PHD, Kent State University, 2009, College of Arts and Sciences / Department of Physics

The study of quantum phase transitions (QPT) provides a new route to find and understand unconventional phases in condensed matter physics. The presently studied alloy, Ni(1-x)Vx, offers an opportunity to investigate a ferromagnetic quantum phase transition, a transition from a ferromagnetic ordered state into a paramagnetic state at T = 0 K, by varying the vanadium concentration, x. Magnetization and transport measurements are used to probe the critical behavior of the phase transition and characterize the onset of “unconventional behavior” such as non-Fermi liquid behavior, which signals a deviation from Fermi liquid theory, a fundamental concept in metals. Towards 11.2 % vanadium, the Curie temperature (Tc) is reduced to zero from its pure nickel value of Tc = 627 K. The critical behavior of the phase transition in samples with the higher nickel content (x < 11%) at a finite Tc essentially follows theories as expected for weak itinerant magnets. The samples with more vanadium (x > 11.2%) do not show a conventional ferromagnetic transition or the typical properties of an ordinary paramagnet. Instead, we see evidence for power laws with unusual exponents in the temperature dependence of the magnetization and the resistivity due to an inhomogeneous magnetic moment distribution. We compare our data findings with recent theories addressing a new critical scenario, quantum phase transitions with disorder. One signature is a Quantum Griffiths' phase which is observed as power laws with non-universal exponents heading towards a T → 0 instability. At very low temperatures, the quantum Griffiths phase in Ni-V leads to the formation of a frozen cluster glass phase. To our knowledge, our compound is the first to experimentally show all signatures of a quantum Griffiths phase in an extended regime, and therefore provides an ideal model system for a disordered itinerant 3-d Heisenberg system.

Committee: Almut Schroeder Dr. (Advisor); Carmen Almasan Dr. (Committee Member); David Allender Dr. (Committee Member); Songping Huang Dr. (Committee Member); Robert Twieg Dr. (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2017, Physics

Classical interatomic potentials are an important bridge between nano-scale and meso-scale properties of materials, and facilitate an understanding of deformation and phase transitions at the atomistic level. This work presents the development of two semi-empirical interatomic potentials, one for the body-centered cubic metal tungsten and another for multi-phase titanium-niobium alloys. Accurate density functional theory calculations constitute large databases of forces, stresses and energies to which the empirical models are fit using an evolutionary algorithm. Accuracy of the potentials is verified by comparison with experiment and first-principles calculations for numerous structural, elastic and thermal properties. The models are used to investigate structural phase transitions under high pressure, in the case of tungsten, and chemical disorder, in the case of titanium-niobium. The presented models provide improved descriptions of these technologically important metals over existing classical potentials.

Committee: John Wilkins (Advisor); Maryam Ghazisaeidi (Committee Member); Richard Kass (Committee Member); Fengyuan Yang (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Metallurgy

Master of Science, Miami University, 2017, Physics

An experimental investigation of the structural, magnetic and transport properties of four Heusler alloy systems (Ni2Mn1-xCrxIn, Ni2Mn1-xCrxGa, Ni2Mn0.4-xFexCr0.6Ga, and Ni2Mn0.55CoxCr0.45-xGa) has been performed. The polycrystalline samples have been characterized in detail by means of x-ray diffraction, scanning electron microscopy, magnetic, calorimetric, and transport measurements. The study revealed multiple phase transitions in the alloys and novel phenomena in the vicinity of these transitions, including magnetocaloric effects and large changes in electrical resistivity. The experimental results serve as a foundation for conjectures regarding fundamental interactions in Heusler alloys.

