnetic field is up to 35 T and the temperature is down to 300 mK. The oscillatory signal of the magnetization is detected, which is the so called "de Hass van Alphen (dHvA)" effect. By using dHvA effect, many important...
Show moreThis thesis studied the electronic structure at the Fermi level and the topological character of topological semimetals via torque magnetometry. Torque magnetometry measures the anisotropic magnetization of the sample in a tilted magnetic field. In our measurements, the magnetic field is up to 35 T and the temperature is down to 300 mK. The oscillatory signal of the magnetization is detected, which is the so called "de Hass van Alphen (dHvA)" effect. By using dHvA effect, many important parameters such as the geometry of Fermi surfaces, effective masses, quantum mobilities, Land ́e g factors and Berry's phases. It is especially important for the Berry's phase extraction. It is known that if there is a cyclotron orbit encircling a Dirac node, a non-trivial Berry's phase π can be extracted and a trivial Berry's phase 0 is expected for a conventional parabolic band. In the study of MAl3, we provided a detailed study of the dHvA oscillations and provided a comparison with the calculated band structures. The angular dependence of their Fermi surface cross-sectional areas reveals a remarkably good agreement with our first-principles calculations. dHvA supports the existence of tilted Dirac cones with Dirac type-II nodes located at 100, 230 and 250 meV above the Fermi level EF for VAl3,NbAl3 and TaAl3 respectively, in agreement with the prediction of broken Lorentz invariance in these compounds. However, for all three compounds we find that the cyclotron orbits on their FSs, including an orbit nearly enclosing the Dirac type-II node, yield trivial Berry phases. We showed that if one would like to derive a convincing Berry's phase from quantum oscillations one has to take into account the spin dephasing term in the LK formalism, and the precise location between the cyclotron orbit and the Dirac node. M2Te2X is studied via both torque magnetometry and angle-resolved photoemission spectroscopy (ARPES). Bulk two-dimensional Fermi surfaces are well-described by the dHvA oscillations and first principles calculations. Intriguingly, slab electronic structure calculations predict Dirac-like surface states at different locations within the Brillouin zone, which is consistent with ARPES observations.
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Date Issued: 2019
Identifier: 2019_Summer_Chen_fsu_0071E_15394
Format: Thesis

Title: The Sunyaev-Zel'Dovich Effect in Galaxy Clusters.
Creator: Fuzia, Brittany, Huffenberger, Kevin M., Zhao, Peixiang, Collins, David C. (David Christopher), Murphy, Jeremiah Wayne, Prosper, Harrison B., Florida State University, College...
Show moreFuzia, Brittany, Huffenberger, Kevin M., Zhao, Peixiang, Collins, David C. (David Christopher), Murphy, Jeremiah Wayne, Prosper, Harrison B., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Galaxy clusters are the largest gravitationally bound structures in the Universe, and they provide valuable insights into astrophysics and cosmology. One method of studying galaxy clusters - the Sunyaev-Zel'dovich Effect - involves using the Cosmic Microwave Background (CMB) as a backlight. As CMB photons pass through the hot electron gas of the intracluster medium, a small portion are scattered and gain energy. This causes a distortion in the spectrum of the CMB which depends mainly on the...
Show moreGalaxy clusters are the largest gravitationally bound structures in the Universe, and they provide valuable insights into astrophysics and cosmology. One method of studying galaxy clusters - the Sunyaev-Zel'dovich Effect - involves using the Cosmic Microwave Background (CMB) as a backlight. As CMB photons pass through the hot electron gas of the intracluster medium, a small portion are scattered and gain energy. This causes a distortion in the spectrum of the CMB which depends mainly on the total mass of the galaxy cluster and is independent of redshift, making it an important means of probing high-redshift systems. Only recently have telescopes become sensitive enough to detect the Sunyaev-Zel'dovich effect for low masses. In this thesis, we discuss the work done using the Sunyaev-Zel'dovich effect to study low-mass galaxy clusters and to identify new galaxy clusters in CMB maps.
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Date Issued: 2019
Identifier: 2019_Summer_Fuzia_fsu_0071E_15345
Format: Thesis

Title: Study of the ¹⁸Ne(α, p)²¹Na Reaction with ANASEN and Its Significance in the Breakout from the Hot-CNO Cycle.
Creator: Anastasiou, Maria, Weidenhoever, Ingo Ludwing M., Albrecht-Schmitt, Thomas E., Huffenberger, Kevin M., Riley, Mark A., Volya, Alexander, Florida State University, College of...
Show moreAnastasiou, Maria, Weidenhoever, Ingo Ludwing M., Albrecht-Schmitt, Thomas E., Huffenberger, Kevin M., Riley, Mark A., Volya, Alexander, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: 18Ne(α,p)21Na reaction is one of the reactions providing a pathway for breakout from the hot CNO cycles to the rp-process in Type I X-ray bursts. The actual conditions under which the breakout occurs depend critically on the thermonuclear reaction rate. This rate has not been sufficiently determined yet over the temperatures present under X-ray burst conditions. We study the direct 18Ne(α,p)21Na reaction with the Array for Nuclear Astrophysics and Structure with Exotic Nuclei (ANASEN), using...
Show more18Ne(α,p)21Na reaction is one of the reactions providing a pathway for breakout from the hot CNO cycles to the rp-process in Type I X-ray bursts. The actual conditions under which the breakout occurs depend critically on the thermonuclear reaction rate. This rate has not been sufficiently determined yet over the temperatures present under X-ray burst conditions. We study the direct 18Ne(α,p)21Na reaction with the Array for Nuclear Astrophysics and Structure with Exotic Nuclei (ANASEN), using a helium gas target and an 18Ne radioactive beam. ANASEN is an active gas target detection system that uses tracking of the light reaction products in conjunction with energy measurements in Silicon detectors. The position information required for the tracking is provided by a Multi-Anode Proportional Counter in combination with the Silicon detectors. From the tracking the location of the interaction is obtained, which is directly correlated to the energy of the beam particle. While the beam is losing energy while traveling in the gas target, a wide range of reaction energies can be measured simultaneously and without changing the accelerator parameters. The difficulty of this particular experiment lies on the fact that we are trying to detect one single proton from the 18Ne(α,p)21Na reaction per event. Proton background is caused by fusion evaporation reactions of the 18Ne with the CO2 quenching gas added on the 4He target gas. For the first time in the ANASEN setup, we have implemented a cylindrical Ion Chamber for coincident heavy-recoil detection, which was successfully used to suppress such background events. The 18Ne(α,p)21Na cross section was measured in the context of this dissertation. The experiment allows for a determination of the cross section down to reaction energies ∼2 MeV in the center-of- mass system. The results are compared to the previous (α,p) reaction measurement, as well as to the time-inverse (p,α) reaction measurement and theoretically calculated cross sections. Our work resolves significant inconsistencies between the experimental information on the 18Ne(α,p) reaction and the indirect information available, giving larger credence to the use of such indirect methods. At the same time, more sensitive measurements of the 18Ne(α,p) reaction are needed to provide experimental information on the reaction energies below 2 MeV, most important for the break-out phase of X-ray bursts. The experimental techniques developed in this work would have to be applied to a beam of 18Ne with significantly higher quality and intensity.
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Date Issued: 2019
Identifier: 2019_Summer_Anastasiou_fsu_0071E_15357
Format: Thesis

Title: Topological Materials and Their Interfaces.
Creator: Aryal, Niraj, Manousakis, Efstratios, Dalal, Naresh S., Bonesteel, N. E., Berg, Bernd A., Balicas, Luis, Florida State University, College of Arts and Sciences, Department of...
Show moreAryal, Niraj, Manousakis, Efstratios, Dalal, Naresh S., Bonesteel, N. E., Berg, Bernd A., Balicas, Luis, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: This dissertation is a theoretical and computational examination of electronic properties of topological materials, such as topological insulators and Weyl semimetals. Our work is motivated by various experimental observations and theoretical predictions about the presence of exotic electronic properties and transport phenomena in different topological materials. These materials have been a subject of intense research since last decade because their low-energy dispersion can be described by...
Show moreThis dissertation is a theoretical and computational examination of electronic properties of topological materials, such as topological insulators and Weyl semimetals. Our work is motivated by various experimental observations and theoretical predictions about the presence of exotic electronic properties and transport phenomena in different topological materials. These materials have been a subject of intense research since last decade because their low-energy dispersion can be described by Dirac and Weyl equations and they are predicted to have many exciting properties of both fundamental and practical significance. In this dissertation, we examine theoretical predictions and experimental measurements using density functional theory (DFT) based methods and using calculations based on a model Hamiltonian. In the first half of this dissertation, we present our detailed results for the Weyl semimetal candidate T[subscript d]-MoTe₂ using DFT and DFT+U methods while making careful comparison to different experiments in order to validate our results. We also address the fate of the Weyl fermions as a function of the Hubbard U using both DFT and a model Hamiltonian approach. Moreover, from our calculations, we predict that the system could be in close vicinity of a Lifshitz transition. Such a prediction can be experimentally verified by means of doping or electrostatic gating. In the second half of this dissertation, we present our results for the surface and interface states of the prototypical topological insulator material Bi₂Se₃. We study interfaces of the topological insulator with different conventional insulators using both DFT and model Hamiltonian calculations and examine the fate of the topological states at the interface. Moreover, we predict the occurrence of a topological phase transition in the interface geometry from our calculations and point towards a few directions where some of these predictions could be verified experimentally. We conclude by presenting implications for further work.
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Date Issued: 2019
Identifier: 2019_Summer_Aryal_fsu_0071E_15363
Format: Thesis

Title: Experimental Efforts to Study the Nuclear Structure of ³³P and ³⁸Cl and a Theoretical Endeavor to Develop an Empirical Shell-Model Interaction.
Creator: Lubna, Rebeka Sultana, Tabor, Samuel L., Tripathi, Vandana, Albrecht-Schmitt, Thomas E., Volya, Alexander, Reina, Laura, Florida State University, College of Arts and Sciences,...
Show moreLubna, Rebeka Sultana, Tabor, Samuel L., Tripathi, Vandana, Albrecht-Schmitt, Thomas E., Volya, Alexander, Reina, Laura, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: In this thesis, the excited states ³³P were populated by the ¹⁸O+¹⁸O reaction at E[subscript lab]=24 MeV. The GAMMASPEHRE array was used along with the Microball particle detector array to detect γ transitions in coincidence with the charged particles emitted from the compound nucleus ³⁶S. The use of Microball enabled the selection of the proton emission channel. It also helped in determining the position and energy of the emitted proton; which eventually helped in calculating more precise...
Show moreIn this thesis, the excited states ³³P were populated by the ¹⁸O+¹⁸O reaction at E[subscript lab]=24 MeV. The GAMMASPEHRE array was used along with the Microball particle detector array to detect γ transitions in coincidence with the charged particles emitted from the compound nucleus ³⁶S. The use of Microball enabled the selection of the proton emission channel. It also helped in determining the position and energy of the emitted proton; which eventually helped in calculating more precise direction of the recoils to achieve better Doppler corrections. 16 new transitions and 13 new states were observed in ³³P for the first time. The nearly 4π geometry of GAMMASPEHRE allowed the measurement of γ-ray angular distributions leading to spin suggestions for many states. In a separate experiment conducted at the John D. Fox laboratory in Florida State University, the higher-spin structure of ³⁸Cl (N = 21) was investigated following the ²⁶Mg(¹⁴C, pn) reaction at 30 and 37 MeV. The outgoing protons were detected in an E ‒ ΔE Si telescope placed at 0° close to the target with a Ta beam stopper between the target and telescope. Multiple γ rays were detected in time coincidence with the protons using an enhanced version of the FSU γ detection array. A total of 11 new γ transitions and 6 new states were reported for the first time. DCO ratio analysis and measurement of polarization asymmetry for the emitted γ transitions were performed to assign spins and parities to a number of states. The level scheme was extended up to 8420 keV with a likely spin of 10 ħ. A new empirical shell model interaction was developed in the spsdfp model space. This FSU interaction was built by fitting to the energies of 270 experimental states from ¹³C to ⁵¹Ti. Calculations using the FSU interaction reproduced observed energy states of ³³P and ³⁸Cl rather well, including other spectroscopic properties. The interaction has been used to predict the intruder states of other sd-shell nuclei, along with the configurations of the nuclei belong to the Island of Inversion region of the nuclear landscape.
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Date Issued: 2019
Identifier: 2019_Summer_Lubna_fsu_0071E_15247
Format: Thesis

Title: Developing Multi-Frequency EPR Methods for Studying Protein-Lipid Interactions on the HIV Membrane.
Creator: Hayati, Zahra, Song, Likai, Hill, S. (Stephen Olof), Cross, Timothy A., Bonesteel, N. E., Murphy, Jeremiah Wayne, Florida State University, College of Arts and Sciences,...
Show moreHayati, Zahra, Song, Likai, Hill, S. (Stephen Olof), Cross, Timothy A., Bonesteel, N. E., Murphy, Jeremiah Wayne, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Electron paramagnetic resonance (EPR) spectroscopy is a powerful technique to study biomolecules. EPR has been employed to investigate the structure and dynamics of biological membranes, membrane proteins, and protein-lipid interactions. Multi-frequency and high-field experiments enhance the ability of EPR to observe spin dynamics at different time scales and to obtain high-resolution spectra, including 𝑔 anisotropy sensitivity. However, the lack of available techniques and instruments...
Show moreElectron paramagnetic resonance (EPR) spectroscopy is a powerful technique to study biomolecules. EPR has been employed to investigate the structure and dynamics of biological membranes, membrane proteins, and protein-lipid interactions. Multi-frequency and high-field experiments enhance the ability of EPR to observe spin dynamics at different time scales and to obtain high-resolution spectra, including 𝑔 anisotropy sensitivity. However, the lack of available techniques and instruments hinders the application of high-field EPR for biological studies. In this work, we focused on the development of multi-frequency EPR methods in order to characterize protein-lipid interactions on the human immunodeficiency virus (HIV) membrane. HIV infects T cells through a process of membrane fusion. The membrane-bound regions of a viral surface protein gp41, including the membrane proximal ectodomain region (MPER) and the transmembrane region (TM), facilitate membrane fusion and have been targeted for vaccine and drug development. This fusion process is mediated by the raft-like viral membrane that contains a high quantity of cholesterol. The molecular details of gp41-viral membrane interactions during the fusion process are still unclear. To investigate these interactions, we introduced and further developed several EPR methods: 1) Determining peptide-induced lipid orientational disorder using magnetically aligned bicelles (Chapter 3). Bicelles with 20 mol% cholesterol were applied to elucidate how the MPER/TM disrupts the lipid orientational order of the viral membrane. 2) Analyzing the lipid lateral ordering of raft-like lipids using EPR at 94 GHz (Chapter 4). The results demonstrated that this parameter, which reflects the order of the lipids along the membrane plane, has a high sensitivity to lipid motion changes as a result of protein-lipid interactions or lipid composition/position differences. 3) Development of magnetically aligned bicelles containing raft-like lipids using multi-frequency EPR (Chapter 5). For the first time, raft- like bicelles were aligned in the magnetic field with an optimized q value and lipid ratios. These aligned membranes enable studies of membrane properties and protein associations, including the MPER/TM. Other methods involved in this study included lipid fluidity measurements, a fairly new membrane permeability assay, and the application of spin-spin distance measurements to determine peptide self-association and oligomerization. Using these methods, we demonstrated that the MPER interacts strongly with the viral membrane, perturbs the bilayer, and induces significant lipid mobility, membrane permeability, and lipid orientational order changes. The MPER-induced membrane property changes are modulated by the cholesterol content and the TM. Cholesterol inhibits MPER-lipid interactions and promotes MPER/TM oligomerization. The TM stabilizes the MPER on the membrane and abolishes the inhibition effect of cholesterol. In summary, we have improved multi-frequency EPR techniques to study proteins and membranes, and elucidated the mechanism of‎ the‎ MPER/TM‎ of‎ HIV‎ gp41’s‎ interaction with the viral membrane.
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Date Issued: 2019
Identifier: 2019_Fall_Hayati_fsu_0071E_15110
Format: Thesis

