Chemistry & Biochemistry Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2752

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    How Non-Hermitian Superfluids are Special? Theory and Experiments
    (2024) Tao, Junheng; Spielman, Ian Bairstow; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Ultracold atoms emerge as a promising advanced platform for researching the principles of quantum mechanics. Its development of scientific understanding and technology enriches the toolbox for quantum simulations and quantum computations. In this dissertation work, we describe the methods we applied to build our new high-resolution 87Rb Bose-Einstein condensate (BEC) machine integrated with versatile quantum control and measurement tools. Then we describe the applications of these tools to the research of novel superfluidity and non-Hermitian physics. Superfluids and normal fluids were often studied in the context of Landau’s two-fluid model, where the normal fluid stemmed from thermally excited atoms in a superfluid background. But can there be normal fluids in the ground state of a pure BEC, at near zero temperature? Our work addressed the understanding of this scenario, and then measured the anisotropic superfluid density in a density-modulated BEC, where the result matched the prediction of the Leggett formula proposed for supersolids. We further considered and measured this BEC in rotation and found a non-classical moment of inertia that sometimes turns negative. We distinguished the roles of superfluid and normal fluid flows, and linked some features to the dipolar and spin-orbit coupled supersolids. As a second direction, we describe our capability to create non-Hermiticity with Raman lasers, digital-micromirror device (DMD), and microwave, and present our work in engineering the real space non-Hermitian skin effect with a spin-orbit coupled BEC. By use of a spin-dependent dissipative channel, we realized an imaginary gauge potential which led to nonreciprocal transport in the flat box trap. We studied the system dynamics by quenching the dissipation, and further prepared stationary edge states. We link our discoveries to a non-Hermitian topological class characterized by a quantized winding number. Finally, we discuss the exciting promises of using these tools to study many-body physics open quantum systems.
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    Progress in Nitrogen Vacancy Nuclear Magnetic Resonance Detection
    (2023) Huckestein, Emma Kaye; Walsworth, Ronald L; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytic tool of use in the physics, chemistry, and biology disciplines, yet the resource costs to buy, maintain, and use the spectrometer limit the tool's accessibility and the limited sensitivity and spectral resolution limit its application space. In recent years, Nitrogen Vacancy (NV) centers have emerged as an alternative NMR sensor due to their atomic-scale resolution and minimal resource costs. However, NV-NMR similarly suffers from limited sensitivity and spectral resolution due to the technical challenges associated with increasing the applied magnetic field. In this work, the sensitivity of an existing NV-NMR setup is characterized to determine the experimental modifications necessary for measurements at higher magnetic fields (>0.5 T). As a consequence of this characterization, a coplanar waveguide integrated with a microfluidic channel is designed. Finally, metabolomics, particularly spheroids, are reviewed for a potential high-impact NV-NMR application given historically relevant sample concentration sensitivities.
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    EXPERIMENTAL MEASUREMENTS AND ANALYSIS OF RELATIVISTIC THOMSON SCATTERING PRODUCED BY INTENSE LASERS
    (2023) He, Calvin Z; Hill, Wendell T.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The viability of using relativistic Thomson scattering (RTS) as an intensity gauge of intense(I ≳ 2 × 1018 W/cm2) lasers is explored. Theory motivating measurement of the RTS second harmonic spectrum as well as angular distribution as possible diagnostics of the laser intensity are developed and presented. Experiments conducted at the Centro de Láseres Pulsados (CLPU) laser facility in Salamanca, Spain measuring the second harmonic spectrum radiated along E as well as the angular distribution along the E-k plane are described and their results summarized. We conclude that measurements of the second harmonic spectrum provide a means for determining the laser peak intensity via the corresponding Doppler shifted onset wavelength, provided that the intensity is in the range of ∼ 10^18 to ∼ 10^19 W/cm^2. Measurement of the angular distribution along the E-k plane, on the other hand, does not provide features that are useful for indicating laser intensity. Possible paths forward in the study of RTS are discussed.
