Physics
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Item LOCAL MOLECULAR FIELD THEORY FOR NON-EQUILIBRIUM SYSTEMS(2019) Baker III, Edward Bigelow; Weeks, John D; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Local Molecular Field (LMF) theory is a framework for modeling the long range forces of a statistical system using a mimic system with a modified Hamiltonian that includes a self consistent molecular potential. This theory was formulated in the equilibrium context, being an extension of the Weeks Chandler Andersen (WCA) theory to inhomogeneous systems. This thesis extends the framework further into the nonequilibrium regime. It is first shown that the equilibrium derivation can be generalized readily by using a nonequilibrium ensemble average and its relevant equations of motion. Specifically, the equations of interest are fluid dynamics equations which can be generated as moments of the BBGKY hierarchy. Although this approach works well, for the application to simulations it is desirable to approximate the LMF potential dynamically during a single simulation, instead of a nonequilibrium ensemble. This goal was pursued with a variety of techniques, the most promising of which is a nonequilibrium force balance approach to dynamically approximate the relevant ensemble averages. This method views a quantity such as the particle density as a field, and uses the statistical equations of motion to propagate the field, with the forces in the equations computed from simulation. These results should help LMF theory become more useful in practice, in addition to furthering the theoretical understanding of near equilibrium molecular fluids.Item Topological dispersion relations in spin-orbit coupled Bose gases(2019) Valdes Curiel, Ana; Spielman, Ian; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum degenerate gases have proven to be an ideal platform for the simulation of complex quantum systems. Due to their high level of control it is possible to readily design and implement systems with effective Hamiltonians in the laboratory. This thesis presents new tools for the characterization and control of engineered quantum systems and describes their application in the realization of a topological system with Rashba-type spin-orbit coupling. The underlying properties of these engineered systems depend on their underlying energies. I describe a Fourier transform spectroscopy technique for characterizing the single particle spectrum of a quantum system. We tested Fourier spectroscopy by measuring the dispersion relation of a spin-1 spin-orbit coupled Bose-Einstein condensate (BEC) and found good agreement with our predictions. Decoherence due to uncontrolled fluctuations of the environment presents fundamental obstacles in quantum science. I describe an implementation of continuous dynamical decoupling (CDD) in a spin-1 BEC. We applied a strong radio-frequency (RF) magnetic field to the ground state hyperfine manifold of Rubidium-87 atoms, generating a dynamically protected dressed system that was first-order insensitive to changes in magnetic field. The CDD states constitute effective clock states and we observed a reduction in sensitivity to magnetic field of up to four orders of magnitude. We additionally show that the CDD states can be coupled in a fully connected geometry and thus enable the implementation of new models not possible using the bare atomic states. Finally, I describe a new realization of Rashba-type SOC using Raman coupled CDD states. Our system had non-trivial topology but no underlying crystalline structure that yields integer valued Chern numbers in conventional materials. We validated our procedure using Fourier transform spectroscopy to measure the full dispersion relation containing only a single Dirac point. We measured the quantum geometry underlying the dispersion relation and obtained the topological index using matter-wave interferometry. In contrast to crystalline materials, where topological indices take on integer values, our continuum system reveals an unconventional half-integer Chern number.Item Machine Learning Approaches for Data-Driven Analysis and Forecasting of High-Dimensional Chaotic Dynamical Systems(2019) Pathak, Jaideep; Ott, Edward; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We consider problems in the forecasting of large, complex, spatiotemporal chaotic systems and the possibility that machine learning might be a useful tool for significant improvement of such forecasts. Focusing on weather forecasting as perhaps the most important example of such systems, we note that physics-based weather models have substantial error due to various factors including imperfect modeling of subgrid-scale dynamics and incomplete knowledge of physical processes. In this thesis, we ask if machine learning can potentially correct for such knowledge deficits. First, we demonstrate the effectiveness of using machine learning for model- free prediction of spatiotemporally chaotic systems of arbitrarily large spatial extent and attractor dimension purely from observations of the system’s past evolution. We present a parallel scheme with an example implementation based on the reservoir computing paradigm and demonstrate the scalability