Title: Electrical Properties Mapping and Coil Characterization at High Magnetic Fields.
Creator: Amouzandeh, Ghoncheh, Grant, Samuel C., Hill, S. (Stephen Olof), Levenson, Cathy W., Brey, William W., Boebinger, Gregory S., Reina, Laura, Florida State University, College of...
Show moreAmouzandeh, Ghoncheh, Grant, Samuel C., Hill, S. (Stephen Olof), Levenson, Cathy W., Brey, William W., Boebinger, Gregory S., Reina, Laura, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Electrical properties (EP), namely conductivity and permittivity, can provide endogenous (having an internal origin) contrast for tissue characterization. EP of the biological tissue are strongly related to essential determinants of the physiological state of tissue such as ionic concentration and mobility, water content and cell structure. As a result, non-invasive measurement of EP has been increasingly used in neuroimaging and other areas. Magnetic resonance electrical property tomography ...
Show moreElectrical properties (EP), namely conductivity and permittivity, can provide endogenous (having an internal origin) contrast for tissue characterization. EP of the biological tissue are strongly related to essential determinants of the physiological state of tissue such as ionic concentration and mobility, water content and cell structure. As a result, non-invasive measurement of EP has been increasingly used in neuroimaging and other areas. Magnetic resonance electrical property tomography (EPT) is a recently introduced technique that can provide maps of EP from conventional MRI data by measuring the distortions induced on the radio frequency (RF) field (B1+). Although its feasibility has been shown at clinical field strengths (1-7T), the application of EPT to routine medical protocols is limited, partly due to reconstruction inaccuracies and variations. In this dissertation, the first application of EPT at 21.1 T (900 MHz), the highest magnetic field available for MRI, is presented with a focus on pre-clinical research. The ultra-high pre-clinical field provides improved signal-to-noise ratios and higher interaction between sample EP and the applied RF field that can enhance EPT accuracy and precision. Helmholtz-based EPT was implemented in its full-form, which demands the complex B1+ field, and a simplified form requiring either just the B1+ field phase for conductivity or the B1+ field magnitude for permittivity. Experiments were conducted at 21.1 T using birdcage and saddle coils operated in linear or quadrature transceive mode, respectively. Feasibility and accuracy of EPT approaches at this field were evaluated using a phantom, ex and in vivo Sprague-Dawley rats under the conditions of naïve and ischemic stroke via transient middle cerebral artery occlusion. Different conductivity reconstruction approaches applied to the phantom displayed average errors of 23-86% to target values. Permittivity reconstructions showed higher agreement and an average 5-8% error to the target depending on the reconstruction approach. The full-form technique generated from the linear birdcage provided the best accuracy for the EP of the phantom. Phase-based conductivity and magnitude-based permittivity mapping provided reasonable estimates but also demonstrated the limitations of Helmholtz-based EPT at 21.1 T. Conductivity and permittivity of ex and in vivo rodent brains also were measured. With the aim to demonstrate the applicability of EPT for ischemic stroke studies, EP of the in vivo rat brain with and without ischemia were measured. The findings demonstrate significantly elevated conductivity and permittivity in the ischemic stroke lesion compared to the contralateral non-pathological side correlated with the increased sodium content and the influx of water intracellularly following ischemia. Permittivity reconstruction was improved significantly over lower fields, suggesting a novel metric for in vivo brain studies. The last section of this dissertation aims to address the implementation of high temperature superconducting (HTS) coils for nuclear magnetic resonance (NMR) spectroscopy with a particular focus on the transmit coil’s characteristics for 13C NMR. NMR is widely used to study the molecular structure and dynamics of molecules in solution, and 13C NMR is critical for structural elucidation in organic chemistry. However, the low sensitivity of NMR has meant that relatively large amounts of the sample or alternate techniques are needed to improve sensitivity. Replacing the normal-metal pickup coils with thin-film HTS resonators has been shown to increase the sensitivity of NMR and reduce the amount of sample required. It also would be convenient and beneficial to use HTS resonators to excite as well as to detect the NMR signal. However, producing a sufficiently strong and rapidly switched excitation field is more challenging with thin-film HTS resonators than with the normal metal coils that they would replace. While double-sided HTS resonators can significantly increase the achievable RF field, the high Q factor of the HTS resonator limits the pulse bandwidth and the minimum dead time following a pulse before reception can begin. This study explored several important aspects of the use of HTS resonators as NMR excitation coils. The presented analysis showed non-linearity in the coil’s response and current compression when high power levels are applied to the coil. Additionally, time domain representation of the excitation pulses generated by the coil showed long ring-up and ring-down times as well as distorted pulse shapes at different power levels. The Fourier transformation of these pulses displayed the limited bandwidth of the coil, which can be problematic for exciting and receiving the whole 13C spectrum. To mitigate the elongated pulse shapes, a shorted stub was added to the transmission path. The result demonstrated improved pulse shapes and reduced phase transients. A similar technique is expected to be applied to the HTS NMR probe to increase the applicability of HTS resonators for transmission.
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Date Issued: 2019
Identifier: 2019_Fall_Amouzandeh_fsu_0071E_15334
Format: Thesis

Title: The Weak Nuclear Form Factor: Nuclear Structure & Coherent Elastic Neutrino-Nucleus Scattering.
Creator: Hernandez, Jesse A. (Jesse Antonio), Piekarewicz, Jorge, Capstick, Simon, Wiedenhoever, Ingo Ludwing M., Florida State University, College of Arts and Sciences, Department of...
Show moreHernandez, Jesse A. (Jesse Antonio), Piekarewicz, Jorge, Capstick, Simon, Wiedenhoever, Ingo Ludwing M., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: The weak nuclear form factor is the final missing piece to complete our understanding of the nuclear structure and the Coherent Elastic Neutrino Nucleus Scattering (CEvNS) cross section. The weak form factor is dominated by the neutron distributions of the atomic nucleus, which are poorly known. The complex scalar and vector potentials within the nucleus are provided using a Relativistic Mean Field (RMF) approach. These potentials are used to calculate the point neutron and proton...
Show moreThe weak nuclear form factor is the final missing piece to complete our understanding of the nuclear structure and the Coherent Elastic Neutrino Nucleus Scattering (CEvNS) cross section. The weak form factor is dominated by the neutron distributions of the atomic nucleus, which are poorly known. The complex scalar and vector potentials within the nucleus are provided using a Relativistic Mean Field (RMF) approach. These potentials are used to calculate the point neutron and proton distributions, and in conjunction with single-nucleon electric Sachs form factors---obtained from data---are used to predict the weak form factor. It is determined that the radius of the proton distributions agree within 1% to the experimental values. The agreement of the proton distributions gives confidence that the predicted neutron distributions are computed correctly. It is also determined that to constrain the weak form factor, next-generation measurements need to be at 1% error. The correction due to the weak form factor for CEvNS is determined to be approximately between 10-40% for neutron number, N, ranging from 20 to 126. The findings show that the precise measurement of the weak form factor is necessary for the accurate determination of the CEvNS cross section and our comprehension of the nuclear structure as a whole. Most importantly, the predicted quantities and estimated errors give guidance to experiments measuring the weak form factor.
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Date Issued: 2019
Identifier: 2019_Fall_Hernandez_fsu_0071N_15473
Format: Thesis

Title: Clustering in Light Nuclei with Configuration Interaction Approaches.
Creator: Kravvaris, Konstantinos, Volya, Alexander, Kopriva, David A., Weidenhoever, Ingo Ludwing M., Capstick, Simon, Reina, Laura, Florida State University, College of Arts and...
Show moreKravvaris, Konstantinos, Volya, Alexander, Kopriva, David A., Weidenhoever, Ingo Ludwing M., Capstick, Simon, Reina, Laura, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: The formation of sub-structures within an atomic nucleus, appropriately termed nuclear clustering, is one of the core questions of nuclear many-body physics. In this thesis, we put forward a new method for the study of nuclear clustering relying on the completely microscopic Configuration Interaction approach. We construct reaction cluster channels in a Harmonic Oscillator many-body basis that respect the symmetries of the Hamiltonian, are fully antisymmetrized, and carry a separable and...
Show moreThe formation of sub-structures within an atomic nucleus, appropriately termed nuclear clustering, is one of the core questions of nuclear many-body physics. In this thesis, we put forward a new method for the study of nuclear clustering relying on the completely microscopic Configuration Interaction approach. We construct reaction cluster channels in a Harmonic Oscillator many-body basis that respect the symmetries of the Hamiltonian, are fully antisymmetrized, and carry a separable and controlled Center of Mass component. Such channels are then used to explore cluster signatures in Configuration Interaction many-body wavefunctions. The Resonating Group Method is then applied, utilizing the reaction channels as a basis to capture the essential cluster characteristics of the system. We investigate the emergence of nuclear clustering in 2α, 2α+n, 2α+2n and 3α systems using a No Core Shell Model approach from first principles, and traditional Shell Model studies of clustering in heavier nuclei.
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Date Issued: 2018
Identifier: 2018_Su_Kravvaris_fsu_0071E_14611
Format: Thesis

Title: The Search for N* Resonances: Measurement of Differential Cross Sections and Polarization Observables for γp → pω and γp → K0Σ+ Using Circularly-Polarized Photons at CLAS, Jefferson Lab.
Creator: Akbar, Zulkaida, Crede, Volker, Meyer-Bäse, Anke, Piekarewicz, Jorge, Eugenio, Paul Michael, Adams, Todd, Florida State University, College of Arts and Sciences, Department of...
Show moreAkbar, Zulkaida, Crede, Volker, Meyer-Bäse, Anke, Piekarewicz, Jorge, Eugenio, Paul Michael, Adams, Todd, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: The study of baryon resonances offers a deeper understanding of the strong interaction, since the dynamics and relevant degrees of freedom hidden within them are reflected by the properties of these states. The baryon resonances have been fairly accurately predicted in the low-energy region by constituent quark models and lattice quantum chromodynamics. However, most of the predicted higher-lying excited resonances (center-of-mass energies above 1.7 GeV/c²) and experimental findings do not...
Show moreThe study of baryon resonances offers a deeper understanding of the strong interaction, since the dynamics and relevant degrees of freedom hidden within them are reflected by the properties of these states. The baryon resonances have been fairly accurately predicted in the low-energy region by constituent quark models and lattice quantum chromodynamics. However, most of the predicted higher-lying excited resonances (center-of-mass energies above 1.7 GeV/c²) and experimental findings do not match up. The model calculations predict more baryon resonances than have been experimentally observed. Quark model calculations have suggested that some of the unobserved resonances couple strongly to γp reactions. The higher-lying excited are also generally predicted to have strong couplings to final states involving a heavier meson, e.g. one of the vector mesons, ρ, ω, ϕ. The excited states of the nucleon are usually found as broadly overlapping resonances, which may decay into a multitude of finasl states involving mesons and baryons. Polarization observables make it possible to isolate singleresonance contributions from other interference terms. This works presents measurements of the helicity asymmetry, E, for the reaction γp → pω in the energy range 1.1 GeV < Eγ < 2.3 GeV, differential cross sections, and spin density matrix elements, also for the reaction γp → pω in the energy range 1.5 GeV < Eγ < 5.4 GeV. Photoproduction of nucleon resonances in their decay to strange particles also offers attractive possibilities because the strange quark in the particle generates another degree of freedom and gives additional information not available from the nucleon-nucleon scattering. Thus, we have also extracted the helicity asymmetry, E, for the reaction γp → K⁰Σ⁺ in the energy range 1.1 GeV < Eγ < 2.1 GeV, differential cross sections, and recoil hyperon polarization, P, also for the reaction γp → K⁰Σ⁺ in the energy range 1.15 GeV < Eγ < 3.0 GeV. The data were collected at Jefferson Lab, using the CLAS detector, as part of the g9a and g12 experiments. Both experiments, as part of the N* spectroscopy program at Jefferson Laboratory, accumulated photoproduction data using circularly-polarized photons incident on a longitudinally-polarized butanol target in the g9a experiment and un-polarized liquid hydrogen target for the g12 experiment. A partial-wave analysis to the E data for the reaction γp → pω within the Bonn-Gatchina framework found dominant contributions from the 3/2⁺ near threshold, which is identified with the sub-treshold N(1720)3/2⁺ resonance. Some additional resonances and the t-channel π and pomeron exchange are needed to describe the data.
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Date Issued: 2018
Identifier: 2018_Su_Akbar_fsu_0071E_14714
Format: Thesis

Title: Spin Transport in Silicon Nanowires with an Intrinsic Axial Doping Gradient.
Creator: Kountouriotis, Konstantinos, Xiong, Peng, Lenhert, Steven John, Hill, S., Crede, Volker, Schlottmann, Pedro U., Florida State University, College of Arts and Sciences,...
Show moreKountouriotis, Konstantinos, Xiong, Peng, Lenhert, Steven John, Hill, S., Crede, Volker, Schlottmann, Pedro U., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: This dissertation is focused on electrical spin injection and detection at the nanoscale dimensions that semiconductor nanowires offer. Semiconductor spintronics is the natural extension of metallic spintronics for applications in semiconductor industry. After the tremendous impact of the giant magnetoresistance effect (GMR) in hard disk read heads, semiconductor spintronics has been thought as the key ingredient for the realization of spin field-effect transistors (Spin-FETs). The advantages...
Show moreThis dissertation is focused on electrical spin injection and detection at the nanoscale dimensions that semiconductor nanowires offer. Semiconductor spintronics is the natural extension of metallic spintronics for applications in semiconductor industry. After the tremendous impact of the giant magnetoresistance effect (GMR) in hard disk read heads, semiconductor spintronics has been thought as the key ingredient for the realization of spin field-effect transistors (Spin-FETs). The advantages of spintronic devices would include non-volatility, enhanced data processing speeds, decreased electric power consumption and facilitation of quantum computation. The primary goal of this research is to study spin dynamics and spin-polarized transport in semiconductor nanowire (NW) channels, specifically in phosphorus (P) doped silicon (Si) nanowires (NWs). The interest in one-dimensional (1D) nanoscopic devices is driven by the rich spin-dependent physics quantum confinement engenders, and the eventual miniaturization of the spintronic devices down to nanoscales. One of the most important aspects to achieve efficient spin injection from a ferromagnet (FM) into a semiconductor (SC) is the interface between the two materials. This study is focused primarily on this effect and how it can be tuned. In this work, we peform systematic spin transport measurements on a unique type of P-doped Si NWs which exhibit an inherent doping gradient along the axial direction. On a single NW, we place a series of FM electrodes, which form contacts that evolve from Ohmic-like to Schottky barriers of increasing heights and widths due to the pronounced doping gradient. This facilitates rigorous investigation of the dependence of the spin signal on the nature of the FM/SC interface. The advantage of using a single NW to study the afformentioned effects is that possible complications during the fabrication process are minimized compared to experiments that use multiple different devices to perform such experiments. 2-terminal (2T), nonlocal 4-terminal (4T) and 3-terminal (3T) spin valve (SV) measurements using different configurations of FM electrodes were performed on the Si NWs. In addition, 3T and nonlocal 4T Hanle measurements were performed. The collected data reveal distinct correlations between the spin signals and the injector and detector interfacial properties. These results were possible due to the unique inhomogeneous doping profile of our Si NWs. This study reveals a distinct correlation between the spin signals and the FM/Si NW injector interfacial properties. Specifically, we observe a decreasing injected current spin polarization due to diminishing contribution of the d-electrons, thus the necessary tunneling contact for efficient spin injection and its properties are being investigated and analyzed. The results demonstrate that there is an optimal window of interface resistance parameters for maximum injected current spin polarization. In addition, they suggest a new approach for maximizing the spin signals by making devices with asymmetric interfaces. To the best of our knowledge, this is the first report of electrical spin injection in SC channels with asymmetric interfaces.
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Date Issued: 2018
Identifier: 2018_Su_Kountouriotis_fsu_0071E_14593
Format: Thesis