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    Phase Transitions Affected by Molecular Interconversion
    (2023) Longo, Thomas; Anisimov, Mikhail; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Typically, pure substances may be found with only one gaseous or liquid state, while their solid state may exist in various polymorphic states. The existence of two distinct liquid forms in a single component substance is more unusual since liquids lack the long-range order common to crystals. Yet, the existence of multiple amorphous states in a single component substance, a phenomenon known as "liquid polyamorphism," has been observed or predicted in a wide variety of substances. In contrast to standard phase transitions, it has been suggested that polyamorphic liquid-liquid transitions are caused by the interconversion of molecular or supramolecular states. To investigate this phenomenon, a nonequilibrium thermodynamic model was developed to quantitatively describe the interplay between the dynamics of molecular interconversion and fluid-phase separation. The theory has been compared to a variety of interconverting systems, and has demonstrated a quantitative agreement with the results of Monte Carlo and Molecular Dynamics simulations. In this thesis, it is shown that there are two major effects of molecular interconversion on the thermodynamics and the kinetics of fluid-phase separation: if the system evolves to an equilibrium state, then the growth of one of the alternative phases may result in the destruction of phase coexistence - a phenomenon referred to as "phase amplification." It is demonstrated that depending on the experimental or simulation conditions, either phase separation or phase amplification would be observed. Previous studies of polyamorphic substances report conflicting observations of phase formation, which may be explained by the possibility of phase amplification occurring. Alternatively, if the system evolves to a nonequilibrium steady state, the phase domain growth could be restricted at a mesoscopic length scale. This phenomenon (referred to as "microphase separation") is one of the simplest examples of steady-state dissipative structures, and may be applicable to active matter systems, hydrodynamic instabilities, and bifurcations in chemical reactions, in which the nonequilibrium conditions could be imposed by an external flux of matter or energy.
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    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.
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    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.
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    Nuclear Structure Studies of ${^78,80}$Ge and Adjacent Nuclei
    (2018) Forney, Anne Marie; Walters, William B; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The main topic of this thesis concerns the unusual features of the nuclear structure of $^{78}$Ge. The discovery of a sequence of levels separated by one unit of angular momentum for which the expected crossover transitions carrying two units of angular momentum are not observed. This result stands in contrast to the neighboring even-even Ge and Se nuclei, as well as the results of most model calculations. The level structures of adjacent $^{82,80}$Ge are also studied to place the results for $^{78}$Ge in context. Likewise, shell-model calculations are performed as comparisons with the structures for all three nuclei. The data come from experiments at Argonne National Laboratory using the Gammasphere detector array. These experiments are important because of the broad interest in the structure of $^{76}$Ge and neighboring nuclei, owing to a world-wide effort in the search for neutrinoless double $\beta$ decay. Owing to the unusual features of this level sequence, it is labelled as a $\kappa$ band, taken from the Greek word καινουργιος, meaning new. The results pose a challenge to theorists to find ways to develop models that can fit both these features, as well as the other aspects of the structure of $^{78}$Ge. In addition, this study is important determining why no sequence like this has been found in any of the adjacent nuclei.
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    DYNAMICS OF LARGE SYSTEMS OF NONLINEARLY EVOLVING UNITS
    (2017) Lu, Zhixin; Ott, Edward; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The dynamics of large systems of many nonlinearly evolving units is a general research area that has great importance for many areas in science and technology, including biology, computation by artificial neural networks, statistical mechanics, flocking in animal groups, the dynamics of coupled neurons in the brain, and many others. While universal principles and techniques are largely lacking in this broad area of research, there is still one particular phenomenon that seems to be broadly applicable. In particular, this is the idea of emergence, by which is meant macroscopic behaviors that “emerge” from a large system of many “smaller or simpler entities such that ... large entities” [i.e., macroscopic behaviors] arise which “exhibit properties the smaller/simpler entities do not exhibit.” [1]. In this thesis we investigate mechanisms and manifestations of emergence in four dynamical systems consisting many nonlinearly evolving units. These four systems are as follows. (a) We first study the motion of a large ensemble of many noninteracting particles in a slowly changing Hamiltonian system that undergoes a separatrix crossing. In such systems, we find that separatrix-crossing induces a counterintuitive effect. Specifically, numerical simulation of two sets of densely sprinkled initial conditions on two energy curves appears to suggest that the two energy curves, one originally enclosing the other, seemingly interchange their positions. This, however, is topologically forbidden. We resolve this paradox by introducing a numerical simulation method we call “robust” and study its consequences. (b) We next study the collective dynamics of oscillatory pacemaker neurons in Suprachiasmatic Nucleus (SCN), which, through synchrony, govern the circadian rhythm of mammals. We start from a high-dimensional description of the many coupled oscillatory neuronal units within the SCN. This description is based on a forced Kuramoto model. We then reduce the system dimensionality by using the Ott Antonsen Ansatz and obtain a low-dimensional macroscopic description. Using this reduced macroscopic system, we explain the east-west asymmetry of jet-lag recovery and discus the consequences of our findings. (c) Thirdly, we study neuron firing in integrate-and-fire neural networks. We build a discrete-state/discrete-time model with both excitatory and inhibitory neurons and find a phase transition between avalanching dynamics and ceaseless firing dynamics. Power-law firing avalanche size/duration distributions are observed at critical parameter values. Furthermore, in this critical regime we find the same power law exponents as those observed from experiments and previous, more restricted, simulation studies. We also employ a mean-field method and show that inhibitory neurons in this system promote robustness of the criticality (i.e., an enhanced range of system parameter where power-law avalanche statistics applies). (d) Lastly, we study the dynamics of “reservoir computing networks” (RCN’s), which is a recurrent neural network (RNN) scheme for machine learning. The ad- vantage of RCN’s over traditional RNN’s is that the training is done only on the output layer, usually via a simple least-square method. We show that RCN’s are very effective for inferring unmeasured state variables of dynamical systems whose system state is only partially measured. Using the examples of the Lorenz system and the Rossler system we demonstrate the potential of an RCN to perform as an universal model-free “observer”.