of our scheme using the Kuramoto-Sivashinsky equation as an example of a spatiotemporally chaotic system. We then demonstrate the use of machine learning for inferring fundamental properties of dynamical systems, namely the Lyapunov exponents, purely from observed data. We obtain results of unprecedented fidelity with our novel technique, making it possible to find the Lyapunov exponents of large systems where previously known techniques have failed. Next, we propose a general method that combines a physics-informed knowledge-based model and a machine learning technique to build a hybrid forecasting scheme. We further extend our hybrid forecasting approach to the difficult case where only partial measurements of the state of the dynamical system are available. For this purpose, we propose a novel technique that combines machine learning with a data assimilation method called an Ensemble Transform Kalman Filter (ETKF).Item Many-Body Dephasing in a Cryogenic Trapped Ion Quantum Simulator(2019) Kaplan, Harvey B.; Monroe, Christopher R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)While realizing a fully functional quantum computer presents a long term technical goal, in the present, there are small to mid-sized quantum simulators (up to $\sim 100$ qubits), that are capable of approaching specialized problems. The quantum simulator discussed here uses trapped ions to act as qubits and is housed in a cryogenically cooled vacuum chamber in order to reduce the background pressure, thereby increasing ion chain length and life-time. The details of performance and characterization of this cryogenic apparatus are discussed, and this system is used to study many-body dephasing in a finite-sized quantum spin system. How a closed quantum many-body system relaxes and dephases as a function of time is important to understand when dealing with many-body spin systems. In this work, the first experimental observation of persistent temporal fluctuations after a quantum quench is presented with a tunable long-range interacting transverse-field Ising Hamiltonian. The fluctuations in the average magnetization of a finite-size system of spin-$1/2$ particles are measured presenting a direct measurement of relaxation dynamics in a non-integrable system. This experiment is in the regime where the properties of the system are closely related to the integrable Hamiltonian with global coupling. The system size is varied in order to investigate the dependence on finite-size scaling, and the system size scaling exponent extracted from the measured fluctuations is consistent with theoretical prediction.Item BOSE EINSTEIN CONDENSATES IN DYNAMICALLY CONTROLLED OPTICAL LATTICES(2019) Smith, Zachary Schutz; Rolston, Steven L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ultracold atomic gases are often used as quantum simulators, where the ability to precisely control and modulate the potential landscape allows many model Hamiltonians to be realized. Recently, dynamic control over these potentials has been leveraged to extend the kinds of systems that can be explored. Our lab has constructed an optical lattice generator capable of dynamically altering the spacing, phase, and amplitude of an optical lattice at RF timescales. The rapid timescales allows the construction of a time-averaged disordered potential from individual flashes of optical lattice, producing layered system exhibiting a Griffiths phase. A future experiment where the optical lattice is periodically expanded and contracted is proposed, and a numerical treatment is presented suggesting interesting structure in the tunneling dynamics.Item STABILITY AND SCALING OF NEURONAL AVALANCHES AND THEIR RELATIONSHIP TO NEURONAL OSCILLATIONS(2019) Miller, Stephanie Regina; Roy, Rajarshi; Biophysics (BIPH); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The generation of cortical dynamics in awake mammals is not yet fully understood. However, it is known that neurons leverage distinct organizational schemes to achieve behavior and cognitive function, and that this precise spatiotemporal organization may go awry in illness. In 2003, a form of scale-free synchrony termed “neuronal avalanches” was first observed by Beggs & Plenz in cultured cortical tissue and later confirmed in rodents, nonhuman primates, and humans. In this dissertation, we draw from monkey and rodent studies to demonstrate that neuronal avalanches capture key features of neural population activity and constitute a robust and stable (e.g. self-organized) indicator of balanced excitation and inhibition in cortical networks. We also show for the first time that neuronal avalanches and oscillations co-exist in frontal cortex of nonhuman primates and identify the avalanche temporal shape as a biomarker predicated upon critical systems theory. Finally, we present progress towards characterizing altered avalanche dynamics in a developmental mouse model for schizophrenia using 2-photon calcium imaging in awake animals.Item PARTICLE HEATING AND ENERGY PARTITION IN RECONNECTION WITH A GUIDE FIELD(2019) Zhang, Qile; Drake, James F; Swisdak, Michael M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Kinetic Riemann simulations have been completed to explore particle heating during