Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Ultracold Gases in a Two-Frequency Breathing Lattice(2024) Dewan, Aftaab; Rolston, Steven L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Driven systems have been of particular interest in the field of ultracold atomic gases. Theprecise control and relative purity allows for construction of many novel Hamiltonians. One such system is the ‘breathing’ lattice, where both the frequency and amplitude is modulated in time, much like an accordion. We present the results of a phenomenological investigation of a proposed experiment, one where we apply a two-frequency breathing lattice to an atomic system. The results are surprising, as they indicate the possibility of a phase-dependent transition between nearest-neighbour and beyond nearest-neighbour interactions.Item Topology from Quantum Dynamics of Ultracold Atoms(2023) Reid, Graham Hair; Rolston, Steven L; Spielman, Ian B; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ultracold atoms are a versatile platform for studying quantum physics in the lab. Usingcarefully chosen external fields, these systems can be engineered to obey a wide range of effective Hamiltonians, making them an ideal system for quantum simulation experiments studying exotic forms of matter. In this work, we describe experiments using 87Rb Bose–Einstein condensates (BECs) to study exotic topological matter based on out-of-equilibrium effects. The topological states are prepared through the quantum dynamics of the ultracold atom system subjected to a highly tunable lattice potential described by the bipartite Rice–Mele (RM) model, created by combining dressing from a radiofrequency (RF) magnetic field and laser fields driving Raman transitions. We describe a form of crystal momentum-resolved quantum state tomography, which functions by diabatically changing the lattice parameters, used to reconstruct the full pseudospin quantum state. This allows us to calculate topological invariants characterizing the system. We apply these techniques to study out-of-equilibrium states of our lattice system, described by various combinations of sublattice, time-reversal and particle-hole symmetry. Afterquenching between lattice configurations, we observe the resulting time-evolution and follow the Zak phase and winding number. Depending on the symmetry configuration, the Zak phase may evolve continuously. In contrast, the winding number may jump between integer values when sublattice symmetry is transiently present in the time-evolving state. We observe a scenario where the winding number changes by ±2, yielding values that are not present in the native RM Hamiltonian. Finally, we describe a modulation protocol in which the configuration of the bipartite latticeis periodically switched, resulting in the Floquet eigenstates of the system having pseudospin-momentum locked linear dispersion, analogous to massless particles described by the Dirac equation. We modulate our lattice configuration to experimentally realize the Floquet system and quantify the drift velocity associated with the bands at zero crystal momentum. The linear dispersion of Floquet bands derives from nontrivial topology defined over the micromotion of the system, which we measure using our pseudospin quantum state tomography, in very good agreement with theory.Item Floquet Heating and Relaxation of Interacting Bose Einstein Condensates(2022) Maslek, James; Porto, James V; Rolston, Steve; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Floquet’s theorem says that any unitary, periodically driven system can be described by an effective time-independent Hamiltonian, where the effective Hamiltonian can have completely different properties than the static, undriven system. Floquet engineering makes use of this idea to simulate new Hamiltonians that would otherwise not be possible in the undriven case. For interacting systems, this approach can be used to realize interesting correlated many-body states, but drive-induced heating must be understood and mitigated. Cold atoms in optical lattices provide a controllable, well-isolated system in which these ideas can and have been realized. I describe research into two areas of Floquet engineering for interacting Bose-Einstein condensates in periodically driven optical lattices. The first half of this thesis focuses on the study of heating mechanisms for condensates in periodically driven lattices. In the weakly interacting limit, one might expect that heating could be described with a Fermi Golden Rule approach. Parametric driving of fluctuations in the condensate, however, can lead to runaway heating that cannot be described perturbatively. We experimentally study heating in shaken 2D square lattices and demonstrate heating consistent with the theoretical predictions of parametric instabilities. The second half of this thesis describes experiments that realize Floquet-induced effective staggered magnetic fields, and the relaxation dynamics of interacting particles subject to these fields. Interestingly, we observe pre-thermal relaxation dynamics, where an initially heated cloud suddenly subject to the effective Hamiltonian condenses into a state governed by the drive-induced effective Hamilton on a timescale faster than heating.Item Energy absorption and diffusion in chaotic systems under rapid periodic driving(2022) Hodson, Wade Daniel; Jarzynski, Christopher; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we study energy absorption in classical chaotic, ergodic systems subject to rapid periodic driving, and in related systems. Under a rapid periodic drive, we find that the energy evolution of chaotic systems appears as a random walk in energy space, which can be described as a process of energy diffusion. We characterize this process, and show that it generally predicts three stages of energy evolution: Initial relaxation to a prethermal state, followed by slow evolution of the system’s energy probability distribution in accordance with a Fokker-Planck equation, followed by either unbounded energy absorption or relaxation to an infinite temperature state. We then study the energy diffusion model in detail in driven billiard systems specifically; in particular, we obtain numerical results which corroborate the energy diffusion description for a specific choice of billiard. This is followed by an analysis of energy diffusion in one-dimensional oscillator systems subject to weak, correlated noise. Finally, we begin to investigate energy absorption in periodically driven quantum chaotic systems, i.e., quantum systems with a classical chaotic analogue. We invoke tools from Floquet theory and random matrix theory to investigate whether the classical energy diffusion framework can be applied to quantum systems, and under what conditions. We conclude with a discussion of potential models for energy absorption in quantum chaotic systems, and with an overview of open questions and directions for future work.