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UMSL Bulletin 2023-2024
The 2023-2024 Bulletin and Course Catalog for the University of Missouri St. Louis.https://irl.umsl.edu/bulletin/1088/thumbnail.jp
MXene Materials for CO(2) Capture and Transformation
[eng] MXene materials, the low-dimensional counterpart of transition metal carbides and nitrides present suitable properties as catalysts in heterogeneous catalysis given the similar properties with transition metal carbides (TMC) but with the advantage of presenting high surface areas. Thus, MXene easily activate the CO2 molecule, being an emerging alternative as a catalyst to the processes of transformation of CO2 into chemical products of added value. In this Doctoral Thesis, three main topics are addressed, related to this. First, the abatement of CO2 by nitride MXenes, using kinetic phase diagrams to identify the conditions of pressure and temperature whether or not the CO2 is captured by the MXene. Second, the distinction between the different atomic stacking of the MXene, using vibrational modes of the CO2 molecule and finally, the Reverse Water Gas Shift reaction on the Mo2C MXene as catalyst, performing the analysis of the thermodynamic, kinetic and microkinetic aspects.[cat] Els materials coneguts com MXene són la contrapart de baixa dimensionalitat dels carburs i nitrurs de metalls de transició. Aquests materials presenten propietats adequades per a catàlisi heterogènia donades les propietats similars a les dels carburs de metalls de transició (TMC), però amb l'avantatge de presentar àrees superficials més grans. D’aquesta manera, els MXene activen fàcilment la molècula de CO2, sent així una alternativa emergent com a catalitzador pels processos de transformació de CO2 en productes químics de valor afegit. En aquesta tesi doctoral, s'aborden tres temes principals relacionats amb això. Primer, la reducció de CO2 per part de MXenes de nitrogen, utilitzant diagrames de fase cinètica per identificar les condicions de pressió i temperatura on el CO2 és capturat o no pel MXene. En segon lloc, la distinció entre els diferents apilaments atòmics del MXene, utilitzant modes vibracionals de la molècula de CO2 i, finalment, la reacció inversa de desplaçament del gas d'aigua al MXene Mo2C com a catalitzador, realitzant l'anàlisi dels aspectes termodinàmics, cinètics i microcinètics
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Fourth Order Dispersion in Nonlinear Media
In recent years, there has been an explosion of interest in media bearing quarticdispersion. After the experimental realization of so-called pure-quartic solitons, asignificant number of studies followed both for bright and for dark solitonic struc-tures exploring the properties of not only quartic, but also setic, octic, decic etc.dispersion, but also examining the competition between, e.g., quadratic and quarticdispersion among others.In the first chapter of this Thesis, we consider the interaction of solitary waves ina model involving the well-known φ4 Klein-Gordon theory, bearing both Laplacian and biharmonic terms with different prefactors. As a result of the competition ofthe respective linear operators, we obtain three distinct cases as we vary the modelparameters. In the first the biharmonic effect dominates, yielding an oscillatoryinter-wave interaction; in the third the harmonic effect prevails yielding exponen-tial interactions, while we find an intriguing linearly modulated exponential effectin the critical second case, separating the above two regimes. For each case, wecalculate the force between the kink and antikink when initially separated with suf-ficient distance. Being able to write the acceleration as a function of the separationdistance, and its corresponding ordinary differential equation, we test the corre-sponding predictions, finding very good agreement, where appropriate, with thecorresponding partial differential equation results. Where the two findings differ,we explain the source of disparities. Finally, we offer a first glimpse of the interplayof harmonic and biharmonic effects on the results of kink-antikink collisions andthe corresponding single- and multi-bounce windows.In the next two Chapters, we explore the competition of quadratic and quar-tic dispersion in producing kink-like solitary waves in a model of the nonlinearSchroedinger type bearing cubic nonlinearity. We present 6 families of multikink so-lutions and explore their bifurcations as a prototypical parameter is varied, namelythe strength of the quadratic dispersion. We reveal a rich bifurcation structure forthe system, connecting two-kink states with ones involving 4-, as well as 6-kinks.The stability of all of these states is explored. For each family, we discuss a “lowerbranch” adhering to the energy landscape of the 2-kink states (also discussed inthe previous Chapter). We also, however, study in detail the “upper branches”bearing higher numbers of kinks. In addition to computing the stationary statesand analyzing their stability at the PDE level, we develop an effective particle the-ory that is shown to be surprisingly efficient in capturing the kink equilibria and normal (as well as unstable) modes. Finally, the results of the bifurcation analysisare corroborated with direct numerical simulations involving the excitation of thestates in a targeted way in order to explore their instability-induced dynamics.While the previous two studies were focused on the one-dimensional problem,in the fourth and final chapter, we explore a two-dimensional realm. More specif-ically, we provide a characterization of the ground states of a higher-dimensionalquadratic-quartic model of the nonlinear Schr ̈odinger class with a combination of afocusing biharmonic operator with either an isotropic or an anisotropic defocusingLaplacian operator (at the linear level) and power-law nonlinearity. Examiningprincipally the prototypical example of dimension d = 2, we find that instabilityarises beyond a certain threshold coefficient of the Laplacian between the cubic andquintic cases, while all solutions are stable for powers below the cubic. Above thequintic, and up to a critical nonlinearity exponent p, there exists a progressivelynarrowing range of stable frequencies. Finally, above the critical p all solutionsare unstable. The picture is rather similar in the anisotropic case, with the dif-ference that even before the cubic case, the numerical computations suggest aninterval of unstable frequencies. Our analysis generalizes the relevant observationsfor arbitrary combinations of Laplacian prefactor b and nonlinearity power p.We conclude the thesis with a summary of its main findings, as well as with anoutlook towards interesting future problem