Committee: Mahmud Khan Ph.D. (Advisor); Herbert Jaeger Ph.D. (Committee Member); Khalid Eid Ph.D. (Committee Member) Subjects: Physics

Master of Science, Miami University, 2020, Physics

The phase transitions and associated magnetic and transport properties of Ni2Mn0.70Cu0.30Ga, Ni2Mn0.70Cu0.25Cr0.05Ga and Ni2Mn0.70Cu0.30Ga0.95In0.05 have been investigated via x-ray diffraction, scanning electron microscopy, magnetic, calorimetric, and electrical resistivity measurements. While Ni2Mn0.70Cu0.30Ga exhibited a tetragonal structure at room temperature, cubic and orthorhombic phases coexisted in the other two samples. The scanning electron microscope micrographs indicated that no impurity phases existed in the samples. All three samples exhibited the first order martensitic phase transformation upon cooling and heating. The phase transitions were accompanied by large magnetic entropy changes and anomalies in the electrical resistivity data. For a field change of 50 kOe, peak magnetic entropy changes of -17 J kg-1K-1, -39 J kg-1K-1, and -26 J kg-1K -1 were observed for Ni2Mn0.70Cu0.30Ga, Ni2Mn0.70Cu0.25Cr0.05Ga and Ni2Mn0.70Cu0.30Ga0.95In0.05 respectively, when the measurements were done while heating. When the measurements were done while cooling from 400 K to lower temperatures, the peak values were -33 Jkg-1K-1, -17 Jkg-1K-1, and -15 Jkg-1K-1 for Ni2Mn0.70Cu0.30Ga, Ni2Mn0.70Cu0.25Cr0.05Ga and Ni2Mn0.70Cu0.30Ga0.95In0.05 respectively. The experimental results are discussed taking the decoupling and coupling of the phase transitions into consideration.

Committee: Mahmud Khan PhD (Advisor); Herbert Jaeger PhD (Committee Member); Stephen Alexander PhD (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Akron, 2007, Polymer Engineering

A free energy functional has been formulated based on an order parameter approach to describe the competition between liquid-liquid phase separation and solid-liquid phase separation. In the free energy description, the assumption of complete solvent rejection from the crystalline phase that is inherent in the Flory diluent theory was removed as solvent has been found to reside in the crystalline phase in the form of intercalates. Using this approach,we have calculated various phase diagrams in binary blends of crystalline and amorphous polymers that show upper or lower critical solution temperature. Also, the discrepancy in the χ values obtained from different experimental methods reported in the literature for the polymer blend of poly(vinylidenefluoride) and poly(methylmethacrylate) has been discussed in the context of the present model. Experimental phase diagram for the polymer blend of poly(caprolactone) and polystyrene has also been calculated. Of particular importance is that the crystalline phase concentration as a function of temperature has been calculated using free energy minimization methods instead of assuming it to be pure. In the limit of complete immiscibility of the solvent in the crystalline phase, the Flory diluent theory is recovered. The model is extended to binary crystalline blends and the formation of eutectic, peritectic and azeotrope phase diagrams has been explained on the basis of departure from ideal solid solution behavior. Experimental eutectic phase diagram from literature of a binary blend of crystalline polymer poly(caprolactone) and trioxane were recalculated using the aforementioned approach. Furthermore, simulations on the spatio temporal dynamics of crystallization in blends of crystalline and amorphous polymers were carried out using the Ginzburg-Landau approach. These simulations have provided insight into the distribution of the amorphous polymer in the blends during the crystallization process. The simulated results are in (open full item for complete abstract)

Committee: Thein Kyu (Advisor) Subjects:

Doctor of Philosophy, The Ohio State University, 2024, Physics

Phase transitions occur in many classical and quantum systems, and are the subject of many an open problem in physics. In the past decade, the conformal bootstrap has provided new perspectives for looking at the critical point of a transition. With this formalism, it is possible to exploit the conformal symmetry intrinsically present at the critical point, and derive general results about classes of transitions that obey the same symmetries. This thesis presents the application of this method to two problems of note in classical and quantum phase transitions. The first is a classical model of O(N) spins in the presence of a boundary. We use the numerical conformal bootstrap to prove rigorously the existence of a new boundary phase in three-dimensional Heisenberg (O(3)) and O(4) magnets, deemed the extraordinary-log universality class. The results agree well with a parallel numerical study but are more rigorous due to the bounded nature of the error. The second case is the inherently quantum problem of Anderson transitions between metals and insulators. It has been discovered that at criticality, the wavefunctions describe multifractal objects, that are described by infinitely many fractal dimensions. We use analytical constraints from conformal symmetry to predict the form of these fractal parameters in dimensions greater than two. Our exact prediction, which works in arbitrary dimensions, can be used as a probe for conformal symmetry at Anderson transitions. By studying these two problems, we demonstrate the power of conformal symmetry as a non-perturbative tool in the theory of phase transitions in arbitrary dimensions. Throughout the thesis, we have extended the domain of applicability of traditional bootstrap techniques for the purpose of non-unitary and non-positive systems.