Title: Lateral P-N Junctions Based on 2-D Materials.
Creator: Memaran, Shahriar, Balicas, Luis, Rikvold, Per Arne, Shatruk, Mykhailo, Manousakis, Efstratios, Cao, Jianming, Almaraz-Calderon, Sergio J., Florida State University, College of...
Show moreMemaran, Shahriar, Balicas, Luis, Rikvold, Per Arne, Shatruk, Mykhailo, Manousakis, Efstratios, Cao, Jianming, Almaraz-Calderon, Sergio J., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: The discovery of graphene marked a turning point in research and interest towards 2 -D materials. Among them, Transition Metal Dichalcogenides (TMDs) and Metal Monochalcogenides (MM) have seen an upturn in interest owing to their versatile properties. Although, they have been studied for many years in bulk form, recent advances in nano-technology enabled new opportunities to study the role of atomically thin materials. In recent years much work has been dedicated to development of their...
Show moreThe discovery of graphene marked a turning point in research and interest towards 2 -D materials. Among them, Transition Metal Dichalcogenides (TMDs) and Metal Monochalcogenides (MM) have seen an upturn in interest owing to their versatile properties. Although, they have been studied for many years in bulk form, recent advances in nano-technology enabled new opportunities to study the role of atomically thin materials. In recent years much work has been dedicated to development of their application for the next generation of electronic and optoelectronic devices, and we are witnessing the dawn of the exploration of their properties. In Chapter 1 a brief introduction of highlighted properties of the newly emerged 2 -D materials and their heterostructures is provided. Chapter 2 focuses on field-effect transistor response of few atomic layers of MoSe2, MoTe2 and WSe2. In contrast to previous reports on MoSe2 FETs electrically contacted with Ni, MoSe2 FETs electrically contacted with Ti display ambipolar behavior with current ON to OFF ratios up to 10^6 for both hole and electron channels when applying a small excitation voltage. For both channels the Hall effect indicates Hall mobilities H = 250 cm^2/V.s. Our MoTe2 field-effect transistors are observed to be hole-doped, displaying ON/OFF ratios surpassing 10^6 and typical subthreshold swings of ~140mV per decade. Both field-effect and Hall mobilities indicate maximum values approaching or surpassing 10 cm^2/V.s, which are comparable to figures previously reported for single or bilayer MoS2 and/or for MoSe2 exfoliated onto SiO2 at room temperature and without the use of dielectric engineering. Temperature dependent comparison between field-effect and Hall mobilities in field effect transistors based on few-layered WSe2 exfoliated onto SiO2 is also reported. We observe maximum hole mobilities approaching 350 cm^2/V.s at T = 300 K. The hole Hall mobility reaches a maximum value of 650 cm^2/V.s as T is lowered below ~150 K, indicating that insofar WSe2- based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides. Chapter 3 evaluates electrostatically gated p-n junctions based on MoSe2 and the photovoltaic response of electrostatically generated p-n junctions composed of approximately 10 atomic layers of MoSe2 stacked onto dielectric h-BN is presented. In addition to ideal diode-like response, we find that these junctions can yield photovoltaic effciencies exceeding 14% under standard solar simulator spectrum with fill factors values of about 70 %. Chapter 4 presents electrical and optical characterization of monolayer and bilayer lateral heterostructures of MoS2-WS2 and MoSe2-WSe2, grown by a one-pot chemical vapor deposition (CVD) synthesis approach, using a single heterogeneous solid source, a newly developed CVD growth method that eliminates the need for the exchange of multiple sources which leads to sample air exposure. The structures show a diode like response which is enhanced under optical illumination. Additionally, bilayer lateral heterostructures exhibit a clear photovoltaic response to optical excitation.
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Date Issued: 2018
Identifier: 2018_Sp_Memaran_fsu_0071E_14363
Format: Thesis

Title: High Magnetic Field Studies of Doped Plutonium and Uranium Based Superconductors.
Creator: Wartenbe, Mark, Boebinger, Gregory S., Siegrist, Theo, McDonald, Ross, Baumbach, Ryan E., Dobrosavljević, Vladimir, Cao, Jianming, Florida State University, College of Arts and...
Show moreWartenbe, Mark, Boebinger, Gregory S., Siegrist, Theo, McDonald, Ross, Baumbach, Ryan E., Dobrosavljević, Vladimir, Cao, Jianming, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Two heavy Fermion superconducting compounds, URu₂Si₂ and PuRhIn₅, were investigated by the techniques of chemical substitution and application of high magnetic fields. These materials are particularity interesting for their unique electronic and ordered state behavior (hidden order and superconducting phases) and proximity to magnetism. It is thought the superconductivity of these materials is unconventional and that they exhibit some features that are associated with quantum criticality,...
Show moreTwo heavy Fermion superconducting compounds, URu₂Si₂ and PuRhIn₅, were investigated by the techniques of chemical substitution and application of high magnetic fields. These materials are particularity interesting for their unique electronic and ordered state behavior (hidden order and superconducting phases) and proximity to magnetism. It is thought the superconductivity of these materials is unconventional and that they exhibit some features that are associated with quantum criticality, especially for PuRhIn₅. In earlier work URu₂Si₂ was doped with the non-isoelectric element phosphorus to produce the doping series URu₂Si₂₋ₓPₓ. During the current study, single crystals of the series were placed in high pulsed magnetic fields (up to 65T) and the evolution of the field induced phases was observed. The parent compound exhibits five unique phases in field as does the doped series up to approximately x=0.3. At this concentration and at zero field the hidden order phase is destroyed and any higher doping exhibit no ordered ground state. Over this x-range there is only one field induced state. Further increasing x (x > 0.26) pushes the system into an antiferromagnetic ground state, which has some high field ordering but at a higher magnetic fields than the lower doped compounds. This behavior is similar to the effect of Co, Rh and Ir substitution, which are also non-isoelectronic dopants that add electrons. This is in contrast to isoelectric doping (using Fe or Os) in which produce effects in the material similar to applied pressure. From this, it appears that the effects of non-isoelectric dopants might be attributed to band filling. The hidden order state of the parent compound URu₂Si₂ was also investigated with an optical magnetostriction technique in high magnetic fields. A transition from a quadratic to linear field response is seen in the signal while still in the hidden order state. This behavior is unusual and possible explanations include partial polarization of the Fermi surface and quadrupolar interactions. The Pu based superconductor PuRhIn₅ was doped with Cd and placed in high magnetic fields. Pu is both a radiological and toxicity hazard. As a result, a significant part of this project was spent controlling these hazards while enabling measurements. The phase diagram of PuRh(In₁₋ₓCdₓ)₅ in the T-x-H phase diagram was mapped and the optical magnetostriction technique was applied to the parent compound. From this data the electronic Grüneisen ratio was determined. Applying scaling arguments it was shown that the Grüneisen data is consistent with proximity to a quantum critical point, which is though to figure heavily in unconventional superconducting systems.
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Date Issued: 2018
Identifier: 2018_Sp_Wartenbe_fsu_0071E_14506
Format: Thesis

Title: Transport and Tunneling Investigation of Strongly Correlated Superconducting and Magnetic Thin Films.
Creator: Hu, Longqian, Xiong, Peng, Guan, Jingjiao, Adams, Todd, Chiorescu, Irinel, Schlottmann, Pedro U., Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: Complex physical phenomena, such as superconductivity, colossal magnetoresistance (CMR) effect, multi-ferroics, metal-insulator transition, quantum phase transition, etc. in strongly correlated materials have been enduring topics in condensed matter physics. The overarching theme of this dissertation is the study of transport and electronic states in emergent phases with distinct magnetic and electronic properties in strongly correlated materials in thin film forms. At first, we have...
Show moreComplex physical phenomena, such as superconductivity, colossal magnetoresistance (CMR) effect, multi-ferroics, metal-insulator transition, quantum phase transition, etc. in strongly correlated materials have been enduring topics in condensed matter physics. The overarching theme of this dissertation is the study of transport and electronic states in emergent phases with distinct magnetic and electronic properties in strongly correlated materials in thin film forms. At first, we have investigated anisotropic electronic phase separation (EPS) of optimally doped La2/3Ca1/3MnO3 (LCMO) thin films under various degrees of anisotropic strain by static magnetotransport and dynamic relaxation measurements. Distinct from the prototype perovskite manganite LPCMO with well-known micrometer scale EPS, the bulk optimally doped LCMO does not exhibit the large-scale EPS near a transition from paramagnetic insulating phase (PMI) to ferromagnetic metallic (FMM) phase at a high temperature. Through epitaxial growth of LCMO thin films on NGO (001) substrates and post-growth annealing, an antiferromagnetic insulating phase is induced in the FMM ground state and results in a large-scale EPS of coexisting AFI and FMM phases below the bulk metal insulator transition (MIT). Substantial resistivity anisotropies along the two orthogonal in-plane directions in the EPS region were experimentally probed by static temperature and magnetic field dependent resistivity measurements. More strikingly, with increasing annealing time, resistivity along the tensile-strained [010] direction becomes progressively larger than that along the compressive-strained [100] direction in the EPS region. The enhanced resistivity anisotropy suggests that the EPS is characterized by phase-separated FMM entities with a preferred orientation along [100] direction, possibly due to the deformation and rotation of the MnO6 octahedra under the enhanced anisotropic strain via the post-growth annealing. Furthermore, the EPS was found to exhibit glass-like behavior. The resistivity measured at fixed temperatures relaxes logarithmically over a long period of time. The relaxation behavior also shows a coherent enhancement with increasing annealing time. By fitting the relaxation data to a phenomenological model, the fitted parameters, resistive viscosity and characteristic relaxation time were found to evolve with temperature, showing a close correlation with the static measurements in the EPS states. In another project, we have investigated the superconductor-insulator quantum phase transitions tuned by disorder (d), magnetic impurity (MI) and magnetic field (B) in ultrathin Pb films by electrical transport measurement and single electron tunneling spectroscopy. In the past decade, the investigation of SITs in homogeneous thin films by transport measurement from our group has provided valuable insights to the mechanisms of SITs. There are two main theoretical models to explain SITs. The first one emphasizes that a transition from a superconducting state to a fermionic insulator without the existence of the superconducting order parameter, the formation of Cooper pairs is completely suppressed at the transition. The other one calls for a bosonic insulator with localized Cooper pairs. d-tuned and MI-tuned SITs well fit the fermionic framework, and both share common transport features, such as a sharp resistive transition to the superconducting state, a well-defined phase boundary, and a weakly insulating state near the phase boundary. While B-tuned SITs are a canonical example of the bosonic model. The resistive transition to the superconducting state is broadened by an application of magnetic field. Rather than a clear phase boundary near the transition, emerged resistive reentrance and double reentrance indicate phase fluctuation of the superconducting parameter is the main driving force for the transition, suggesting the survival of Cooper pairs in the insulating phase. Electron tunneling spectroscopy has been proposed to directly probe the existence and evolution of the superconductivity in these SITs. For B-tuned SITs, the existence of Cooper pairs is supposed to be detected even in the global insulating phase of thin films. More importantly, the approach also allows us to compare the evolutions of the normal state density of state among these SITs, particularly for d-tuned and MI-tuned SITs. Transport results show that MI has little influence on the normal state sheet resistance near the transition, while increasing disorder gradually raises the normal state sheet resistance. These observations suggest that the normal state density of states behaves differently in the two transitions. The experimental setup is a dilution refrigerator incorporated with in situ quench condensation, electrical measurement, and sample rotation, enabling us to achieve and tune SITs in the same sample by different parameters, and systematically check and compare the evolution of the density of states in the SITs. Up to now, we have performed transport and tunneling measurements for d-tuned SITs in homogeneous Pb films in a 4He quench probe and the modified dilution refrigerator. The transport results are consistent with previous experiments from our group. Increasing disorder leads to a SIT characterized with a sharp resistive transition to a zero resistance state, a well-defined phase boundary, and a gradual reduction of the superconducting critical temperature. The preliminary tunneling testing in the quench probe successfully reveals the suppression of the superconducting energy gap and the normal state density of state by the increasing disorder. In the modified dilution refrigerator, we still observed a concomitant suppression of the normal state density of states. Unfortunately, we were not able to reproduce valid tunneling spectra to study the evolution of the superconducting energy gap near the Fermi level. Possible reasons for the unsatisfying tunneling results are discussed at the end.
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Date Issued: 2018
Identifier: 2018_Sp_Hu_fsu_0071E_14482
Format: Thesis

Title: Nanoscale Thermal Transport and Ultrafast Lattice Dynamics in Semiconductor Nanostructures.
Creator: Gorfien, Matthew Charles, Cao, Jianming, Yang, Wei, Xiong, Peng, Riley, Mark A., Bonesteel, N. E., Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: This dissertation presents the recent developments and experiments performed using the third generation femtosecond electron diffractometer in Professor Jianming Cao's group as well as experiments performed using the previous second generation diffractometer now located at Shanghai Jiao Tong University. Two techniques of ultrafast electron diffraction (UED), time-resolved reflection high energy electron diffraction (Tr-RHEED) and time-resolved transmission electron diffraction (Tr-TED) were...
Show moreThis dissertation presents the recent developments and experiments performed using the third generation femtosecond electron diffractometer in Professor Jianming Cao's group as well as experiments performed using the previous second generation diffractometer now located at Shanghai Jiao Tong University. Two techniques of ultrafast electron diffraction (UED), time-resolved reflection high energy electron diffraction (Tr-RHEED) and time-resolved transmission electron diffraction (Tr-TED) were developed and applied to study the ultrafast lattice dynamics in semiconductor nanostructures. Tr-RHEED provides the ability to directly monitor the thermal transport across an interface inside a semiconductor quantum well (QW) by measuring the temperature evolution of the first few atomic layers. Tr-TED allows for a measurement of the laser-induced ultrafast structural dynamics of 5 nm PbSe quantum dots (QDs) in real time by diffracting through the entire sample thickness. In the first project, the setup of the first Tr-RHEED experiments and the first successful collection of Tr-RHEED data in our laboratory's history is discussed. The ultrafast temperature evolution of the GaAs nanofilm was measured and numerically modeled using the well known heat conduction equation and also a three-temperature model. These models were fit to the experimental data, allowing for the extraction of the thermal boundary conductance (TBC) and providing a method of measuring TBC in epitaxially grown semiconductor heterostructures. Surprisingly, the TBC was found to increase with increasing temperature even for temperatures above the Debye temperature, opening up questions about the exact mechanisms governing heat transfer at interfaces between very similar semiconductor nanoscale materials. In the second project, we directly monitored the lattice dynamics in PbSe quantum dots induced by laser excitation using Tr-TED. The energy relaxation between the carriers and the lattice took place within 10 ps, showing no evidence of any significant phonon bottleneck effect. Meanwhile, the lattice dilation exhibited some unusual features that could not be explained by the available mechanisms of photon-induced acoustic vibrations in semiconductors alone. The heat transport between the QDs and the substrate deviates significantly from Fourier's Law, which furthers studies about the heat transfer under nonequilibrium conditions in nanoscale materials. In addition to the UED projects, femtosecond transient spectroscopy (FTS) experiments were set up and tested on 20 nm gold nanofilms for various optical excitation laser fluences. The experimental data obtained agrees well with many previous published results. The well known two-temperature model (TTM) was used to describe the temperature evolution and the energy redistribution from the electronic to lattice systems. Using similar experimental and data analysis techniques to the ones developed in this dissertation will pave the way for future FTS experiments performed in conjunction to UED experiments to gain a more complete picture of the ultrafast dynamics in carriers and phonons in complex materials.
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Date Issued: 2018
Identifier: 2018_Fall_Gorfien_fsu_0071E_14841
Format: Thesis