GNN-Assisted Phase Space Integration with Application to Atomistics
Overcoming the time scale limitations of atomistics can be achieved by
switching from the state-space representation of Molecular Dynamics (MD) to a
statistical-mechanics-based representation in phase space, where approximations
such as maximum-entropy or Gaussian phase packets (GPP) evolve the atomistic
ensemble in a time-coarsened fashion. In practice, this requires the
computation of expensive high-dimensional integrals over all of phase space of
an atomistic ensemble. This, in turn, is commonly accomplished efficiently by
low-order numerical quadrature. We show that numerical quadrature in this
context, unfortunately, comes with a set of inherent problems, which corrupt
the accuracy of simulations -- especially when dealing with crystal lattices
with imperfections. As a remedy, we demonstrate that Graph Neural Networks,
trained on Monte-Carlo data, can serve as a replacement for commonly used
numerical quadrature rules, overcoming their deficiencies and significantly
improving the accuracy. This is showcased by three benchmarks: the thermal
expansion of copper, the martensitic phase transition of iron, and the energy
of grain boundaries. We illustrate the benefits of the proposed technique over
classically used third- and fifth-order Gaussian quadrature, we highlight the
impact on time-coarsened atomistic predictions, and we discuss the
computational efficiency. The latter is of general importance when performing
frequent evaluation of phase space or other high-dimensional integrals, which
is why the proposed framework promises applications beyond the scope of
atomistics
Benchmarking highly entangled states on a 60-atom analog quantum simulator
Quantum systems have entered a competitive regime where classical computers
must make approximations to represent highly entangled quantum states. However,
in this beyond-classically-exact regime, fidelity comparisons between quantum
and classical systems have so far been limited to digital quantum devices, and
it remains unsolved how to estimate the actual entanglement content of
experiments. Here we perform fidelity benchmarking and mixed-state entanglement
estimation with a 60-atom analog Rydberg quantum simulator, reaching a high
entanglement entropy regime where exact classical simulation becomes
impractical. Our benchmarking protocol involves extrapolation from comparisons
against many approximate classical algorithms with varying entanglement limits.
We then develop and demonstrate an estimator of the experimental mixed-state
entanglement, finding our experiment is competitive with state-of-the-art
digital quantum devices performing random circuit evolution. Finally, we
compare the experimental fidelity against that achieved by various approximate
classical algorithms, and find that only one, which we introduce here, is able
to keep pace with the experiment on the classical hardware we employ. Our
results enable a new paradigm for evaluating the performance of both analog and
digital quantum devices in the beyond-classically-exact regime, and highlight
the evolving divide between quantum and classical systems.Comment: ALS, ZC, and JC contributed equall
Clockwork Cosmology
The higher order generalisation of the clockwork mechanism to gravitational
interactions provides a means to generate an exponentially suppressed coupling
to matter from a fundamental theory of multiple interacting gravitons, without
introducing large hierarchies in the underlying potential and without the need
for a dilaton, suggesting a possible application to the hierarchy problem. We
work in the framework of ghost free multi-gravity with "nearest-neighbour"
interactions, and present a formalism by which one is able to construct
potentials such that the theory will always exhibit this clockwork effect. We
also consider cosmological solutions to the general theory, where all metrics
are of FRW form, with site-dependent scale factors/lapses. We demonstrate the
existence of multiple deSitter vacua where all metrics share the same Hubble
parameter, and we solve the modified Einstein equations numerically for an
example clockwork model constructed using our formalism, finding that the
evolution of the metric that matter couples to is essentially equivalent to
that of general relativity at the modified Planck scale. It is important to
stress that while we focus on the application to clockwork theories, our work
is entirely general and facilitates finding cosmological solutions to any ghost
free multi-gravity theory with "nearest-neighbour" interactions. Moreover, we
clarify previous work on the continuum limit of the theory, which is
generically a scalar-tensor braneworld, using the Randall-Sundrum model as a
special case and showing how the discrete-clockwork cosmological results map to
the continuum results in the appropriate limit.Comment: 48 pages, 4 figure
Analog Photonics Computing for Information Processing, Inference and Optimisation
This review presents an overview of the current state-of-the-art in photonics
computing, which leverages photons, photons coupled with matter, and
optics-related technologies for effective and efficient computational purposes.