Committee: Ilya Gruzberg (Advisor); Marc Bockrath (Committee Member); Samir Mathur (Committee Member); Yuanming Lu (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, The Ohio State University, 2022, Chemistry

Perovskite-based materials provide an exemplary platform to explore fundamental structure-property relationships. The relative simplicity of the perovskite crystal structure and a plethora of interesting properties related to purposes such as optoelectronics, multiferroics, and superconductivity, has sparked immense research interest. For halide perovskite variants, research has focused primarily on APbX3 (A = Cs+, CH3NH3+, NH2CHNH2+; X = Cl−, Br−, I−) perovskites for photovoltaic applications. One of the most impressive features of the halide perovskite is the ease of chemical substitution, which has resulted in a wide variety of structural variations. Halide perovskite materials containing transition metals offer a broader range of applications due to the possibility of diamagnetic and paramagnetic electronic configurations. Here we explore halide perovskite derivatives containing transition metals to understand the optical and magnetic properties that arise in low dimensional crystal structures. Following the first introductory chapter, Chapter 2 looks to expand on the known compositional space of the (CH3NH3)2M′MʺX6 halide double perovskites by introducing Rh3+ at the Mʺ site. Here, we synthesized (CH3NH¬3)2M′RhX6 (M′ = Na+, Ag+; X = Cl−, Br−) and looked to understand the optical properties that arise from the one-dimensional chain structure of these hexagonal 2H perovskite variants. This sets the foundation for Chapter 3, which explores two-dimensional layered perovskites containing the Ag–Rh–X inorganic framework. By replacing CH3NH3+ with a bulky organic cation, the crystal structure transitions from one-dimensional chains to two-dimensional layers. The optical properties are further explored in comparison to the previously studied hexagonal perovskites. Changes in the absorption spectra are explained by changes in the octahedral connectivity and ability of orbitals to hybridize. Chapter 4 focuses on the structural phase transitions and magnetic propertie (open full item for complete abstract)

Committee: Patrick Woodward (Advisor); Yiying Wu (Committee Member); Joshua Goldberger (Committee Member) Subjects: Chemistry; Materials Science

PHD, Kent State University, 2022, College of Arts and Sciences / Department of Physics

We performed single-molecule studies to investigate the impact of several prominent small molecules (the oxazole telomestatin derivative L2H2-6OTD, pyridostatin, and Phen-DC 3) on intermolecular G-quadruplex (i-GQ) formation between two guanine-rich DNA strands that have 3-GGG repeats in one strand and 1-GGG repeat in the other (3+1 GGG), or 2-GGG repeats in each strand (2+2 GGG). Such structures are not only physiologically significant but have recently found use in various biotechnology applications, ranging from DNA-based wires to chemical sensors. Understanding the extent of stability imparted by small molecules on i-GQ structures has implications for these applications. The small molecules resulted in different levels of enhancement in i-GQ formation, depending on the small molecule and arrangement of GGG repeats. The largest enhancement we observed was in the 3+1 GGG arrangement, where i-GQ formation increased by an order of magnitude, in the presence of L2H2-6OTD. On the other hand, the enhancement was limited to three-fold with Pyridostatin (PDS) or less for the other small molecules in the 2+2 GGG case. By demonstrating detection of i-GQ formation at the single-molecule level, our studies illustrate the feasibility to develop more sensitive sensors that could operate with limited quantities of materials. In another study, although its mesomorphic properties have been studied for many years, only recently has the molecule of life begun to reveal the true range of its rich liquid crystalline (LC) behavior. End-to-end interactions between concentrated, ultra-short DNA duplexes – self-assembling to form longer aggregates that then organize into LC phases – and the incorporation of flexible single-stranded “gap” regions in otherwise fully-paired duplexes – leading to the first convincing evidence of an elementary lamellar (smectic-A) phase in DNA solutions – are two exciting developments that have opened new avenues for discovery. In this dissertation, w (open full item for complete abstract)

