PHD, Kent State University, 2019, College of Arts and Sciences / Department of Physics

The study of quantum phase transitions (QPT) is a promising route to comprehend the origin of unconventional properties in strongly correlated electron systems. Recent theories predict a new quantum critical point (QCP) in disordered itinerant system with exotic properties such as observable quantum Griffiths phase (QGP). The binary alloy Ni-V presents such QGP and looks like the best example to study a ferromagnetic QPT with controlled disorder. In this work, we investigate further Ni-V with advanced local methods using neutron scattering and μSR techniques. Initial doubts on sample quality and sample-dependent impact on different magnetic phases and disordered scenarios can be resolved. A structural characterization indicates that the investigated Ni-V samples show a high-quality chemical structure with expected random atomic distribution. We provide direct evidence of the "disorder" with μSR methods. We clarify essential details of the Ni-V phase diagram such as the nature and the boundaries of ferromagnetic and cluster glass phases. These new data reinforce the location of the QCP and the limits of the QGP in Ni-V. The main results are consistent with theories predicting an infinite randomness quantum critical point associated with a QGP.

Committee: Almut Schroeder PhD (Committee Chair); Carmen C. Almasan PhD (Committee Member); Maxim Dzero PhD (Committee Member); Songping Huang PhD (Committee Member); Michael Tubergen PhD (Committee Member) Subjects: Condensed Matter Physics; Experiments; Physics

PHD, Kent State University, 2016, College of Arts and Sciences / Department of Physics

We performed magnetoresistivity measurements on samples of Ce1-xYbxCoIn5 over a range of doping, temperature, field, and pressure. We were able to reveal information on the quantum critical nature of the system and on the relationship between the quantum critical fluctuations and the Kondo coherence of the system. At x = 0.07, the system is extremely close to a zero-field quantum critical point. Pressure is applied here to investigate the effect that it has on the physics of the material and to gain understanding as to the origin of its properties. It is revealed that the quantum fluctuations are related to the heavy quasiparticles and are weakened with pressure, although the quantum critical field is pressure-independent. A mysterious T1/2 term in the low-temperature resistivity is shown to be due to scattering processes involving light electrons from a smaller Fermi surface. Further doping shows that the quantum critical point is suppressed to zero field at x = 0.09. Measurements at x = 0.16 show no evidence of quantum critical behavior. An in-depth analysis of the magnetoresistivity reveals two main components - one due to a coherence-related order formed within the system and one due to the formation of Kondo singlets behaving independently of each other at higher fields. Through these components, we are able to determine the Kondo temperature TK as a function of doping and pressure. Our measurements also indicate that TK and Tcoh are independent of each other.

Committee: Carmen Almasan (Advisor); Almut Schroeder (Committee Member); James Gleeson (Committee Member); Robin Selinger (Committee Member) Subjects: Condensed Matter Physics; Physics

PhD, University of Cincinnati, 2009, Arts and Sciences : Physics

This work presents extensions of the numerical methods for strongly correlated electron systems. The first part of the thesis discusses extensions and applications of the quantum cluster theories to the systems of classical spins. It is shown that such extensions can provide faster convergence through better estimation of the effects of fluctuations, yet they can also possess shortcomings which limit their application in the studies of the phase transitions. The second part of the thesis is dedicated to the numerical studies of the Hubbard model. Present Quantum Monte Carlo methods are reviewed and relationships among them are elucidated. The final part of the thesis contains the application of the developed numerical methods to investigate the phase diagram of the two-dimensional Hubbard model, especially the evidence of the Quantum Critical Point (QCP) at a finite doping. High accuracy results for thermodynamic quantities are presented in support of the existence of the QCP at a finite doping in two-dimensional Hubbard model. The relation of the QCP to the charge fluctuations is revealed and a mechanism that relates QCP to incipient phase separation is proposed.

Committee: Michael Ma PhD (Committee Chair); Leigh Smith PhD (Committee Member); L.C.R. Wijewardhana PhD (Committee Member); Mark Jarrell PhD (Committee Member) Subjects: Condensation

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

The physical properties of the heavy fermion compound CeNi2Ge2, have been investigated in this dissertation. Although heavy fermion systems are strongly correlated electron systems with hybridization of conduction and f-electrons, they usually still follow a Landau Fermi liquid description with high renormalized fermion masses. Modifying the electron correlations by chemical substitution or pressure, heavy fermions can be driven through a magnetic quantum critical point changing, e.g., from a magnetic ordered into a nonmagnetic ground state. Close to such a quantum critical point, Fermi liquid description breaks down and signs of “non-Fermi liquid” behavior appear. Signs for non-Fermi liquid properties are increased low energy excitations resulting in power laws when approaching lower temperatures as observed in the specific heat coefficient which is otherwise temperature independent in Fermi liquid, and in electric resistivity that displays exponents less than two. CeNi2Ge2 is one of the few heavy fermion compounds that shows these signs of non-Fermi liquid behavior without external pressure or chemical substitution. It has been suggested that non-Fermi liquid behavior is caused by quantum critical magnetic fluctuations close to an antiferromagnetic quantum critical point. In order to classify the nature of this non-Fermi liquid behavior, neutron scattering experiments were performed to find and characterize the relevant critical magnetic fluctuations. Several single crystals grown in collaboration with Leiden University and polycrystals grown in our lab have been studied. Beside the neutron scattering measurements performed on all available samples, we also carried out low temperature characterization measurements on part of these samples in dilution refrigerator and SQUID magnetometer. Electrical resistivity measurements have confirmed results reported by other researchers. Signs of non-Fermi liquid behavior below T=10K appear with the temperature dependenc (open full item for complete abstract)

Committee: Almut Schroeder PhD (Committee Chair); Brett Ellman PhD (Committee Member); Edgar Kooijman PhD (Committee Member); Khandker Quader PhD (Committee Member); Mietek Jaroniec PhD (Committee Member) Subjects: Physics