Title: Predicting the Thermodynamic Properties of Proteins Using Computer Simulations.
Creator: Nguemaha, Valery Marcel, Piekarewicz, Jorge, Stagg, Scott, Zhou, Huan-Xiang, Xiong, Peng, Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: Protein molecules, sometimes referred to as the molecules of life, are the drivers of virtually every biological function. In this dissertation, we describe a series of computational studies to dissect the mystery of complex protein molecules. We consider a large collection of protein systems, ranging from globular proteins to Intrinsically Disordered Proteins (IDPs) with a focus on predicting thermodynamic observables that can be quantitatively compared with experimental data. In the first...
Show moreProtein molecules, sometimes referred to as the molecules of life, are the drivers of virtually every biological function. In this dissertation, we describe a series of computational studies to dissect the mystery of complex protein molecules. We consider a large collection of protein systems, ranging from globular proteins to Intrinsically Disordered Proteins (IDPs) with a focus on predicting thermodynamic observables that can be quantitatively compared with experimental data. In the first part of this dissertation, we study the effects of the phenomenon of macromolecular crowding and how it affects the properties of two different groups of proteins. First, we investigate the effects of crowding on globular proteins by calculating the free energy of all-atom proteins in crowded environments. Second, we study how crowding affect the conformational ensembles of disordered proteins with a focus on comparing computations with experiments. In the second part of this dissertation, we apply Monte Carlo simulation techniques to study protein droplet formation and Liquid Liquid Phase Separation in protein systems.
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Date Issued: 2018
Identifier: 2018_Fall_Nguemaha_fsu_0071E_14858
Format: Thesis

Title: Scale Setting and Topological Observables in Pure SU(2) LGT.
Creator: Clarke, David A. (David Anthony), Berg, Bernd A., Reina, Laura, Albrecht-Schmitt, Thomas E., Yohay, Rachel, Höflich, Peter, Florida State University, College of Arts and...
Show moreClarke, David A. (David Anthony), Berg, Bernd A., Reina, Laura, Albrecht-Schmitt, Thomas E., Yohay, Rachel, Höflich, Peter, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: In this dissertation, we investigate the approach of pure SU(2) lattice gauge theory to its continuum limit using the deconfinement temperature, six gradient scales, and six cooling scales. We find that cooling scales exhibit similarly good scaling behavior as gradient scales, while being computationally more efficient. In addition, we estimate systematic error in continuum limit extrapolations of scale ratios by comparing standard scaling to asymptotic scaling. Finally we study topological...
Show moreIn this dissertation, we investigate the approach of pure SU(2) lattice gauge theory to its continuum limit using the deconfinement temperature, six gradient scales, and six cooling scales. We find that cooling scales exhibit similarly good scaling behavior as gradient scales, while being computationally more efficient. In addition, we estimate systematic error in continuum limit extrapolations of scale ratios by comparing standard scaling to asymptotic scaling. Finally we study topological observables in pure SU(2) using cooling to smooth the gauge fields, and investigate the sensitivity of cooling scales to topological charge. We find that large numbers of cooling sweeps lead to metastable charge sectors, without destroying physical instantons, provided the lattice spacing is fine enough and the volume is large enough. Continuum limit estimates of the topological susceptibility are obtained, of which we favor χ 1/4 /T c = 0.643(12). Differences between cooling scales in different topological sectors turn out to be too small to be detectable within our statistical error.
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Date Issued: 2018
Identifier: 2018_Fall_Clarke_fsu_0071E_14832
Format: Thesis

Title: The Interplay of Orders in La-214 Cuprates.
Creator: Baity, Paul G. (Paul Gabriel), Popović, Dragana, Dobrosavljević, Vladimir, Salters, Vincent J. M., Piekarewicz, Jorge, Chiorescu, Irinel, Florida State University, College of...
Show moreBaity, Paul G. (Paul Gabriel), Popović, Dragana, Dobrosavljević, Vladimir, Salters, Vincent J. M., Piekarewicz, Jorge, Chiorescu, Irinel, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Despite over thirty years of research, the origin of high-temperature superconductivity remains unsolved. In these thirty years, the phase diagram for the rst-discovered high-temperature superconductors, the cuprates, has been found to be rather complex and exhibits many different phases such as antiferromagnetism, charge density waves, spin density waves, nematicity, the pseudogap, and of course, superconductivity. Furthermore, several structural instabilities can manifest that affect the...
Show moreDespite over thirty years of research, the origin of high-temperature superconductivity remains unsolved. In these thirty years, the phase diagram for the rst-discovered high-temperature superconductors, the cuprates, has been found to be rather complex and exhibits many different phases such as antiferromagnetism, charge density waves, spin density waves, nematicity, the pseudogap, and of course, superconductivity. Furthermore, several structural instabilities can manifest that affect the stability of these phases. In the La-214 cuprates, for example, it is known the concomitant charge and spin orders (or stripe order) are stabilized by a low-temperature tetragonal structure. The stripe order coincides with a suppression of the superconducting critical temperature, leading the conclusion that these phases either compete or are intertwined. Since the stability of the low-temperature tetragonal structure, and therefore stripes, can be controlled by various dopants, the La-214 cuprates can be used to investigate how these orders intertwine. In this thesis, both striped and unstriped La-214 compounds have been investigated to understand the interplay of these various orders: superconductivity, stripes, and structure. In three distinct studies, using various charge transport techniques, the interplay between these orders is shown to lead to interesting and unexpected behavior. The first study reveals static charge order is in fact a fluctuating order pinned by the structure. The second study shows the two-dimensional nature of the superconductivity in the absence of stripe order, which is speculated to decouple CuO2 planes. Finally, the third study reveals the existence of a hidden order of Cooper pairs in the T=0 field-driven superconducting-normal-state transition when stripes are present. The culmination of these distinct studies lead to a better understanding of the physics of cuprates through the interplay of their various orders, and thus the general phase diagram of high-temperature superconductivity.
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Date Issued: 2018
Identifier: 2018_Sp_Baity_fsu_0071E_14433
Format: Thesis

Title: Theoretical and Experimental Studies of Mononuclear Trigonal Bipyramidal Single Molecule Magnets.
Creator: Bhaskaran, Lakshmi, Hill, S., Shatruk, Mykhailo, Cao, Jianming, Bonesteel, N. E., Piekarewicz, Jorge, Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: This dissertation presents studies on mononuclear single molecule magnets (SMMs) with magnetic properties arising from transition metal ions in trigonal bipyramidal (TBP) coordination environments. We use both experimental and theoretical methods to elucidate the effects of coordination geometry on the magnetic anisotropy of a SMM. The role of an axial magnetic anisotropy is to pin the magnetic moment of the metal ion in one of two preferred orientations, either parallel or anti-parallel to...
Show moreThis dissertation presents studies on mononuclear single molecule magnets (SMMs) with magnetic properties arising from transition metal ions in trigonal bipyramidal (TBP) coordination environments. We use both experimental and theoretical methods to elucidate the effects of coordination geometry on the magnetic anisotropy of a SMM. The role of an axial magnetic anisotropy is to pin the magnetic moment of the metal ion in one of two preferred orientations, either parallel or anti-parallel to the magnetic easy-axis. For transition metals, maximization of the axial magnetic anisotropy requires stabilization of an unquenched orbital moment that can couple to a ligand field. SMMs with giant magnetic anisotropy play an important role both in terms of fundamental scientific reasons and potential application in information technologies. Thus the studies presented in this dissertation attempt to explore some of the interesting physics in these compounds. The presence of orbitally degenerate states and unquenched orbital momentum pushes the limits of spin-only model. To overcome this limitation, we propose a phenomenological spin-orbit model based on point charge approximation with the goal to investigate orbitally degenerate mononuclear compounds. As an application of our model, we consider two test compounds : Iron(II) and Nickel(II) ions in trigonal bipyramidal (TBP) environments, where we find that the high symmetry configuration supports a large magnetic anisotropy in the absence of Jahn-Teller distortion. The motivation for our phenomenological model stemmed from our detailed EPR measurements performed on a mononuclear Nickel(II) SMM in a TBP environment that revealed an unprecedented magnetic anisotropy, reaching the limits of applicability of the familiar spin-only description. The axial anisotropy estimated for this complex was found to the be the largest so far for a mononuclear Nickel(II) complex; and, importantly, only a very small degree of axial symmetry breaking was detected. This was most likely considered to be due to the unquenched orbital moment in the ground states of the Nickel(II) ion. To further confirm this prediction we performed theoretical studies of the SMM using the phenomenological spin-orbit model. This study showed the suppression of Jahn-Teller effects in trigonal bipyramidal Nickel(II) complex because of rigid, bulky axial ligands. To further understand the effects of using bulky ligands in TBP coordination environments we performed experimental and theoretical studies on a mononuclear Iron(II) SMM. Although the ground states of this complex is also orbitally degenerate, our investigation showed reduced axial magnetic anisotropy compared to Nickel(II) with a very small transverse component. Our phenomenological investigation of the ground states revealed that the magnitude of the first order contribution is strongly dependent on the bond angles, and the spin-orbit coupling constant also plays a significant role in achieving large magnetic anisotropy. Finally, we also explore the effects of different ligand types in Cobalt(II) mononuclear complexes in TBP coordination environments. In these Kramers systems we and that the combination of a 3-fold symmetric ligand and a trigonal space group gives rise to an increase in the easy-plane magnetic anisotropy, while keeping the rhombicity of the system close to zero. This is particularly interesting for quantum information processing, especially in relation to molecules with a large spin ground state characterized by a large easy-plane anisotropy.
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Date Issued: 2018
Identifier: 2018_Su_Bhaskaran_fsu_0071E_14700
Format: Thesis

Title: EPR Study of Molecular Qubits Based on Lanthanide Nanomagnets.
Creator: Komijani, Dorsa, Hill, S., Shatruk, Mykhailo, Piekarewicz, Jorge, Van Tol, Johan, Schlottmann, Pedro U., Florida State University, College of Arts and Sciences, Department of...
Show moreKomijani, Dorsa, Hill, S., Shatruk, Mykhailo, Piekarewicz, Jorge, Van Tol, Johan, Schlottmann, Pedro U., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Molecular magnets containing one or more open shell elements are one of the proposed systems for quantum information processing that can be chemically engineered to have well-separated and stable quantum states. This dissertation explores mono- and dinuclear lanthanide nanomagnets for quantum information processing and spintronics by means of continuous-wave and pulsed electron paramagnetic resonance (EPR) spectroscopy. EPR is a powerful and sensitive technique that allows us to probe the ﬁne...
Show moreMolecular magnets containing one or more open shell elements are one of the proposed systems for quantum information processing that can be chemically engineered to have well-separated and stable quantum states. This dissertation explores mono- and dinuclear lanthanide nanomagnets for quantum information processing and spintronics by means of continuous-wave and pulsed electron paramagnetic resonance (EPR) spectroscopy. EPR is a powerful and sensitive technique that allows us to probe the ﬁne structure of the ground spin state in molecular magnets. In Chapter 2, we demonstrate that atomic clock transitions (ACTs) can be employed as a means of enhancing the coherence of molecular spin qubits without resorting to extreme dilution, which can be impractical at the stage of device design for multi-qubit gate operations. This approach is illustrated with a holmium molecular nanomagnet in which long coherence times (up to 8.4 microseconds at 5 kelvin) are obtained at unusually high concentrations. ACTs are realized in the vicinity of avoided level crossings within the ground doublet of the holmium compound, at which several sources of decoherence are mitigated. In Chapter 3, we continue the pulsed EPR study presented in Chapter 2 to introduce electro-nuclear atomic clock transitions in a hybrid system with both large electronic and nuclear moment. We demonstrate an enhancement in the coherence time of hybrid transitions that involve coupled dynamics of electron and nuclear spins. This is signiﬁcant for applications in hybrid magnetic qubits, where manipulation of the nuclear spin is controlled by EPR pulses. In Chapter 4, we report single-crystal and powder high-ﬁeld EPR (HF-EPR) measurements on a neutral [TbPc2]0 complex for which the organic bis-phthalocyaninato (Pc2) ligand is open shell, i.e., it carries an unpaired electron. A highly anisotropic EPR signal can be attributed to the radical, suggesting an appreciable interaction with the Ising-like Tb(III) ion. Analysis of the results unambiguously demonstrate that the radical-Tb(III) coupling is due to a ferromagnetic exchange interaction. The essential physics is captured via an eﬀective spin Hamiltonian in which the exchange is assumed to be isotropic, while the magnetic anisotropy is folded entirely into the single-ion properties of the terbium ion. In Chapter 4, we investigate lanthanide-radical interactions across the lanthanide (Ln) series in double-decker compounds (LnPc2). We further discuss the eﬀect of free ion anisotropy on the magnetic properties of the lanthanide ion and the exchange coupled radical. Finally, in Chapter 6, we present HF-EPR studies of a series of symmetric and asymmetric triple-decker compounds that are potential candidates for two-qubit gate operations. Triple-decker compounds contain two lanthanide ions in each molecule that are linked by a phthalocyanine (Pc) ring sandwiched by Pc or porphyrin on the top and bottom. We further show that the two inequivalent sites required for two-qubit gates can be chemically engineered by adjusting the coordination symmetry for one of the ions in these dinuclear compounds.
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Date Issued: 2018
Identifier: 2018_Su_Komijani_fsu_0071E_14626
Format: Thesis