Committee: Hamza Balci (Committee Chair); Samuel Sprunt (Committee Member); Michael Tubergen (Committee Member); Sanjaya Abeysirigunawardena (Committee Member); Antal Jákli (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2020, Physics

We probe the effects of applying an external magnetic field to Kitaev's spin S=1/2 model on a honeycomb lattice [1] and study this model's low-lying energy excitations across varying field strengths (and field orientations) using exact diagonalization. We go beyond Kitaev's original third order perturbation result (as well as some later studies [2]) in order to explore the effects of significantly stronger fields (up to the fully polarized limit), and we discover the presence of a novel gapless quantum spin liquid (QSL) phase sandwiched between Kitaev's lower-field gapped QSL phase and the forced ferromagnetic phase, for fields pointing along [111]. We discern this new intermediate-field strength gapless QSL by identifying signature features in the Kitaev honeycomb model's low-lying energy spectrum, topological entanglement entropy, on-site dynamical response, real-time on-site dynamics, and the average of plaquette flux operator expectations. We further explore the evolution of these measures away from a field pointing along [111] to gauge the robustness of the intermediate-field strength gapless QSL phase along varying field orientations. Finally, and in order to characterize the newfound intermediate-field gapless QSL phase, we perform a symmetry analysis of the Kitaev honeycomb model and derive a spinon hopping Hamiltonian consisting of five parameters. We vary these parameters to construct a many-body trial wavefunction whose energy, magnetization, and various other physical properties we calculate. We obtain the many-body ground state wavefunction in the completely polarized limit by minimizing the energy and then evolve away from this limit towards lower field strengths to find pockets at the M-points of the noninteracting Hamiltonian's band structure.

Committee: Nandini Trivedi (Advisor); Yuan-Ming Lu (Committee Member); Rolando Valdes Aguilar (Committee Member); Yuri Kovchegov (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, Case Western Reserve University, 2020, Physics

In modern electronics, the miniaturization of electronic devices is becoming ever so important for which nanostructures have become the center of focus. Understanding the electron transport of nanostructures is imperative in incorporating or assembling them into electronic devices. Three main studies are discussed in here, covering two aspects of electron transport. Two of those studies are focused on quantum transport involving InSe where Rashba spin-orbit coupling and the effect of Coulomb interactions are explored. The other study is oriented towards exploring the possibility to harness the reversible phase change of Ag2Te nanowires as nanoscale memory devices. The findings of these studies may play a key role in devising future electronic, spintronic and memory devices.

Committee: Xuan Gao (Advisor) Subjects: Condensed Matter Physics; Materials Science; Physics

Doctor of Philosophy, The Ohio State University, 2019, Physics

Superconductivity has been at the heart of research into quantum phenomena since its discovery over a century ago. More recently efforts have been made to understand the nature of the quantum phase transition (QPT) that separates the superconducting and insulating phases in certain 2D materials at zero temperature. This superconductor-insulator transition (SIT) has been theoretically and experimentally proven to be driven by quantum fluctuations of the superconducting phase instead of the breakup of Cooper pairs. In this thesis we present a study of quantum fluctuations across the SIT and how they can be imaged in both theoretical simulations and experimental measurements. We begin with an overview of the field from a historical perspective, describing the development of the theory of SITs driven by experiments on thin films. We present the Josephson junction array (JJA) model as a paradigm to investigate the quantum phase fluctuation-driven SIT using quantum Monte Carlo (QMC) techniques. We explore the manifestation of quantum fluctuations across the SIT in three different local measurements: the diamagnetic susceptibility χ(r), two-particle density of states P(r, ω), and compressibility κ(r), revealed through their local maps and calculated via QMC. χ(r) probes the system's ability to generate diamagnetic currents and its local map displays growing fluctuations upon increasing both temperature the quantum tuning parameter g. Remarkably, however, these fluctuations persist well below Tc as the SIT is approached, indicating the quantum nature of these fluctuations. We compare our results to SQUID susceptometry measurements performed on thin-film NbTiN and find good qualitative agreement. The map of κ(r) paints a similar picture when tuned via g, but in contrast to χ(r), we find a fundamental difference in its evolution with temperature, providing a complementary local probe to χ(r). P(r, ω), obtained using Maximum Entropy analytic continuation of imaginary time Q (open full item for complete abstract)