It covers the history and development of photonics computing and modern
analogue computing platforms and architectures, focusing on optimization tasks
and neural network implementations. The authors examine special-purpose
optimizers, mathematical descriptions of photonics optimizers, and their
various interconnections. Disparate applications are discussed, including
direct encoding, logistics, finance, phase retrieval, machine learning, neural
networks, probabilistic graphical models, and image processing, among many
others. The main directions of technological advancement and associated
challenges in photonics computing are explored, along with an assessment of its
efficiency. Finally, the paper discusses prospects and the field of optical
quantum computing, providing insights into the potential applications of this
technology.Comment: Invited submission by Journal of Advanced Quantum Technologies;
accepted version 5/06/202
Gradient flows of interacting Laguerre cells as discrete porous media flows
We study a class of discrete models in which a collection of particles
evolves in time following the gradient flow of an energy depending on the cell
areas of an associated Laguerre (i.e. a weighted Voronoi) tessellation. We
consider the high number of cell limit of such systems and, using a modulated
energy argument, we prove convergence towards smooth solutions of nonlinear
diffusion PDEs of porous medium type
Precision Studies of QCD in the Low Energy Domain of the EIC
The manuscript focuses on the high impact science of the EIC with objective
to identify a portion of the science program for QCD precision studies that
requires or greatly benefits from high luminosity and low center-of-mass
energies. The science topics include (1) Generalized Parton Distributions, 3D
imagining and mechanical properties of the nucleon (2) mass and spin of the
nucleon (3) Momentum dependence of the nucleon in semi-inclusive deep inelastic
scattering (4) Exotic meson spectroscopy (5) Science highlights of nuclei (6)
Precision studies of Lattice QCD in the EIC era (7) Science of far-forward
particle detection (8) Radiative effects and corrections (9) Artificial
Intelligence (10) EIC interaction regions for high impact science program with
discovery potential. This paper documents the scientific basis for supporting
such a program and helps to define the path toward the realization of the
second EIC interaction region.Comment: 103 pages,47 figure
Application of multi-scale computational techniques to complex materials systems
The applications of computational materials science are ever-increasing, connecting fields far beyond traditional subfields in materials science. This dissertation demonstrates the broad scope of multi-scale computational techniques by investigating multiple unrelated complex material systems, namely scandate thermionic cathodes and the metallic foam component of micrometeoroid and orbital debris (MMOD) shielding. Sc-containing scandate cathodes have been widely reported to exhibit superior properties compared to previous thermionic cathodes; however, knowledge of their precise operating mechanism remains elusive. Here, quantum mechanical calculations were utilized to map the phase space of stable, highly-faceted and chemically-complex W nanoparticles, accounting for both finite temperature and chemical environment. The precise processing conditions required to form the characteristic W nanoparticle observed experimentally were then distilled. Metallic foams, a central component of MMOD shielding, also represent a highly-complex materials system, albeit at a far higher length scale than W nanoparticles. The non-periodic, randomly-oriented constituent ligaments of metallic foams and similar materials create a significant variability in properties that is generally difficult to model. Rather than homogenizing the material such that its unique characteristic structural features are neglected, here, a stochastic modeling approach is applied that integrates complex geometric structure and utilizes continuum calculations to predict the resulting probabilistic distributions of elastic properties. Though different in many aspects, scandate cathodes and metallic foams are united by complexity that is impractical, even dangerous, to ignore and well-suited to exploration with multi-scale computational methods
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