Title: Experimental Investigations of Mass-7 Destruction in Deuteron Induced Reactions with Respect to Standard Big Bang Nucleosynthesis.
Creator: Rijal, Nabin, Weidenhoever, Ingo Ludwing M., Humayun, Munir, Volya, Alexander, Höflich, Peter, Almaraz-Calderon, Sergio J., Florida State University, College of Arts and...
Show moreRijal, Nabin, Weidenhoever, Ingo Ludwing M., Humayun, Munir, Volya, Alexander, Höflich, Peter, Almaraz-Calderon, Sergio J., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: The Big-Bang theory is the most widely accepted description of the origin of the Universe, creating elementary particles and synthesizing light nuclei up to mass-7 within the first few minutes of the Big-Bang. Its parameters have been recently precisely determined through he multi-years WMAP (Wilkinson Microwave Anisotropy Probe, NASA mission) followed by Planck (ESA mission). These results tightly constrain the baryonic density in the Universe with the highest precision ever achieved,...
Show moreThe Big-Bang theory is the most widely accepted description of the origin of the Universe, creating elementary particles and synthesizing light nuclei up to mass-7 within the first few minutes of the Big-Bang. Its parameters have been recently precisely determined through he multi-years WMAP (Wilkinson Microwave Anisotropy Probe, NASA mission) followed by Planck (ESA mission). These results tightly constrain the baryonic density in the Universe with the highest precision ever achieved, resulting in more constraints in the light element abundance predicted in the Standard Big Bang Nucleosynthesis (SBBN). The observed amount of all other light elements (H, D, 3He, 4He) agrees well with the SBBN calculation but the 7Li is overpredicted by a factor of 3 − 4, which is referred as the Primordial Lithium Problem. The Lithium problem is the disagreement in the theoretical and observed amount of 7Li in the Cosmos which has the key importance in the Big Bang Nucleosynthesis theory which can’t be explained by uncertainties in the main reactions included in the SBBN. In this work, we performed experiments on certain nuclear reactions, 7Be + d and 7Li + d, at energies relevant to SBBN, which could destroy a fraction of the mass-7 nuclei in the conditions of the Big Bang and could offer an explanation of the observed deviation from the prediction of SBBN. In preparation for the main experiment of this dissertation 7Be+d -> p+2 alpha, we performed an experiment for mirror nuclear reaction 7Li + d -> n + 2 alpha, with ANASEN in gas target mode using deuterium as an active gas target and detected 2 -particles in coincidence. In particular, we investigated the 7Be+d reaction at SBBN energies using a radioactive 7Be beam and deuterium gas target, stopping the beam in the target gas inside the ANASEN (Array for Nuclear Astrophysics and Structures with Exotic Nuclei) detector at the Florida State University (FSU). ANASEN is an active target detector system which tracks the charged particles with cylindrical configuration of a position sensitive proportional counter, a Silicon detector array backed up by Caesium Iodide (CsI) detectors; one of the first detectors of its kind. The experiment measured a continuous excitation function by slowing down the beam particles in the target gas down to very low energies into the Gamow window. Our experimental set-up provided a high detection efficiency for all relevant reaction channels focusing on the lowest energies, relevant to the Big Bang Nucleosynthesis (BBN). Results of these experiments after a comprehensive data analysis are documented in this dissertation.
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Date Issued: 2018
Identifier: 2018_Su_RIJAL_fsu_0071E_14658
Format: Thesis

Title: Direct Observation of Structural Defects in Pyrochlore Yb₂Ti₂O₇ by Atomic Resolution Scanning Transmission Electron Microscopy.
Creator: Shafieizadeh, Zahra, Xin, Yan, Chiorescu, Irinel, Andrei, Petru, Reina, Laura, Van Winkle, David H., Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: In transition metal oxides, such as A2B2O7, the B-site cations typically govern many of the fundamentally and technologically interesting properties. The cubic pyrochlore oxides, A2B2O7, have attracted much attention over the past 20 years. However, even after years of theoretical and experimental study of pyrochlore compounds many fundamental questions still remain about the nature of the magnetic ground states in this series. In the pyrochlore structure, both the rare earth and the...
Show moreIn transition metal oxides, such as A2B2O7, the B-site cations typically govern many of the fundamentally and technologically interesting properties. The cubic pyrochlore oxides, A2B2O7, have attracted much attention over the past 20 years. However, even after years of theoretical and experimental study of pyrochlore compounds many fundamental questions still remain about the nature of the magnetic ground states in this series. In the pyrochlore structure, both the rare earth and the transition metal sublattices have a topology consisting of corner-sharing tetrahedra and are, thus, geometrically frustrated. Yb2Ti2O7 is part of a series of compounds called the rare-earth titanates. It’s been noticed that its magnetic ground states are sample dependent and shows broad specific heat peak in colored single crystals. It could have long-range ordered collinear ferromagnetic state, or non-collinear ferromagnetic fluctuations, or short ranged fluctuations. Atomic resolution high-angle-annular-dark-field STEM (HAADF-STEM) imaging is a powerful tool to observe and obtain information on defect structures. The primary aim of this thesis is, therefore, to study defects in Yb2Ti2O7 samples by quantitative scanning transmission electron microscopy at atomic resolution. We study three types of samples, polycrystalline sample and yellowish single crystal grown by conventional floating zone (FZ) method and high quality colorless single crystal grown by the traveling solvent floating zone (TSFZ) technique. We compared the atomic structure of them using STEM and found different types of defects: Yb atoms “stuffing” into Ti sites, extended defects such as anti-phase boundaries (APB) and dissociated superdislocations and grain boundary defects were discovered for the first time and found to be prevalent in the yellowish single crystal grown by FZ, contrary to the few found in high quality colorless transparent single crystals or stoichiometric white polycrystalline samples. These results are discussed along with the results from simulation. Also, EELS results showed that the Ti valence was not decreased in defected areas and its value is 4+. These results explain the varied magnetic properties and broad specific heat peak of yellowish Yb2Ti2O7 single crystals grown by conventional FZ method and provide new insights into understanding the roles of defects on physical properties of the geometrically frustrated pyrochlore compounds.
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Date Issued: 2018
Identifier: 2018_Su_Shafieizadeh_fsu_0071E_14721
Format: Thesis

Title: Exploring the Nuclear Structure of the A = 39 Isobars.
Creator: Abromeit, Brittany Lynn, Tabor, Samuel L., Humayun, Munir, Hsiao, Eric Y., Riley, Mark A., Volya, Alexander, Florida State University, College of Arts and Sciences, Department...
Show moreAbromeit, Brittany Lynn, Tabor, Samuel L., Humayun, Munir, Hsiao, Eric Y., Riley, Mark A., Volya, Alexander, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Investigation of nuclei with neutron and proton imbalance is at the forefront of nuclear physics research today, along with how the nuclear structure varies with the movement of the nucleons. Experimental data and theoretical models work hand-in-hand to understand the structure of these nuclei. Two of the A = 39 isobars, residing in limbo between the sd- and fp-shells are the isotopes of interest for this study. With 24 neutrons, eight neutrons more than the stable isotope of phosphorous, ³⁹P...
Show moreInvestigation of nuclei with neutron and proton imbalance is at the forefront of nuclear physics research today, along with how the nuclear structure varies with the movement of the nucleons. Experimental data and theoretical models work hand-in-hand to understand the structure of these nuclei. Two of the A = 39 isobars, residing in limbo between the sd- and fp-shells are the isotopes of interest for this study. With 24 neutrons, eight neutrons more than the stable isotope of phosphorous, ³⁹P is considered a neutron-rich exotic nucleus, one of which has not been extensively studied until now. Since the late 1990s only two experiments have been conducted, producing the three known gamma-rays and one tentative gamma-ray for ³⁹P. Except for half-life measurements and mass measurements, no further studies of ³⁹P have been done prior to a beta-gamma coincidence experiment conducted in this study at the National Superconducting Cyclotron Laboratory (NSCL) using the Beta Counting System. Continuing up the isobaric chain from ³⁹P toward stability, ³⁹Ar is reached. Unlike ³⁹P, ³⁹Ar is nested between two of the stable isotopes of argon. Being closer to stability, it has been studied more extensively using transfer reactions, however, limited information on high-spin states is produced from only two prior experiments. Despite the more extensive study of ³⁹Ar, the observed gamma transitions are only well known for low-spin states except for one band of high-spin states which have been studied heavily using the gamma spectroscopy setup at the John D. Fox Superconducting Linear Accelerator Laboratory at Florida State University. Results for both isotopes, along with comparisons to Shell Model calculations will be presented.
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Date Issued: 2018
Identifier: 2018_Fall_Abromeit_fsu_0071E_14900
Format: Thesis

Title: Exotic Nuclear Deformation and the Evolution of Nuclear Structure with Angular Momentum and Excitation Energy in ¹⁵⁷Ho, ¹⁶⁶Er, and ¹⁶⁹,¹⁷⁰Yb.
Creator: Baron, Jonathan S., Riley, Mark A., Steinbock, Oliver, Tabor, Samuel L., Volya, Alexander, Reina, Laura, Florida State University, College of Arts and Sciences, Department of...
Show moreBaron, Jonathan S., Riley, Mark A., Steinbock, Oliver, Tabor, Samuel L., Volya, Alexander, Reina, Laura, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Nuclei in the light rare-earth, for values of N ≥ 90, are textbook examples of the evolution of nuclear structure with respect to excitation energy and angular momentum in deformed nuclei. In the high-spin region (J ≥ 10ħ), effects such as backbends and shape changes occur, ending with termination of the lower energy collective structures. First backbends occur before 20ħ Findings also reflect a spectacular return to collectivity in the "ultra-high spin" region (J ≥ 50ħ). Thanks to recent...
Show moreNuclei in the light rare-earth, for values of N ≥ 90, are textbook examples of the evolution of nuclear structure with respect to excitation energy and angular momentum in deformed nuclei. In the high-spin region (J ≥ 10ħ), effects such as backbends and shape changes occur, ending with termination of the lower energy collective structures. First backbends occur before 20ħ Findings also reflect a spectacular return to collectivity in the "ultra-high spin" region (J ≥ 50ħ). Thanks to recent developments in both detectors and accelerators, gamma-ray spectroscopy has been able to probe the upper ends of the high-spin region, and begin probing into the ultra-high spin regime. Data from two experiments form the basis of this work. One study was an ultra-high-spin analysis of the Z=67, N=90 ¹⁵⁷Ho nucleus at Argonne National Laboratory with Gammasphere. The findings were that remarkable correlations were observed to the neighboring isotone, ¹⁵⁸Er, in which termination states and ultra-high spin structures had been previously observed. A high-spin investigation of Ytterbium (Yb) and Erbium (Er) isotopes was performed at FSU. This study used an intense radioactive ¹⁴C beam, available at FSU, in order to study these heavy, neutron-rich nuclei in regions of angular momentum and excitation energy not attainable with stable beams. The reactions of ¹⁷⁰Er(¹⁴C,5n/4n/α4n) generated new information in ¹⁶⁹Yb, ¹⁷⁰Yb, and ¹⁶⁶Er, respectively. Due to the recently upgraded digital FSU Gamma-Ray Array, aided by JBSMILE, triple ᵧ-ray coincidences were able to be viewed in these nuclei for the first time at FSU. The result was the investigation of rotational alignments in both the yrast and non-yrast multi-quasiparticle bands in these nuclei.
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Date Issued: 2018
Identifier: 2018_Fall_Baron_fsu_0071E_14887
Format: Thesis

Title: Transport Properties of Semimetallic Transition Metal Dichalcogenides.
Creator: Zhou, Qiong, Balicas, Luis, Bonesteel, N. E., Andrei, Petru, Xiong, Peng, Wahl, Horst, Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: The Weyl semimetal requires the breaking of either the time-reversal symmetry (TRS) or the lattice inversion symmetry. When the TRS and inversion symmetry coexist, a pair of degenerate Weyl points may exist, leading to the related Dirac semimetal phase. In other words, a Dirac semimetallic state can be regarded as two copies of Weyl semimetal states. In this dissertation, we study tellurium based compounds like the Weyl semimetal candidate MoTe2 and the Dirac semimetal candidate PtTe2 within...
Show moreThe Weyl semimetal requires the breaking of either the time-reversal symmetry (TRS) or the lattice inversion symmetry. When the TRS and inversion symmetry coexist, a pair of degenerate Weyl points may exist, leading to the related Dirac semimetal phase. In other words, a Dirac semimetallic state can be regarded as two copies of Weyl semimetal states. In this dissertation, we study tellurium based compounds like the Weyl semimetal candidate MoTe2 and the Dirac semimetal candidate PtTe2 within the transition metal dichalcogenides family. Firstly, we report a systematic study on the Hall-effect of the semi-metallic state of bulk MoTe2, which was recently claimed to be a candidate for a novel type of Weyl semi-metallic state. The temperature (T) dependence of the carrier densities and of their mobilities, as estimated from a numerical analysis based on the isotropic two-carrier model, indicates that its exceedingly large and non-saturating magnetoresistance may be attributed to a near perfect compensation between the densities of electrons and holes at low temperatures. A sudden increase in hole density, with a concomitant rapid increase in the electron mobility below T ∼ 40 K, leads to comparable densities of electrons and holes at low temperatures suggesting a possible electronic phase-transition around this temperature. Secondly, the electronic structure of semi-metallic transition-metal dichalcogenides, such as WTe2 and orthorhombic γ−MoTe2, are claimed to contain pairs of Weyl points or linearly touching electron and hole pockets associated with a non-trivial Chern number. For this reason, these compounds were recently claimed to conform to a new class, deemed type-II, of Weyl semi-metallic systems. A series of angle resolved photoemission experiments (ARPES) claim a broad agreement with these predictions detecting, for example, topological Fermi arcs at the surface of these crystals. We synthesized single-crystals of semi-metallic MoTe2 through a Te flux method to validate these predictions through measurements of its bulk Fermi surface (FS) via quantum oscillatory phenomena. We find that the superconducting transition temperature of γ−MoTe2 depends on disorder as quantified by the ratio between the room- and low-temperature resistivities, suggesting the possibility of an unconventional superconducting pairing symmetry. Similarly to WTe2, the magnetoresistivity of γ−MoTe2 does not saturate at high magnetic fields and can easily surpass 106 %. Remarkably, the analysis of the de Haas-van Alphen (dHvA) signal superimposed onto the magnetic torque, indicates that the geometry of its FS is markedly distinct from the calculated one. The dHvA signal also reveals that the FS is affected by the Zeeman-effect precluding the extraction of the Berry-phase. A direct comparison between the previous ARPES studies and density-functional-theory (DFT) calculations reveals a disagreement in the position of the valence bands relative to the Fermi level εF . Here, we show that a shift of the DFT valence bands relative to εF , in order to match the ARPES observations, and of the DFT electron bands to explain some of the observed dHvA frequencies, leads to a good agreement between the calculations and the angular dependence of the FS cross-sectional areas observed experimentally. However, this relative displacement between electron- and hole-bands eliminates their crossings and, therefore, the Weyl type-II points predicted for γ−MoTe2. Finally, we investigate the electronic structure and transport properties in single crystals of the semi-metallic platinum ditelluride (PtTe2), recently claimed to be a novel type-II Dirac semimetal, via a methodology similar to that applied to γ−MoTe2, i.e. the temperature and angular dependence of the SdH and dHvA effects. Our high-quality PtTe2 crystal displays a large non-saturating magnetoresistance under magnetic field up to 61 T. The dHvA oscillation and SdH effect reveal several high and low frequencies suggesting a rather complex Fermi surface. We also find evidence for a non-trivial Berry phase. The crystal quality improved considerably under subsequent annealing at high-temperatures leading to the observation of linear in field magnetoresistivity. Combined with effective masses in the order of ∼ 0.1 free electron mass, these results further suggest that PtTe2 displays bulk Dirac-like bands.
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Date Issued: 2017
Identifier: FSU_FALL2017_Zhou_fsu_0071E_14145
Format: Thesis