Committee: Nandini Trivedi (Advisor); Yuan-Ming Lu (Committee Member); Rolando Valdes-Aguilar (Committee Member); Richard Furnstahl (Committee Member) Subjects: Physics; Theoretical Physics

Doctor of Philosophy, University of Akron, 2018, Polymer Science

The self-assemble behavior of various giant surfactants with different functionalities and molecular architectures, which consist of compact and rigid molecular nanoparticles (MNPs) as head and flexible polymer chains as tail, is systematically studied. The benefit of utilizing MNPs comparing with AB type di-block copolymer which has become the model for probing microphase separation in bulk state is that the MNPs could be versatile functionalized at will to fabricate functional materials via collective secondary interactions. Herein, we investigated the effect of rigid-rod junction between two blocks of giant surfactants in bulk state using small angle X-ray scattering and transmission electron microscopy. Besides the segmental interaction (Flory-Huggins parameter) and volume fraction of one component (f), the interfacial energy takes a vital role in the process of self-assembly. By way of about 1 nm rigid-rod junction inserted into the giant surfactant system, the pathways of order-order phase transition (OOT) could be varied due to different molecular weight of polystyrene attached inducing diverse levels of gibbs free energy with temperature increasing, one proceeds from lamellar structure (LAM) directly to the hexagonally packed cylinder structure (CYL), one passes by the double gyroid structure (DG), another one travels from LAM to hexagonally perforated layer structure (HPL), then DG, finally reaches to the stable state, CYL that possesses the most curved interfacial area in these previous mentioned structures. And the model which describes these sorts of phase behaviors with interfacial tension and packing frustration competing each other could be verified via the phenomena that curved lamellar defects observed both in bright field TEM image and small angle x-ray scattering data, which could be occurred due to the relaxation of polymer chains promoting the increase of interfacial area without loosening the imbalance of chain crowding because of tightly packe (open full item for complete abstract)

Committee: Stephen Z. D. Cheng (Advisor); Liu Tianbo (Committee Chair); Zhu Yu (Committee Member); Miyoshi Toshikazu (Committee Member); Wesdemiotis Chrys (Committee Member) Subjects: Chemistry; Polymer Chemistry; Polymers

Doctor of Philosophy, Case Western Reserve University, 2017, Physics

This dissertation focuses on liquid crystals (LCs), specifically their chiral properties and interactions with surfaces and nanostructures. Nematic twist cells were filled with a LC doped with the chiral molecule CB15, which compensates for the imposed twist. Using the electroclinic effect (ECE), results indicate that an ECE always exists near the surface in twist cells containing conformationally deracemizable molecules. ECE measurements were also performed to determine the source of the ECE response in a LC doped with chiral periodic mesoporous organosilica (PMO). The data show that the main source of the signal emanates from outside the PMO, and not inside the PMO pores. An ECE also is reported for chiral LCs above their bulk chiral isotropic – nematic phase transition, and is observable in the paranematic layers induced by the planar-aligning substrates. Optical microscopy measurements were performed on smectic-A oily streaks doped with CB15. When chirally doped, the stripe orientation rotated by a temperature dependent angle: This angle increased with concentration, was largest just below the nematic – smectic-A transition, and stabilized to near zero within ~1oC below TNA. This is explained as a manifestation of a surface ECE. Finally, a novel structure in a hybrid aligned system was observed below the smectic-A – smectic-C transition. The structure appeared as periodic dark and light streaks running perpendicular to the oily streaks, and formed by extending discretely from one oily streak to the next, eliminating optical evidence of the oily streaks. At lower temperatures the streaks undulated in a 2D chiral pattern. A possible origin of these streaks is presented.