reconnection with a guide field in the low-beta environment of the inner heliosphere and the solar corona. The reconnection exhaust is bounded by two rotational discontinuities (RDs) with two slow shocks (SSs) that form within the exhaust as in magnetohydrodynamic (MHD) models. At the RDs, ions are accelerated by the magnetic field tension to drive the reconnection outflow as well as flows in the out-of-plane direction. The out-of-plane flows stream toward the midplane and meet to drive the SSs. The turbulence at the SSs is weak so the shocks are laminar and produce little dissipation, which differs greatly from the MHD treatment. Downstream of the SSs the counter-streaming ion beams lead to higher density and therefore to a positive potential between the SSs that confines the downstream electrons to maintain charge neutrality. The potential accelerates electrons from upstream of the SSs to downstream and traps a small fraction but only produces modest electron heating. In the low-beta limit the released magnetic energy is split between bulk flow and ion heating with little energy going to electrons. To firmly establish the laminar nature of reconnection exhausts, we explore the role of instabilities and turbulence in the dynamics. Two-dimensional reconnection and Riemann simulations reveal that the exhaust develops large-amplitude striations resulting from electron-beam-driven ion cyclotron waves. The electron beams driving the instability are injected into the exhaust from one of the RDs. However, in 3D Riemann simulations, the additional dimension results in a strong Buneman instability at the RD, which suppresses electron beam formation. The 3D simulation does reveal a weak ion-ion streaming instability within the exhaust. All these instabilities become weaker with higher ion-to-electron mass ratio due to higher electron thermal speed. We also use a kinetic dispersion relation solver to show that the ion-ion instability will become stable in conditions expected under lower upstream beta. The results suggest that in realistic reconnection exhausts, which have three dimensions and real mass ratio, the kinetic-scale turbulence that develops will be too weak to play a significant role in energy conversion.Item HIGH REPETITION RATE LASER-DRIVEN ELECTRON ACCELERATION TO MEGA-ELECTRON-VOLT ENERGIES(2019) Salehi, Fatholah; Milchberg, Howard M; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Laser-driven particle accelerators have the potential to be a compact and cost-effective replacement for conventional accelerators. Despite the significant achievements of laser wakefield acceleration in the last two decades, more work is required to improve the beam parameters such as the energy spread, emittance, repetition rate, and maximum achievable energy delivered by these advanced accelerators to values comparable to what conventional accelerators offer for various applications. The goal of this dissertation is to lower the threshold of the laser pulse energy required for driving a wakefield and in turn enable higher repetition rate particle acceleration with current laser technology. In the first set of electron acceleration experiments presented in this dissertation, we show that the use of a thin gas jet target with near critical plasma density lowers the critical power for relativistic self-focusing and leads to electron acceleration to the ~MeV range with ~1pC charge per shot, using only ~1mJ energy drive laser pulses delivered by a 1kHz repetition rate laser system. These electron beams accelerated in the self-modulated laser wakefield acceleration (SM-LWFA) regime have a thermal energy distribution and a rather large divergence angle (>150mrad). In order to improve the energy spread and the divergence, in the second set of the experiments, we employ a few-cycle laser pulse with a ~7fs duration and ~2.5mJ energy as the driver, to perform wakefield acceleration in the bubble regime using near critical plasma density targets. The results show a significant improvement in the energy spread and divergence of the beam. The electron bunches from this experiment have a quasi-monoenergetic peak at ~5MeV with an energy spread of ΔE/E≃0.4 and divergence angle of ~15mrad. These results bring us closer to the use of tabletop advanced accelerators for various scientific, medical, and industrial applications.Item TAMING THE SIGN PROBLEM IN LATTICE FIELD THEORY WITH DEFORMED PATH INTEGRAL CONTOURS(2019) Warrington, Neill C; Bedaque, Paulo; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis a generic method for taming the sign problem is developed. The sign problem is the name given to the difficult task of numerically integrating a highly oscillatory integral, and the sign problem inhibits our ability to understand the properties of a wide range of systems of interest in theoretical physics. Particularly notably for nuclear physics, the sign problem prevents the calculation of the properties of QCD at finite baryon density, thereby precluding an under- standing of the dense nuclear matter found in the center of a neutron star. The central idea developed in this thesis is to use the multidimensional generalization of Cauchy’s Integral Theorem to deform the Feynman Path Integral of