Title: Structural Stability and Emergent Phases in Oxygen Deficient Complex Transition Metal Oxides.
Creator: Ghosh, Soham S., Manousakis, Efstratios, Shatruk, Mykhailo, Bonesteel, N. E., Roberts, Winston, Van Winkle, David, Flaherty, Francis A., Florida State University, College of...
Show moreGhosh, Soham S., Manousakis, Efstratios, Shatruk, Mykhailo, Bonesteel, N. E., Roberts, Winston, Van Winkle, David, Flaherty, Francis A., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: This dissertation is a theoretical and computational examination of structural, electronic and magnetic properties of complex transition metal oxide structures. Our work is motivated by experimental observations that transition metal oxides manifest novel properties at surfaces and interfaces that are absent in bulk, and that there exist competing ground states driven by off-stoichiometry, oxygen vacancy and reduction of symmetry. We examine these properties using density functional theory ...
Show moreThis dissertation is a theoretical and computational examination of structural, electronic and magnetic properties of complex transition metal oxide structures. Our work is motivated by experimental observations that transition metal oxides manifest novel properties at surfaces and interfaces that are absent in bulk, and that there exist competing ground states driven by off-stoichiometry, oxygen vacancy and reduction of symmetry. We examine these properties using density functional theory (DFT) within the spin-generalized gradient approximation (Spin-GGA) along with the application of a Hubbard U (GGA + U). We present our detailed results for the following systems: oxygen deficient strontium titanate surface, strontium ruthenate interfaced with ruthenium metal inclusions, and ytterbium titanate with Yb "stuffing". In the course of our work, we cover materials with 3d, 4d and 4f band characters, each of which have different band masses, electron-electron correlations and spin-orbit coupling (SOC) strength. We investigate the role of surface termination, oxygen vacancy doping and cation "stuffing" defects in these metal-oxides and show the emergence of novel properties consistent with experimentally acquired information and possible applications. We conclude by presenting implications for further work.
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Date Issued: 2017
Identifier: FSU_SUMMER2017_Ghosh_fsu_0071E_13962
Format: Thesis

Title: Electronic Tuning in the Hidden Order Compound URu2Si2 through Si → P Substitution.
Creator: Gallagher, Andrew, Baumbach, Ryan Eagle, Hill, Stephen (Professor of Physics), Siegrist, Theo, Boebinger, Gregory S. (Gregory Scott), Piekarewicz, Jorge, Florida State...
Show moreGallagher, Andrew, Baumbach, Ryan Eagle, Hill, Stephen (Professor of Physics), Siegrist, Theo, Boebinger, Gregory S. (Gregory Scott), Piekarewicz, Jorge, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Crystalline materials that include 4f- and 5f-electron elements frequently exhibit a variety of intriguing phenomena including spin and charge orderings, spin and valence fluctuations, heavy fermion behavior, breakdown of Fermi liquid behavior, and unconventional superconductivity. [5, 6, 7, 8, 9, 10, 11, 12, 13] Amongst such materials, the Kondo lattice system URu2Si2 stands out as being particularly unusual. [14, 15, 16] While at high temperature it exhibits behavior that is typical for an...
Show moreCrystalline materials that include 4f- and 5f-electron elements frequently exhibit a variety of intriguing phenomena including spin and charge orderings, spin and valence fluctuations, heavy fermion behavior, breakdown of Fermi liquid behavior, and unconventional superconductivity. [5, 6, 7, 8, 9, 10, 11, 12, 13] Amongst such materials, the Kondo lattice system URu2Si2 stands out as being particularly unusual. [14, 15, 16] While at high temperature it exhibits behavior that is typical for an f-electron lattice immersed in a sea of conduction electrons, at T0 = 17:5 K there is a second order phase transition that is followed by unconventional superconductivity near Tc 1:5 K. [15] Despite three decades of work, the order parameter for the transition at T0 remains unknown and hence, it has been named "hidden order". There have been a multitude of experimental attempts to unravel hidden order, mainly through tuning of the electronic state via pressure, applied magnetic field, and chemical substitution. [17, 18] While these strategies reveal interesting phase diagrams, a longstanding challenge is that any such approach explores the phase space along an unknown vector: i.e., many different factors are affected. To address this issue, we developed a new organizational map for the U-based ThCr2Si2-type compounds that are related to URu2Si2 and thus guided, we explored a new chemical tuning axis: Si -> P. Our studies were enabled by the development of a new molten metal crystal growth method for URu2Si2 which produces high quality single crystals and allows us to introduce high vapor pressure elements, such as phosphorous. [19, 20] Si -> P tuning reveals that while the high temperature Kondo lattice behavior is robust, the low temperature phenomena are remarkably sensitive to electronic tuning. [21, 22] In the URu2Si2-xPx phase diagram we find that while hidden order is monotonically suppressed and destroyed for x < 0.035, the superconducting strength evolves non-monotonically with a maximum near x = 0.01 and that superconductivity is destroyed near x = 0.028. For 0.03 < x < 0.26 there is a region with Kondo coherence but no ordered state. Antiferromagnetism abruptly appears for x = 0.26. This phase diagram differs significantly from those produced by most other tuning strategies in URu2Si2, including applied pressure, and isoelectronic chemical substitution (i.e. Ru -> Fe and Os), where hidden order and magnetism share a common phase boundary. [2, 23, 24] We discuss implications for understanding hidden order, its relationship to magnetism, and prospects for uncovering novel sibling electronic states.
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Date Issued: 2017
Identifier: FSU_SUMMER2017_Gallagher_fsu_0071E_13976
Format: Thesis

Title: Mott Transition in Strongly Correlated Materials: Many-Body Methods and Realistic Materials Simulations.
Creator: Lee, Tsung-Han, Dobrosavljević, Vladimir, Dalal, Naresh S., Manousakis, Efstratios, Balicas, Luis, Piekarewicz, Jorge, Florida State University, College of Arts and Sciences,...
Show moreLee, Tsung-Han, Dobrosavljević, Vladimir, Dalal, Naresh S., Manousakis, Efstratios, Balicas, Luis, Piekarewicz, Jorge, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Strongly correlated materials are a class of materials that cannot be properly described by the Density Functional Theory (DFT), which is a single-particle approximation to the original many-body electronic Hamiltonian. These systems contain d or f orbital electrons, i.e., transition metals, actinides, and lanthanides compounds, for which the electron-electron interaction (correlation) effects are too strong to be described by the single-particle approximation of DFT. Therefore, complementary...
Show moreStrongly correlated materials are a class of materials that cannot be properly described by the Density Functional Theory (DFT), which is a single-particle approximation to the original many-body electronic Hamiltonian. These systems contain d or f orbital electrons, i.e., transition metals, actinides, and lanthanides compounds, for which the electron-electron interaction (correlation) effects are too strong to be described by the single-particle approximation of DFT. Therefore, complementary many-body methods have been developed, at the model Hamiltonians level, to describe these strong correlation effects. Dynamical Mean Field Theory (DMFT) and Rotationally Invariant Slave-Boson (RISB) approaches are two successful methods that can capture the correlation effects for a broad interaction strength. However, these many-body methods, as applied to model Hamiltonians, treat the electronic structure of realistic materials in a phenomenological fashion, which only allow to describe their properties qualitatively. Consequently, the combination of DFT and many body methods, e.g., Local Density Approximation augmented by RISB and DMFT (LDA+RISB and LDA+DMFT), have been recently proposed to combine the advantages of both methods into a quantitative tool to analyze strongly correlated systems. In this dissertation, we studied the possible improvements of these approaches, and tested their accuracy on realistic materials. This dissertation is separated into two parts. In the first part, we studied the extension of DMFT and RISB in three directions. First, we extended DMFT framework to investigate the behavior of the domain wall structure in metal-Mott insulator coexistence regime by studying the unstable solution describing the domain wall. We found that this solution, differing qualitatively from both the metallic and the insulating solutions, displays an insulating-like behavior in resistivity while carrying a weak metallic character in its electronic structure. Second, we improved DMFT to describe a Mott insulator containing spin-propagating and chargeless fermionic excitations, spinons. We found the spinon Fermi-liquid, in the Mott insulating phase, is immiscible to the electron Fermi-liquid, in the metallic phase, due to the strong scattering between spinons in a metal. Third, we proposed a new approach within the slave-boson (Gutzwiller) framework that allows to describe both the low energy quasiparticle excitation and the high energy Hubbard excitation, which cannot be captured within the original slave-boson framework. In the second part, we applied LDA+RISB to realistic materials modeling. First, we tested the accuracy of LDA+RISB on predicting the structure of transition metal compounds, CrO, MnO, FeO, CoO, CoS, and CoSe. Our results display remarkable agreements with the experimental observations. Second, we applied LDA+RISB to analyze the nature of the Am-O chemical bonding in the CsAm(CrO_4)_2 crystal. Our results indicate the Am-O bonding has strongly covalent character, and they also address the importance of the correlation effects to describe the experimentally observed electronic structure. In summary, we proposed three extensions within DMFT and RISB framework, which allow to investigate the domain wall structure in metal-Mott insulator coexistence regime, the metal-to-Mott-insulator transition with spinons excitation in the Mott-insulating phase, and the Hubbard excitation within RISB approach. Furthermore, we demonstrated that LDA+RISB is a reliable approximation to the strongly correlated materials by applying it to the transition metal compounds and the Americian chromate compounds.
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Date Issued: 2017
Identifier: FSU_SUMMER2017_Lee_fsu_0071E_13983
Format: Thesis

Title: Sensitive Spin Detection Using an on-Chip Squid-Waveguide Resonator.
Creator: Yue, Guang, Chiorescu, Irinel, Dalal, Naresh S., Reina, Laura, Schlottmann, Pedro U., Xiong, Peng, Florida State University, College of Arts Sciences, Department of Physics
Abstract/Description: Quantum computing gives novel way of computing using quantum mechanics, which furthers human knowledge and has exciting applications. Quantum systems with diluted spins such as rare earth ions hosted in single crystal, molecule-based magnets etc. are promising qubits candidates to form the basis of a quantum computer. High sensitivity measurement and coherent control of these spin systems are crucial for their practical usage as qubits. The micro-SQUID (direct-current micrometer-sized...
Show moreQuantum computing gives novel way of computing using quantum mechanics, which furthers human knowledge and has exciting applications. Quantum systems with diluted spins such as rare earth ions hosted in single crystal, molecule-based magnets etc. are promising qubits candidates to form the basis of a quantum computer. High sensitivity measurement and coherent control of these spin systems are crucial for their practical usage as qubits. The micro-SQUID (direct-current micrometer-sized Superconducting QUantum Interference Device) is capable to measure magnetization of spin system with high sensitivity. For example, the micro-SQUID technique can measure magnetic moments as small as several thousand μB as shown by the study of [W. Wernsdorfer, Supercond. Sci. Technol. 22, 064013 (2009)]. Here we develop a novel on-chip setup that combines the micro-SQUID sensitivity with microwave excitation. Such setup can be used for electron spin resonance measurements or coherent control of spins utilizing the high sensitivity of micro-SQUID for signal detection. To build the setup, we studied the fabrication process of the micro-SQUID, which is made of weak-linked Josephson junctions. The SQUID as a detector is integrated on the same chip with a shorted coplanar waveguide, so that the microwave pulses can be applied through the waveguide to excite the sample for resonance measurements. The whole device is plasma etched from a thin (∼20nm) niobium film, so that the SQUID can work at in large in-plane magnetic fields of several tesla. In addition, computer simulations are done to find the best design of the waveguide such that the microwave excitation field is sufficiently strong and uniformly applied to the sample. The magnetization curve of Mn₁₂ molecule-based magnet sample is measured to prove the proper working of the micro-SQUID. Electron spin resonance measurement is done on the setup for gadolinium ions diluted in a CaWO₄ single crystal. The measurement shows clear evidence of the resonance signal from the 1st transition of the gadolinium ions' energy levels, which shows the setup is successfully built. Due to the high sensitivity of micro-SQUID and the ability to concentrate microwave energy in small areas of the chip, this setup can detect signals from a small number of spins (10⁷) in a small volume (several μm³).
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Date Issued: 2017
Identifier: FSU_SUMMER2017_Yue_fsu_0071E_13872
Format: Thesis

Title: Tuning the Photoluminescence of Halide Perovskites.
Creator: Knox, Javon M., Gao, Hanwei, Bonesteel, N. E., Van Winkle, David, Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: Wavelength tunability or bandgap tuning of mixed halide perovskite, CsPb(IxBry)3 (y=1-x), can be achieved by changing the composition of iodide and bromide ions but under continuous illumination the photoluminescence (PL) peak position shifts from its initial position. This optical instability is attributed to phase segregation, where until photoexcitation the mixed halide anions migrate to iodide-rich and bromide-rich domains. Recent work has shown PL peak stability for mixed halide...
Show moreWavelength tunability or bandgap tuning of mixed halide perovskite, CsPb(IxBry)3 (y=1-x), can be achieved by changing the composition of iodide and bromide ions but under continuous illumination the photoluminescence (PL) peak position shifts from its initial position. This optical instability is attributed to phase segregation, where until photoexcitation the mixed halide anions migrate to iodide-rich and bromide-rich domains. Recent work has shown PL peak stability for mixed halide composition x > 0.6, however this limits the range of tunability to wavelengths greater than 640 nanometers (nm). Here we create a dual-source vapor-evaporation method to create mixed halide which can suppress the phase segregation and improve the photoluminescence stability.
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Date Issued: 2017
Identifier: FSU_SUMMER2017_Knox_fsu_0071N_14120
Format: Thesis

Title: A Specific Heat Investigation of High-Temperature Superconductors.
Creator: Moir, Camilla Margaret, Boebinger, Gregory S. (Gregory Scott), Siegrist, Theo, Shekhter, Arkady, Bonesteel, N. E., Riley, Mark A., Florida State University, College of Arts and...
Show moreMoir, Camilla Margaret, Boebinger, Gregory S. (Gregory Scott), Siegrist, Theo, Shekhter, Arkady, Bonesteel, N. E., Riley, Mark A., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: The understanding of the electronic systems of materials has not been only the essential, but the driving force, behind the progress of technology for over 100 years. This year marks the 60th anniversary of the revolutionary Bardeen-Cooper-Schrieffer, or BCS, theory which described the creation of Cooper pairs from a Fermi liquid ‘normal’ state through a coupling of conduction elections to phonons. Despite this, it wasn’t until the cuprate La2−xBaxCuO4, the first high-temperature...
Show moreThe understanding of the electronic systems of materials has not been only the essential, but the driving force, behind the progress of technology for over 100 years. This year marks the 60th anniversary of the revolutionary Bardeen-Cooper-Schrieffer, or BCS, theory which described the creation of Cooper pairs from a Fermi liquid ‘normal’ state through a coupling of conduction elections to phonons. Despite this, it wasn’t until the cuprate La2−xBaxCuO4, the first high-temperature superconductor, was discovered in the late 1980’s [1] that the dream of a room temperature superconductor seemed attainable and the ‘Age of the Superconductor’ began. However, the unique properties for which these high-temperature, unconventional superconductors are prized have also obstructed thorough investigation of the electronic behavior underlying their superconductivity and demanded extremely intense magnetic fields, very low temperatures, and thermodynamic measurements in extreme environments in order to fully characterize their electronic systems. It is, therefore, no small thing to flesh out the phase diagrams of these materials whose exotic electronic properties may eventually lead to faster, more compact devices and new methods of digital computation. Despite the difficulties in collecting usefully data on high-temperature superconductors, a vast body of work has amassed and grown with the increasingly intense magnetic fields available. As a result, quasiparticle mass enhancement near optimal doping was recently observed in two major classes of high-temperature superconductors, cuprates [2] and pnictides [3–5]. Because an effective quasiparticle mass accounts for the interactions between an electron and surrounding particles, it is an experimental indicator of enhanced electronic interactions. Enhancement of the quasiparticle effective mass, or increased electronic interactions, is believed to accompany quantum criticality, and the observation of mass enhancement in two very different classes of high-temperature superconductors makes quantum criticality the most promising candidate for universality across the high-temperature superconductors. The study outlined here is an investigation of the properties of three high-temperature superconductors, La2−xSrxCuO4, YBa2Cu3Oδ , and BaFe2(As1−xPx)2, through specific heat and resistivity measurements at very low temperatures, 1.5 K ≤ T ≤ 20 K, and magnetic fields up to 35 T. Such measurements required the construction of instrumentation specifically designed to deal with these extreme environments, and the low thermodynamic signals which are a signature of the cuprate superconductors. In order to understand the unprecedented data collected, novel analysis techniques based on Volovik phenomenology were developed. The procedures for specific heat measurements and the analysis of the resulting data developed for this study and outlined in the following thesis stand as the model for measurement of the normal state density of states of correlated superconductors. I report the observation of a saturation of the specific heat as a function of applied magnetic field in all three compounds, La2−xSrxCuO4, YBa2Cu3Oδ , and BaFe2(As1−xPx)2, indicating superconductivity has been suppressed and from which an effective mass, or sum of quasiparticle masses can be determined. I report that the onset of the normal state corresponds to the onset of finite resistance in La2−xSrxCuO4 and BaFe2(As1−xPx)2. I report enhancement in the sum of quasiparticle masses with doping in BaFe2(As1−xPx)2 that diverges near the predicted quantum critical point at optimum doping and that the dramatic enhancement evidences an orbital selective coupling to quantum fluctuations when compared to previous studies.
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Date Issued: 2017
Identifier: FSU_SUMMER2017_Moir_fsu_0071E_14029
Format: Thesis