Committee: Charles Rosenblatt (Advisor); Emmanuelle Lacaze (Advisor); Petschek Rolfe (Committee Member); Peshek Timothy (Committee Member); Selinger Robin (Committee Member); Tin Padetha (Committee Member); DiLisi Gregory (Committee Member); Carles Pierre (Committee Member) Subjects: Condensed Matter Physics; Physics

Master of Science, University of Toledo, 2015, Chemistry

Negative thermal expansion (NTE) materials have attracted considerable research interest in recent decades. These unique materials shrink when heated, offering a potential means to control the overall thermal expansion of composites. Several families of materials display this behavior, the largest of which is the A2Mo3O12 family (also called the scandium tungstate family), in which A is a trivalent cation and M is molybdenum or tungsten. These materials show NTE in an orthorhombic structure, but many members transform to a monoclinic structure with positive expansion at low temperatures. Many properties of these materials are dependent on their elemental composition, especially the identity of the A3+ cation. This includes the magnitude of NTE, as well as the phase transition behavior as a function of temperature and pressure. It is also possible to create "mixed site" cation A2Mo3O12 materials, in which the A site is occupied by two different cations. These are described as AxA'2-xM3O12 materials, as the composition A:A' can vary. Creating these new compositions may result in different phase transition properties or the ability to tune the NTE properties of these materials. In this work, the focus was on synthesis and characterization of indium gallium molybdate (InxGa2-xM3O12). The non-hydrolytic sol-gel (NHSG) method was used to synthesize indium gallium molybdate while exploring a variety of reaction parameters. While the goal was to create stoichiometric, homogenous materials, it was found that this could not be accomplished using easily accessible parameters during NHSG reactions. However, it was discovered that certain conditions allowed unusually low temperature (230 °C) crystallization of these materials. Similar conditions were explored for single cation A2Mo3O12 materials, and it was determined that crystallization of indium molybdate, iron molybdate, and scandium molybdate was possible at temperatures of 230 or 300 °C. This extremely low temperature (open full item for complete abstract)

Committee: Cora Lind-Kovacs PhD (Committee Chair); Eric Findsen PhD (Committee Member); Jon Kirchhoff PhD (Committee Member) Subjects: Chemistry; Materials Science

Doctor of Philosophy, The Ohio State University, 2014, Physics

Ultracold atomic gas experiments have emerged as a new testing ground for finding elusive, exotic states of matter. One such state that has eluded detection is the Larkin-Ovchinnikov (LO) phase predicted to exist in a system with unequal populations of up and down fermions. This phase is characterized by periodic domain walls across which the order parameter changes sign and the excess polarization is localized Despite fifty years of theoretical and experimental work, there has so far been no unambiguous observation of an LO phase. In this thesis, we propose an experiment in which two fermion clouds, prepared with unequal population imbalances, are allowed to expand and interfere. We show that a pattern of staggered fringes in the interference is unequivocal evidence of LO physics. Finally, we study the superconductor-insulator quantum phase transition. Both superconductivity and localization stand on the shoulders of giants -- the BCS theory of superconductivity and the Anderson theory of localization. Yet, when their combined effects are considered, both paradigms break down, even for s-wave superconductors. In this work, we calculate the dynamical quantities that help guide present and future experiments. Specifically, we calculate the conductivity σ(ω) and the bosonic (pair) spectral function Ρ(ω) from quantum Monte Carlo simulations across clean and disorder-driven superconductor-insulator transitions (SIT). Using these quantities, we identify characteristic energy scales in both the superconducting and insulating phases that vanish at the transition due to enhanced quantum fluctuations, despite the persistence of a robust fermionic gap across the SIT. The clean and disordered transition are compared throughout, and we find that disorder leads to enhanced absorption in σ(ω) at low frequencies and a change in the universality class, although the underlying Τ=Ο quantum critical point remains in both transitions.