lattice fields theories into complexified field space to manifolds upon which the phase oscillations which cause the sign problem are gentle. Doing so allows calculations of theories with sign problems. Two practical manifold deformation methods, the holomorphic gradient flow and the sign-optimized manifold method, are developed. The holomorphic gradient flow, a generalization of the Lefschetz thimble method, continuously deforms the original path integration domain to a complex manifold via an evolution dictated by a complex first order differential equation. The sign-optimized manifold method is a way to generate a manifold with gentle phase oscillations by minimizing the sign problem in a parameterized family of manifolds through stochastic gradient ascent. With an eye toward QCD at finite density, the Cauchy’s Theorem approach is applied to relativistic quantum field theories of fermions at finite density throughout this thesis. Finally, these methods are general and can be applied to both bosonic and fermionic theories, as well as Minkowski path integrals describing real-time dynamics.Item PANDAX-II DARK MATTER DETECTOR AND ITS FIRST RESULTS(2019) TAN, ANDI; Ji, Xiangdong; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The particle physics nature of dark matter (DM) is one of the most fundamental scientific questions nowadays. The leading candidates, weakly interacting massive particles (WIMPs), can be directly detected by looking for WIMP-nucleus scattering events in deep underground laboratories. Since the 1980s, physicists have improved the sensitivity of direct DM detection by about seven orders of magnitude. In the last decades, dual-phase xenon detectors exhibit their advantages in background rejection and scalability and lead the sensitivity in high mass WIMP direct searches. Experiments in XENON and LUX projects have been continuously pushing the exclusion limits of the elastic WIMP-nucleon scattering cross section into the parameter space predicted by various theoretical models. The Particle and astrophysical Xenon (PandaX) project is a series of xenon- based ultra-low background experiments in the China Jinping Underground Laboratory (CJPL) targeting the unknown physics of DM. The first stage of the project, the PandaX-I experiment, with a 120 kg sensitive liquid xenon (LXe) target, performed the WIMP search in 2014 with a 54×80.1 kg-day exposure. PandaX-I reported a strong limit on the WIMP-nucleon cross section for a WIMP mass of less than 10 GeV/c2, strongly disfavoring all positive claims from other experiments. The construction and installation of the second stage, PandaX-II experiment, with a half-ton scale LXe target, commenced after PandaX-I. In 2015, PandaX- II reported a WIMP search result with a 306×19.1 kg-day exposure from a short physics commissioning run with a notable 85Kr background. With 580 kg LXe in the sensitive region, PandaX-II was the largest running dual-phase xenon detector before the XENON1T detector in 2017. PandaX-II reported the most stringent limit on the WIMP-nucleon scattering cross section at 2.5×10−46 cm2 for the WIMP mass 40 GeV/c2 with a total exposure of 33 ton-day in 2016 and updated the limit to 8.6×10−47 in 2017 with a total exposure of 54 ton-day. In this dissertation, I will focus on the PandaX-II experiment, data analysis and its constraints on theoretical models. After a distillation campaign for krypton removal in 2017, the PandaX-II experiment achieved a background level of 0.8×10−3 event/kg/day/keV which was the lowest among similar detectors at the time. Compared to other dual-phase xenon detectors, we drift electrons by applying bias voltages on the electrodes which producing a stronger uniform electric field at a strength about 400 V/cm. After running for more than three years, more than 97% of 110 3′′ photomultiplier tubes (PMTs) perform stably. The analysis processes are continuously improved in various run periods in PandaX-II. The recorded waveforms were processed using a custom-developed software through several steps including hit finding and calculation, signal clustering, and so on to the final pairing analysis of scintillation and ionization signals. In this dissertation, I shall cover some topics in these steps as following. The amplification factors (gains) of the PMTs are calibrated using Light Emitting Diode (LED) light periodically for transforming the recorded waveform to the number of photon electrons (PE) and energy. An inefficiency raised from zero length encoding (ZLE), a data suppression firmware of the data acquisition system, is investigated in run periods with relatively low PMT gain. A data-driven algorithm is developed for the X-Y position reconstruction using the hit pattern of the proportional scintillation on the top PMT array. The mono-energetic events from xenon isotopes are studied to correct the non-uniformity of the detector response, and key parameters are extracted to reconstruct the deposited energy of events. The background analysis is critical for rare-event search experiments. I will present the study on the intrinsic electron recoil backgrounds from krypton, radon and xenon isotopes. The constraints of PandaX-II data on various theoretical models are investigated.