Title: Nuclear Structure Studies of 44S and 26Si.
Creator: Parker, John J. (John Johnston), Wiedenhöver, Ingo, Fuelberg, Henry E., Collins, David C., Tabor, Samuel Lynn, Volya, Alexander, Florida State University, College of Arts and...
Show moreParker, John J. (John Johnston), Wiedenhöver, Ingo, Fuelberg, Henry E., Collins, David C., Tabor, Samuel Lynn, Volya, Alexander, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Experimental results on the nuclear structure of 44S and 26Si will be reported in this thesis. 44S is studied because of its interest in understanding how nuclei behave far from stability. 26Si is studied because of the impact of understanding its nuclear structure can have on the astrophysical 25Al(p,γ) reaction rate. These are two very differently motivated studies and will be described separately in Chapters 2 and 3, respectively. Chapter 2 focuses on the exotic N=28 nucleus, in 44S....
Show moreExperimental results on the nuclear structure of 44S and 26Si will be reported in this thesis. 44S is studied because of its interest in understanding how nuclei behave far from stability. 26Si is studied because of the impact of understanding its nuclear structure can have on the astrophysical 25Al(p,γ) reaction rate. These are two very differently motivated studies and will be described separately in Chapters 2 and 3, respectively. Chapter 2 focuses on the exotic N=28 nucleus, in 44S. Previous experiments observed a 4+ state and suggested that this state may exhibit a hindered E2-decay rate, inconsistent with being a member of the collective ground state band. We populate this state via a two-proton knockout reaction from a beam of exotic 46Ar projectiles delivered from the coupled cyclotron facility and measure its lifetime using the recoil distance method with the GRETINA γ ray spectrometer. The result, 76(14) stat (20) syst ps, implies a hindered transition of B(E2; 4+ →2+1 ) = 0.61(19) single- particle or Weisskopf units strength and supports the interpretation of the 4 + state as a K = 4 isomer, the first example of a high-K isomer in a nucleus of such low mass. Chapter 3 focuses on resonances above the proton threshold in 26Si. Previous experiments have solidified the placement of 3 resonances thought to contribute to the 25Al(p,γ)26Si reaction. A fourth resonance has been suggested by various experiments, but more recent experiments have suggested that this level has been misidentified. We populate excited states in 26Si via the 24Mg(3He,n) reaction at 10 MeV at the John Fox Lab at FSU. Neutron time-of-flight spectroscopy is used to identify which resonance is populated in 26Si and the γ-array at FSU is used to determine how these levels de-excite. The γ ray sensitivity in this experiment is the highest sensitivity reached to date, but a 4th resonance above the proton threshold was not identified, giving further indication that this state may have been misidentified by past experiments.
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Date Issued: 2017
Identifier: FSU_2017SP_ParkerIV_fsu_0071E_13841
Format: Thesis

Title: Surface Modification for Improved Design and Functionality of Nanostructured Materials and Devices.
Creator: Keiper, Timothy David, Xiong, Peng, Chase, P. Bryant, van Winkle, David H., Rikvold, Per Arne, Riley, Mark A., Florida State University, College of Arts and Sciences, Department...
Show moreKeiper, Timothy David, Xiong, Peng, Chase, P. Bryant, van Winkle, David H., Rikvold, Per Arne, Riley, Mark A., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Progress in nanotechnology is trending towards applications which require the integration of soft (organic or biological) and hard (semiconductor or metallic) materials. Many applications for functional nanomaterials are currently being explored, including chemical and biological sensors, flexible electronics, molecular electronics, etc., with researchers aiming to develop new paradigms of nanoelectronics through manipulation of the physical properties by surface treatments. This dissertation...
Show moreProgress in nanotechnology is trending towards applications which require the integration of soft (organic or biological) and hard (semiconductor or metallic) materials. Many applications for functional nanomaterials are currently being explored, including chemical and biological sensors, flexible electronics, molecular electronics, etc., with researchers aiming to develop new paradigms of nanoelectronics through manipulation of the physical properties by surface treatments. This dissertation focuses on two surface modification techniques important for integration of hard and soft materials: thermal annealing and molecular modification of semiconductors. First, the effects of thermal annealing are investigated directly for their implication in the fundamental understanding of transparent conducting oxides with respect to low resistivity contacts for electronic and optoelectronic applications and the response to environmental stimuli for sensing applications. The second focus of this dissertation covers two aspects of the importance of molecular modification on semiconductor systems. The first of these is the formation of self-assembled monolayers in patterned arrays which leads explicitly to the directed self-assembly of nanostructures. The second aspect concerns the modification of the underlying magnetic properties of the preeminent dilute magnetic semiconductor, manganese-doped gallium arsenide. Tin oxide belongs to a class of materials known as transparent conducting oxides which have received extensive interest due to their sensitivity to environmental stimuli and their potential application in transparent and flexible electronics. Nanostructures composed of SnO2 have been demonstrated as an advantageous material for high performance, point-of-care nanoelectronic sensors, capable of detecting and distinguishing gaseous or biomolecular interactions on unprecedented fast timescales. Through bottom-up fabrication techniques, binary oxide nanobelts synthesized through catalyst-free physical vapor deposition are implemented in the field-effect transistor structure. We have discovered that conductivity is absent in as-grown devices. However, utilizing a process for thermal treatment in vacuum and oxygen environments is found to be instrumental in fabricating field-effect transistors with significant conductivity, up to five orders of magnitude above the as-grown devices, for field-effect transistor application. Further investigation by photoluminescence coupled with the annealing parameters reveals that the likely cause of conductance comes from the reduction of surface defect states in the material. Importantly, the annealed material maintains its response to an applied gate potential showing orders of magnitude switching from the 'off' to the 'on' state. In order to show the practical relevance of our improvements on the SnO₂ material, we show our results for implementing the annealed material in biomolecular sensing experiments to detect the presence of streptavidin and Hepatitis C virus. Surface modification was carried out on oxide-free gallium arsenide (in some cases doped with manganese or zinc) through self-assembly of thiol molecules. First, we investigate the ability to pattern via two complementary micro- and nanopatterning techniques, microcontact printing (μCP) and dip-pen nanolithography (DPN). DPN is a unique lithography tool that allows drawing of arbitrary patterns with a molecular ink on a complementary substrate. It is extremely useful in integration of molecular inks within a pre-defined structure. Here, DPN was used to investigate the diffusion of organic molecules from a point source for both a moving and stationary tip on oxide-free GaAs. The diffusion can be calibrated so that intricate patterns down to tens of nanometers can be arbitrarily drawn on the surface. μCP, a less complicated method for large-scale arrayed patterning, is utilized to investigate the deposition of different thiolated molecular inks on GaAs and (Ga,Mn)As. The patterns deposited by μCP provide the template for directed self-assembly of gold nanoparticles. The systems based on these techniques can be extended to many substrate-molecule-nanostructure systems for an incredible variety of applications. Finally, the thiol-(Ga,Mn)As system is studied to determine the effects of molecular modification on the substrates' magnetic properties via modulation of the hole concentration in the wafer. The results for two molecules, one an electron donor and one an electron acceptor, show opposite trends for modulation of both the Curie temperature and the saturation magnetization. We suggest that nanopatterning of electron donor or electron acceptor molecules could lead to the development of reconfigurable nanomagnetic systems in (Ga,Mn)As with potential applications in molecular spintronics or magnetic memory.
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Date Issued: 2017
Identifier: FSU_2017SP_Keiper_fsu_0071E_13728
Format: Thesis

Title: ΛC Semileptonic Decays in a Quark Model.
Creator: Hussain, Md Mozammel, Roberts, Winston, Goldsby, Kenneth A,, Volya, Alexander, Crede, Volker, Owens, Joseph F., Florida State University, College of Arts and Sciences,...
Show moreHussain, Md Mozammel, Roberts, Winston, Goldsby, Kenneth A,, Volya, Alexander, Crede, Volker, Owens, Joseph F., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Hadronic form factors for semileptonic decay of the Λ[subscript c] are calculated in a nonrelativistic quark model. The full quark model wave functions are employed to numerically calculate the form factors to all orders in (1/m[subscript c], 1/m[subscript s]). The form factors satisfy relationships expected from the heavy quark effective theory (HQET) form factors. No other semileptonic decays of Λ[subscript c] has been reported other than the decay to the ground state Λ that implies f = B(Λ...
Show moreHadronic form factors for semileptonic decay of the Λ[subscript c] are calculated in a nonrelativistic quark model. The full quark model wave functions are employed to numerically calculate the form factors to all orders in (1/m[subscript c], 1/m[subscript s]). The form factors satisfy relationships expected from the heavy quark effective theory (HQET) form factors. No other semileptonic decays of Λ[subscript c] has been reported other than the decay to the ground state Λ that implies f = B(Λ[subscript c]⁺ → Λl⁺ν[subscript l])/B(Λ[subscript c]⁺ → X[subscript s]l⁺ν[subscript l]) = 1. In this work, the differential decay rates and branching fractions are calculated for transitions to the ground state and a number of excited states of Λ. The branching fraction of the semileptonic decay width to the total width of Λ[subscript c] has been calculated and compared with other theoretical estimates and experimental results. The branching fractions for Λ[subscript c] → Λ*l⁺ν[subscript l] → Σπl⁺ν[subscript l] and Λ[subscript c] → Λ*l⁺ν[subscript l] → NǨl⁺ν[subscript l] are also calculated. Apart from decays to the ground state Λ(1115), it is found that decays through the Λ(1405) provide a significant portion of the branching fraction Λ[subscript c] → X[subscript s]lν[subscript l]. There are various conjectures on the structure of the Λ(1405) while we treated it as a three quark state. A new estimate for f = B(Λ[subscript c]⁺ → Λl⁺ν[subscript l] is obtained.
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Date Issued: 2017
Identifier: FSU_2017SP_Hussain_fsu_0071E_13920
Format: Thesis

Title: Automated One-Loop QCD and Electroweak Calculations with NLOX.
Creator: Honeywell, Steven Joseph, Reina, Laura, Aluffi, Paolo, Owens, Joseph F., Roberts, Winston, Yohay, Rachel, Florida State University, College of Arts and Sciences, Department of...
Show moreHoneywell, Steven Joseph, Reina, Laura, Aluffi, Paolo, Owens, Joseph F., Roberts, Winston, Yohay, Rachel, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: We introduce a new framework, NLOX, in which one-loop QCD and electroweak corrections to Standard Model processes can be automatically calculated. Within this framework, we calculate the first order of electroweak corrections to the hadronic production of Z + 1b-jet and discuss some of the most relevant theoretical issues related to this process.
Date Issued: 2017
Identifier: FSU_2017SP_Honeywell_fsu_0071E_13868
Format: Thesis

Title: Search for Heavy Stable Charged Particles √ at S = 13 TeV Utilizing a Multivariate Approach.
Creator: Ackert, Andrew Kenjiro, Adams, Todd, Magnan, Jerry F., Prosper, Harrison B., Owens, Joseph F., Piekarewicz, Jorge, Florida State University, College of Arts and Sciences,...
Show moreAckert, Andrew Kenjiro, Adams, Todd, Magnan, Jerry F., Prosper, Harrison B., Owens, Joseph F., Piekarewicz, Jorge, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Heavy stable charged particles (HSCPs) have been searched for at the Large Hadron Collider since its initial data taking in 2010. The search for heavy stable charged particles provide a means of directly probing the new physics realm, as they produce a detector signature unlike any particle discovered to date. The goal of this research is to investigate an idea that was introduced in the later stages of 2010-2012 data taking period. Rather than utilizing the current tight selection on the...
Show moreHeavy stable charged particles (HSCPs) have been searched for at the Large Hadron Collider since its initial data taking in 2010. The search for heavy stable charged particles provide a means of directly probing the new physics realm, as they produce a detector signature unlike any particle discovered to date. The goal of this research is to investigate an idea that was introduced in the later stages of 2010-2012 data taking period. Rather than utilizing the current tight selection on the calculated particle mass the hypothesis is that by incorporating a multivariate approach, specifically an artificial neural network, the remaining selection criteria could be loosened allowing for a greater signal acceptance while maintaining acceptable background rejection via the multivariate discriminator from the artificial neural network. The increase in signal acceptance and retention or increase in background rejection increases the discovery potential for HSCPs and as a secondary objective calculates improved limits on the HSCP signal models. The multivariate approach was developed and tested using 2.5 fb⁻¹ of 2015 data at √s = 13 TeV based on both the past tracker-only and Tracker+TOF HSCP analyses. The multivariate analyses were able to produce improved upper cross section limits on both expected and observed cross sections compared to the past 2015 results. The lower mass limits produced by the multivariate analyses are also improved, but the improvement was found to be less than 5% higher compared to the past 2015 HSCP search results. A final comparison of the multivariate approach to past HSCP searches was conducted on 12.9 fbfb⁻¹ of 2016 data at √s = 13 TeV. No statistically significant excess of data over background prediction is observed. Therefore no evidence of HSCPs is claimed. The tracker-only (Tracker+TOF) multivariate analysis produced lower mass limits of 1870 (1820) GeV for gluinos with 10% R-hadrons produced neutral, 1260 (1210) GeV for stops, 680 (670) GeV for decay-product staus, 340 (370) GeV for directly pair-produced staus, 720 (750) GeV for modified Drell-Yan |Q| = 1e, and 700 (900) GeV for modified Drell-Yan |Q| = 2e. Overall, the multivariate approach produced improved lower mass limits compared to the results from the past 2016 HSCP search. The overall improvements in cross section and mass limits using the multivariate approach produced the best limits on HSCPs to date. Furthermore the multivariate approach is shown as a viable method of searching for HSCPs. With the HSCP search covering a broad range of beyond the Standard Model physics, the lack of evidence and subsequent limits produced place important restrictions on the theoretical models that predict HSCPs.
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Date Issued: 2017
Identifier: FSU_2017SP_Ackert_fsu_0071E_13836
Format: Thesis

Title: Fabrication and Characterization of Heterogeneous Nanowires.
Creator: Barreda Esparza, Jorge Luis, Xiong, Peng, Guan, Jingjiao, Bonesteel, N. E., Chiorescu, Irinel, Piekarewicz, Jorge, Florida State University, College of Arts and Sciences,...
Show moreBarreda Esparza, Jorge Luis, Xiong, Peng, Guan, Jingjiao, Bonesteel, N. E., Chiorescu, Irinel, Piekarewicz, Jorge, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Nanoscience and nanotechnology research has provided us with new paradigms of technologies to improve human life, but still there is plenty of room to expand its frontiers. In order to do so, we need to pursue the development and study of novel nanostructures with the main goal of understanding the physical properties and finding potential applications. Understanding the physics of low-dimensional systems is the first step to fostering the corresponding technological applications. Considering...
Show moreNanoscience and nanotechnology research has provided us with new paradigms of technologies to improve human life, but still there is plenty of room to expand its frontiers. In order to do so, we need to pursue the development and study of novel nanostructures with the main goal of understanding the physical properties and finding potential applications. Understanding the physics of low-dimensional systems is the first step to fostering the corresponding technological applications. Considering this premise, the goal of this dissertation is to study two distinct classes of heterogeneous nanowires (NWs): phosphorous-doped Si NWs with an axial doping gradient and metal NWs grown on DNA templates. The Si NWs grown by vapor-liquid-solid chemical vapor deposition were utilized to fabricate Schottky barrier-limited field-effect transistors (FETs), which have shown significant promise in the areas of electronics and sensing because of their unique characteristics. The idea of utilizing the modulation of the nano Schottky junction at a metal-semiconductor interface promises higher performance for chemical and biomolecular sensor applications when compared to conventional FETs with Ohmic contacts (exponential versus linear responses). However, the fabrication of such asymmetric FETs presents challenges such as reproducibility through complications in the fabrication processes. We have been able to circumvent the fabrication difficulties and reproducibility problems by utilizing our Si nanowires synthesized by a chemical vapor deposition process which yields a pronounced doping gradient along the length of the NWs. This inhomogenous doping in NWs has typically been seen as a detrimental characteristic; however, we have taken advantage of this doping profile as the basis of our approach. The graded doping profile facilitates definition of a series of metal contacts on a single NW that systematically evolve from Ohmic to Schottky with increasing effective barrier height along the axial direction. The study of this systematic variation is presented in this dissertation as a proposal to obtain devices for sensing and electronic applications. The main results of our research is recently published. The fabrication and study of metal NWs is the second effort discussed in this dissertation. The main motivation is to address the fundamental question of whether a true superconducting state could exist in one dimension. The answer to this question lies in the nature of superconducting fluctuations of the order parameter that describe the coherent behavior of the Cooper pairs. In a superconducting system, the order parameter has a well-defined amplitude and phase. The superconducting fluctuations occur in the form of phase slips which can be either thermally activated or quantum mechanical. Although much experimental and theoretical work has been done on the topic, an unambiguous resolution of this issue remains elusive mainly due to the challenge of producing NWs having the dimensions of the cross-section of the NW smaller than the superconducting coherence length or the size of the Cooper pairs. Our approach to overcome the fabrication difficulties to reach the true 1D limit is a unique combination of DNA templates and low temperature quench-condensation for in situ fabrication and measurement of superconducting NWs with a width of just a few nanometers. In this dissertation, details on the fabrication and our initial results demonstrating the capability of our DNA molecular templates to reach small cross-section metal NWs are presented; also, we present systematic characterizations of the electrical properties of metal nanowires with respect to in situ variation of the geometry of the nanowire. This effort has laid a full foundation for a comprehensive examination of superconductivity in 1D reaching unprecedentedly small cross-sections. A manuscript summarizing these results is in preparation.
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Date Issued: 2017
Identifier: FSU_2017SP_Barreda_fsu_0071E_13848
Format: Thesis

Title: The Photoproduction of Strangeness in ΓP → ΛK+Π+Π− with CLAS at Jefferson Lab.
Creator: Al Ghoul, Hussein, Eugenio, Paul Michael, Justus, James, Crede, Volker, Capstick, Simon, Ostrovidov, Alexander, Wahl, Horst D., Florida State University, College of Arts and...
Show moreAl Ghoul, Hussein, Eugenio, Paul Michael, Justus, James, Crede, Volker, Capstick, Simon, Ostrovidov, Alexander, Wahl, Horst D., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: The available information about strange excited mesons is limited and most of the observed states have been reported but not confirmed. While the low mass region (1.0 - 1.5 GeV) has been extensively studied in the past and states such as the K1(1270), K1(1400), and K*(1410) have been confirmed by a handful of experiments, little is known about the spin-parity structure of resonances in the higher K+ π+ π- mass region (1.5-2.0 GeV). Past experiments have used hadron beams to gain access to the...
Show moreThe available information about strange excited mesons is limited and most of the observed states have been reported but not confirmed. While the low mass region (1.0 - 1.5 GeV) has been extensively studied in the past and states such as the K1(1270), K1(1400), and K*(1410) have been confirmed by a handful of experiments, little is known about the spin-parity structure of resonances in the higher K+ π+ π- mass region (1.5-2.0 GeV). Past experiments have used hadron beams to gain access to the K+ π+ π- system, and have provided extensive information about strange states that made mapping their spectrum possible. Except for the K*(892), none of the excited strange states has been photoproduced before. We perform a partial wave analysis on a photoproduced K+ π+ π- system produced off a Λ baryon using the CLAS detector at Jefferson Lab. Using a photon beam incident of a liquid hydrogen target, we are able to reconstruct 16K events of the γ p -> Λ K+ π+ π- topology. Results from initial data selection confirmed the dominance of two decay modes for a K+ π+ π- resonance: the K*(892)π+ and the ρ(770)K+. A PWA was carried out in the helicity formalism using the reflectivity basis in the isobar model to parametrize the decay amplitudes of the resonances. Resonating structures are found in the 1+S wave primarily coupling to K*(892)π+ with a mass of 1.35 GeV/c2 and to ρ(770)K with a lower mass structure around 1.33 GeV/c2 and a higher mass resonance around 1.73 GeV/c2. The 1-P also exhibited a significant resonating behavior with a mass of 1.43 GeV/c2 coupling primarily to the K*(892)π+ decay mode. Also observed, an enhancement around 1.49 GeV/c2 in the 2+D wave strongly coupling to K*2 (1430)π+, and an enhancement in the 2-S wave around 1.76 GeV/c2 coupling primarily to K*2 (1430)π+.
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Date Issued: 2016
Identifier: FSU_2016SP_AlGhoul_fsu_0071E_13108
Format: Thesis

Title: Quantum Oscillations in Two Dimensional Dirac and Weyl Semimetals.
Creator: Das, Suvadip, Manousakis, Efstratios, Bonesteel, N. E., Balicas, Luis, Piekarewicz, Jorge, Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: Since the discovery of the exotic properties of graphene, two dimensional materials such as metal chalcogenides, transition metal oxides and other 2D compounds have gained renewed interest. Graphene, silicene, germanene, graphyne and boron allotropes form a rare class of 2D Dirac materials. The presence of such Dirac points near the Fermi level provides us the option to switch between two carrier types by slightly doping the material and could lead to potential optoelectronic devices....
Show moreSince the discovery of the exotic properties of graphene, two dimensional materials such as metal chalcogenides, transition metal oxides and other 2D compounds have gained renewed interest. Graphene, silicene, germanene, graphyne and boron allotropes form a rare class of 2D Dirac materials. The presence of such Dirac points near the Fermi level provides us the option to switch between two carrier types by slightly doping the material and could lead to potential optoelectronic devices. Recently discovered sister compounds WTe₂[2] and MoTe₂[62] have gained significant impetus for extremely pronounced nonsaturating magnetroresistance and topological semimetal hosting type II Weyl points. Further, a new class of two dimensional materials with multiple Dirac cones were discovered following the compound Zr₂Te₂P , and include the compounds Hf₂Te₂P , Zr₂Te₂As and Ti₂Te₂P. Quantum oscillation studies were performed to investigate the detailed Fermi surfaces and the topological properties such as Berry phase were obtained for the newly discovered two dimensional materials at the National High Magnetic Field Laboratory. In this thesis we will address the electronic structure, density of states and Fermi surface in all of these materials using Density Functional Theory and related methodology. The low energy (near Fermi energy) physics of all the materials studied are characterized by Dirac-like or Weyl- like electronic structure. Namely, the electrons obey Dirac-like or Weyl-like equations where the speed of light is replaced by the Fermi velocity. Furthermore, these materials share fundamental topological properties at the electronic low energy spectrum. Therefore, we wish to undertake the task of studying such fundamental properties from first principles.
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Date Issued: 2016
Identifier: FSU_FA2016_Das_fsu_0071E_13587
Format: Thesis

Title: Experimental and Computational Studies on DNA Electrophoresis in Lyotropic Polymer Liquid Crystals.
Creator: Wei, Ling, Van Winkle, David H., Shanbhag, Sachin, Xiong, Peng, Rikvold, Per Arne, Wahl, Horst D., Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: Electrophoresis as an analytical technique has made considerable contributions to the separations and analysis of macromolecules in biology-related research. Pluronic gels, which are composed of orderly packed spherical micelles assembled by tri-block copolymers, have been developed as novel sieving media to separate oligonucleotides, duplex DNA molecules and proteins, providing ease of manipulations due to their thermo-reversibility and higher resolution in comparison with other polymer gels...
Show moreElectrophoresis as an analytical technique has made considerable contributions to the separations and analysis of macromolecules in biology-related research. Pluronic gels, which are composed of orderly packed spherical micelles assembled by tri-block copolymers, have been developed as novel sieving media to separate oligonucleotides, duplex DNA molecules and proteins, providing ease of manipulations due to their thermo-reversibility and higher resolution in comparison with other polymer gels. Electrophoretic mobility of short double-stranded DNA molecules in pluronic F127 is reported to have a non-monotonic dependence on DNA length, which is not observed in other polymer-based sieving media or explained by any well-developed theories. In this dissertation, the unusual DNA-length dependence of electrophoretic mobility is experimentally investigated in several different pluronic gels, and the DNA dynamics in pluronic liquid crystals is systematically studied by coarse-grained Brownian dynamics simulations. The crystal structures and micelle dimensions of pluronics P105, P123 and F127 are characterized by atomic force microscopy, small-angle x-ray scattering, small-angle neutron scattering and dynamic light scattering. Two-dimensional gel electrophoresis is performed and the electrophoretic mobility of DNA molecules in the size range of 20-500 bp is measured in pluronics P105, P123 and F127. The unusual DNA length-dependent mobility is consistently obtained in three pluronic gels, where the mobility of very short DNA molecules increases with increasing DNA length, and the mobility of long DNA molecules monotonically decreases with DNA length. Superposed on the rising and falling trends are the subtle oscillations of mobility with DNA length in the intermediate regime. Brownian dynamics simulations are implemented to numerically calculate the DNA mobility in pluronic lattices, by including the short-ranged intra-molecular hydrodynamic interactions, and modeling the interactions between DNA molecules and pluronic micelles via a repulsive force and entanglement effect. The rise, fall and oscillations of mobility with DNA length, as obtained in experimental measurements, are reproduced by the Brownian dynamics simulations, and essential physics that dominates the unusual features of mobility is extracted from the simulations. In addition, electric field-dependent mobility of DNA molecules in pluronic lattices is studied by Brownian dynamics simulations, and the conceptual connection between high-field simulations along specific field directions and low-field experiments in bulk gels is established, and the Brownian dynamic simulations are proven to be an appropriate approach to interpret the DNA electrophoretic dynamics in pluronic matrices. Moreover, electrophoretic mobility of duplex DNA flanked by single-stranded overhangs is measured in pluronic gels, and it is shown that the mobility of DNA with overhangs is higher than the corresponding blunt-ended DNA molecules. Brownian dynamics simulations are carried out, and the enhancement of mobility for DNA with overhangs is captured by the simulations. By integrating numerical simulations with experimental measurements, the fundamental physical quantities and interactions that manipulate the DNA electrophoretic migration in pluronic liquid crystals are revealed. Understanding the unusual DNA length-dependent mobility in pluronic gels potentially provides profound insights in designing and optimizing high-performance sieving matrices for size-based separation purposes.
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Date Issued: 2016
Identifier: FSU_FA2016_Wei_fsu_0071E_13585
Format: Thesis

Title: Computational Studies of Equilibrium and Non-Equilibrium Phase Diagrams and Critical Properties of Two Physical and Chemical Model Systems with Both Short-Range and Long-Range Interactions or Reactivities.
Creator: Chan, Chor-Hoi, Rikvold, Per Arne, Shanbhag, Sachin, Brown, Gregory, Capstick, Simon, Xiong, Peng, Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: In this dissertation, we introduce long-range interactions into one equilibrium model (Ising model) and one non-equilibrium system (Ziff-Gulari-Barshad model), and study their phase diagrams and critical properties. A new approach to do Wang-Landau simulation: macroscopically constrained Wang-Landau, is proposed in connection with the former system. Our macroscopically constrained Wang-Landau method breaks a multidimensional random walk process in phase space into many separate, one...
Show moreIn this dissertation, we introduce long-range interactions into one equilibrium model (Ising model) and one non-equilibrium system (Ziff-Gulari-Barshad model), and study their phase diagrams and critical properties. A new approach to do Wang-Landau simulation: macroscopically constrained Wang-Landau, is proposed in connection with the former system. Our macroscopically constrained Wang-Landau method breaks a multidimensional random walk process in phase space into many separate, one-dimensional random walk processes in the energy space. Each of these random walks is constrained to a different value of the macroscopic order parameters. By knowing the distribution of these constrained variables, we can deduce the multi-variable density of states. When the multi-variable density of states for one set of external parameters is obtained, the density of states at any point in the phase diagram can be obtained by simple transformations. After that, all thermodynamic quantities can be obtained. We apply this method to an antiferromagnetic Ising model with a ferromagnetic long-range interaction. The addition of the long-range interaction induces metastable regions in the phase diagram, and a mean-field class critical point emerges for sufficiently strong long-range interaction. We demonstrate how to use the multi-variable density of states obtained to sketch out the complicated phase diagrams for different values of the long-range interaction. We also give free-energy plots, and plots of the distributions of the order parameters of the system for different special points in these phase diagrams. The Ziff-Gulari-Barshad (ZGB) model, a simplified description of the oxidation of carbon monoxide (CO) on a catalyst surface, is widely used to study properties of nonequilibrium phase transitions. Instead of restricting the CO and atomic oxygen (O) to react to form carbon dioxide (CO₂) only when they are adsorbed in close proximity, we consider a modified model that includes an adjustable probability for adsorbed CO and O atoms located far apart on the lattice to react. We employ large-scale Monte Carlo simulations to study the critical properties of this system. We find that the nonequilibrium critical point changes from the two-dimensional Ising universality class to the mean-field universality class upon introducing even a weak long-range reactivity mechanism.
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Date Issued: 2016
Identifier: FSU_FA2016_Chan_fsu_0071E_13552
Format: Thesis

Theses and Dissertations (282)	+ -
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Florida State University (282)	+ -
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Piekarewicz, Jorge (84)	+ -
Xiong, Peng (62)	+ -
Reina, Laura (47)	+ -

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