Committee: Nandini Trivedi (Advisor); Mohit Randeria (Committee Member); Eric Braaten (Committee Member); Thomas Lemberger (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2013, Physics

Ultracold atomic gases may be the ultimate quantum simulator. These isolated systems have the lowest temperatures in the observable universe, and their properties and interactions can be precisely and accurately tuned across a full spectrum of behaviors, from few-body physics to highly-correlated many-body effects. The ability to impose potentials on and tune interactions within ultracold gases to mimic complex systems mean they could become a theorist's playground. One of their great strengths, however, is also one of the largest obstacles to this dream: isolation. This thesis touches on both of these themes. First, methods to characterize phases and quantum critical points, and to construct finite temperature phase diagrams using experimentally accessible observables in the Bose Hubbard model are discussed. Then, the transition from a weakly to a strongly interacting Bose-Fermi mixture in the continuum is analyzed using zero temperature numerical techniques. Real materials can be emulated by ultracold atomic gases loaded into optical lattice potentials. We discuss the characteristics of a single boson species trapped in an optical lattice (described by the Bose Hubbard model) and the hallmarks of the quantum critical region that separates the superfluid and the Mott insulator ground states. We propose a method to map the quantum critical region using the single, experimentally accessible, local quantity R, the ratio of compressibility to local number fluctuations. The procedure to map a phase diagram with R is easily generalized to inhomogeneous systems and generic many-body Hamiltonians. We illustrate it here using quantum Monte Carlo simulations of the 2D Bose Hubbard model. Secondly, we investigate the transition from a degenerate Fermi gas weakly coupled to a Bose Einstein condensate to the strong coupling limit of composite boson-fermion molecules. We propose a variational wave function to investigate the ground state properties of such a Bose-Fermi mixture w (open full item for complete abstract)

Committee: Nandini Trivedi Ph.D. (Advisor); Tin-Lun Ho Ph.D. (Committee Member); Gregory Lafyatis Ph.D. (Committee Member); Richard Furnstahl Ph.D. (Committee Member) Subjects: Condensed Matter Physics

Doctor of Philosophy, The Ohio State University, 2008, Physics

Ecological systems such as forest and lakes can exhibit multiple stable states, abrupt transitions and self-organization as a control parameter is varied. Understanding the dynamics of these systems and devising easily quantifiable measures with predictive capabilities using the theoretical tools of stochastic dynamics and nonequilibrium statistical physics form the focus of this thesis.We study simple ecological models with no spatial degrees of freedom, that show a catastrophic transition as a control parameter is varied and propose a novel early warning signal that exploits two ubiquitous features of ecological systems: nonlinearity and large external fluctuations. It is shown that changes in asymmetry in the distribution of time series data, quantified by changing skewness, is an early warning signal of impending regime shifts. Using simple analytical calculations, model simulations that mimic field measurements and an analysis of real data from abrupt climate change in the Sahara, we study the feasibility of skewness calculations using data available from routine monitoring. We consider a spatially explicit model of collapse of vegetation in one and two spatial dimensions. An analytical calculation based on the mean-field approximation shows that spatial variance and spatial skewness (with an appropriate sign) increase as one approaches the threshold of vegetation collapse. Our numerical calculations show that an increasing spatial variance in conjunction with a reversal in the initial changing trend of spatial skewness is a superior indicator of an impending spatial ecological regime shift when spatially explicit data are available. These results are shown to hold under several different dispersal kernels such as Gaussian, fat tailed and Cauchy. Vegetation in semi-arid regions exhibits striking spatial patterns. Theoretical models often ignore the strong fluctuations in parameters such as those arising from seasonality. We present a fully seasonal rainfall mod (open full item for complete abstract)

Committee: C Jayaprakash PhD (Committee Chair); David Stroud PhD (Committee Member); Dick Furnstahl PhD (Committee Member); Michael Poirier PhD (Committee Member) Subjects: Ecology; Physics

Doctor of Philosophy (PhD), Ohio University, 2002, Physics (Arts and Sciences)