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    Pair creation and pair annihilation in Bose-Einstein condensates
    (2017) Wang, Yi-Hsieh; Clark, Charles W.; Jacobson, Theodore; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This thesis covers three applications of Bose-Einstein condensates and related phenomena, in the theme of pair creation and pair annihilation. First, Bose-Einstein condensates (BEC) are viewed as one of the candidates to implement a sonic black hole. This can lead to the observation of analog Hawking radiation, resulting from a phonon pair creation at a black-hole horizon (BH). Such implementation has been achieved in a resent experiment by J. Steinhauer, in which a black-hole/white-hole pair has been produced. He also reported the observations of self-amplifying Hawking radiation, via a lasing mechanism operating between the black and white-hole horizons. Through our simulations, we find that the observations should be attributed not to the black hole laser effect, but rather to a growing zero-frequency bow wave, generated at the white-hole horizon. The relative motion of the two horizons produces a Doppler shift of the bow wave at the BH, where it stimulates a monochromatic Hawking radiation. We also find that shot-to-shot atom number variations and quantum fluctuations give density-density correlations consistent with those reported in the experiments. In particular, atom number variations can produce a spurious correlation signal. Secondly, a sonic black hole/white hole pair and phonon pair creation can also be realized using a ring-shaped condensate. Here we focus on the phonon spectroscopy of a ring condensate. We probe the phonon excitation spectrum by applying a harmonically driven barrier to a 23Na Bose-Einstein condensate in a ring-shaped trap. When excited resonantly, these wavepackets display a regular periodic structure. The resonant frequencies depend upon the particular configuration of the barrier, but are commensurate with the orbital frequency of a sound wave traveling around the ring. Energy transfer to the condensate over many cycles of the periodic wavepacket motion causes enhanced atom loss from the trap at resonant frequencies. Solutions of the Gross-Pitaevskii (GP) equation exhibit quantitative agreement with the experimental data. Thirdly, positronium (Ps) BECs have been of experimental and theoretical interest due to their potential application as the gain medium of a gamma-ray laser. Ps BECs are intrinsically spinor due to the presence of ortho- (o-Ps) and para-positronium (p-Ps), whose annihilation lifetimes differ by three orders of magnitude. We study the spinor dynamics and annihilation processes in the p-Ps/o-Ps system using both solutions of the GP equations and a rate-equation approach. For an initially unpolarized condensate, there is a threshold density at which spin mixing between o-Ps and p-Ps occurs. Beyond this threshold, there are unstable spatial modes accompanied by spin mixing. To ensure a high production yield above the critical density, a careful choice of external field must be made to avoid the spin mixing instability.
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    Topics in equilibrium and nonequilibrium thermodynamics: computing crystalline free energies and engineering Maxwell’s demon.
    (2015) Lu, Zhiyue; Jarzynski, Christopher; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This dissertation covers two separate topics in statistical physics. The first part of the dissertation focuses on computational methods of obtaining the free energies (or partition functions) of crystalline solids. We describe a method to compute the Helmholtz free energy of a crystalline solid by direct evaluation of the partition function. In the many-dimensional conformation space of all possible arrangements of N particles inside a periodic box, the energy landscape consists of localized islands corresponding to different solid phases. Calculating the partition function for a specific phase involves integrating over the corresponding island. Introducing a natural order parameter that quantifies the net displacement of particles from lattices sites, we write the partition function in terms of a one-dimensional integral along the order parameter, and evaluate this integral using umbrella sampling. We validate the method by computing free energies of both face-centered cubic (FCC) and hexagonal close-packed (HCP) hard sphere crystals with a precision of $10^{-5}k_BT$ per particle. In developing the numerical method, we find several scaling properties of crystalline solids in the thermodynamic limit. Using these scaling properties, we derive an explicit asymptotic formula for the free energy per particle in the thermodynamic limit. In addition, we describe several changes of coordinates that can be used to separate internal degrees of freedom from external, translational degrees of freedom. The second part of the dissertation focuses on engineering idealized physical devices that work as Maxwell's demon. We describe two autonomous mechanical devices that extract energy from a single heat bath and convert it into work, while writing information onto memory registers. Additionally, both devices can operate as Landauer's eraser, namely they can erase information from a memory register, while energy is dissipated into the heat bath. The phase diagrams and the efficiencies of the two models are solved and analyzed. These two models provide concrete physical illustrations of the thermodynamic consequences of information processing.