A pressure-induced polyamorphic phase transition was reported in some amorphous semiconductors about thirty years ago. Investigation of this transition is hindered by difficulties of probing thin films, and hence it is little explored and understood, especially on the basis of theory work. In this dissertation, we perform constant volume and pressure simulations to reveal pressure-induced phase transitions in certain amorphous semiconductors. We find that amorphous Si and Ge exhibit a first order amorphous to amorphous phase transition while amorphous GeSe² agrees with the available experiments. A crystalline grain in paracrystalline Si transforms from diamond phase to a high density amorphous phase with the application of pressure, which is consistent with the experimental investigation of porous silicon. Gibbs free energy calculations predict a pressure-induced amorphization of crystalline Si, in agreement with pressure-temperature phase diagram, and a pressure-induced high-density crystallization of amorphous and paracrystalline Si. We argue that the crystallization of these amorphous semiconductors is kinetically hindered at low-temperatures in the simulations. In this work, we give the detaliled analysis of the structural modification of these amorphous networks, and the pressure-induced changes in electronic and vibrational properties. We find several common features. The high density amorphous phase of amorphous Si, Ge, and paracrystalline Si resembles the metallic toplogy found in their liquid phase. The optical gap of amorphous Si, Ge, and paracrystalline silicon increases, reaches a maximum and then decreases smoothly. On the other hand, the gap of amorphous GeSe² closes gradually under pressure. The localized electronic and vibrational states become extended with increasing pressure. Softening of low-frequency modes is observed in amorphous Si and Ge but those of GeSe² shift to higher frequencies. Metallization is sharp in elemental amorphous semiconducto (open full item for complete abstract)

Committee: David Drabold (Advisor) Subjects: Physics, General

Master of Science, Miami University, 2010, Physics

Metal-Organic Frameworks (MOFs) are a new class of materials notable for a remarkable range of structural conformations, high porosity and acceptance of mobile charged guest species, or counter-ions, within their network of molecular channels. If the counter-ions were highly mobile, the metal-organic framework would represent a new class of superionic conductor. Presented in this study is an attempt to characterize the ionic properties of two particular metal-organic frameworks. The zero-frequency conductance of the samples is found to be minimal, suggesting that the materials studied are not ionic conductors. The ions in each of the MOFs studied exhibit multiple glass transitions. The particular frequency-temperature scaling associated glassy dynamics was observed in the dielectric response.

Committee: Jeffrey Clayhold PhD (Advisor); Michael Pechan PhD (Committee Member); Samir Bali PhD (Committee Member); Herbert Jaeger PhD (Committee Member) Subjects: Physics

Doctor of Philosophy, Case Western Reserve University, 2004, Systems and Control Engineering

Cruse's model of leg coordination (CCM) was derived to account for gaits and gait transitions in arthropods (analogous to, e.g. walktrotgallop in some quadrupeds). It has also been adapted to control locomotion in a series of hexapod robots. CCM is a systems-level, kinematic model that abstracts key physiological and dynamical properties in favor of tractability. A key feature is that gaits emerge from interaction among pairs of legs as effected by a set of coordination mechanisms acting at discrete points in time. We represent CCM networks as systems of coupled hybrid oscillators. Gaits are quantified by a temporal (discrete) phase vector. System trajectories are polyhedral, hence solvable over finite-time, but the presence of the switching automaton renders infinite horizon properties harder to analyze. Via numerical and symbolic simulations, we have mapped out the synchronization behavior of CCM networks of various topologies parametrically. We have developed a section-map analysis approach that exploits the polyhedral geometry of the hybrid state space. Our approach is constructive. Once switching boundaries are appropriately parameterized, we can extract periodic orbits, their domains of admissibility and stability, as well as expressions for the period of oscillation and relative phase of each cycle, parametrically. Applied to 2-oscillator networks, our approach yields excellent agreement with simulation results. A key emergent concept is that of a virtual periodic orbit (VPO). Distinguished from admissible periodic orbits, VPOs do not correspond to any in the underlying hybrid dynamics. However, when stable and close to being admissible, they are canonical precursors for a class of nonsmooth bifurcations and predictive of long transient behavior. Last, we take into consideration the possibility and difficulties of extending our approach to larger networks and to related oscillator-like hybrid dynamical systems with polyhedral trajectories.

Committee: Randall Beer (Advisor) Subjects: