Session F62: Undergrad Research VI

Bispectral Analysis of Local Atomic Environments

In this talk, we explore the use of bispectra as a tool to analyze and cluster local atomic environments, enabling the identification of geometric trends across different material classes. The local environments in a material play a significant role in determining its overall properties. Understanding the complex interplay between local environment geometries and material properties is crucial to designing next-generation materials with desired properties. Bispectra are well-suited to characterizing local environments due to being a rotationally invariant, smooth, invertible (modulo a global rotation), and fixed-length descriptor. We define the bispectrum in this context as the scalars and pseudoscalars resulting from the triple tensor product of a local environment's spherical harmonic expansion with itself.

We apply the bispectra to compare the local environments of a new class of hybrid inorganic-organic materials called MOChAs with those found in other transition-metal and chalcogen-containing materials from the Materials Project and Cambridge Structural Database. MOChAs, or Metal-Organic Chalcogenides Assemblies, are self-assembled hybrid crystals in which low-dimensional transition metal chalcogenide structures are scaffolded by organic ligands. The structural diversity of organic ligands leads to a wide range of possible local environments in MOChAs, which are effectively captured by bispectra. These bispectra can be visualized and clustered, ultimately guiding the design and discovery of new MOChAs.   

Clusters of Quantum Particles due to Strong Interactions: Spectrum and Dynamics

We investigate the static and dynamical properties of one-dimensional quantum models in the presence of weak and strong interparticle interactions. We observe that when the interaction is strong, particles bind together and propagate as a single slow cluster. This behavior is not observed for weak interactions. Our numerical analysis is done for spin-1/2 models with nearest neighbor couplings and for Bose-Hubbard models with onsite interaction. The evaluation of static properties is based on eigenvalues and the structure of the eigenstates, while the analysis of dynamics focuses on the evolution of magnetization, participation ratio, and the probability for a cluster to persist. This study has applications in quantum computation and it can be experimentally implemented with ultracold atoms.   

Failure to Break Clusters of Quantum Particles using Defects: Spectrum and Dynamics

We consider one-dimensional quantum models with strong interparticle interactions, which cause particles to bind together and behave as a single slow cluster. To attempt separating the particles from the clusters, we introduce defects (Zeeman splittings), but despite energy being conserved, they remain unbroken. We also investigate two-dimensional models, which provide more possibilities for spreading of particles, but find that the clusters persist. Static properties are studied via eigenvalues and eigenstates, while dynamical properties are investigated through the evolution of magnetization, participation ratio, and the probability for the clusters to survive. This study has applications in quantum computation and it can be experimentally implemented with ultracold atoms.   

Reexamining the Classical Stern-Gerlach Effect with Simulations

Predictions of the Stern-Gerlach effect with classical magnetic moments have been commonly in error due to overlooking the constraints Maxwell's equations on a non-uniform magnetic field [1]. In addition, the role of classical spin precession in such an experiment has been debated [2,3]. We perform simulations of the Stern-Gerlach effect for ensembles of randomly oriented classical magnetic moments passing through a region of inhomogeneous magnetic field. We also explore the Stern-Gerlach deflections for polarized classical spins. Our results provide clarity to the dialogue in Refs [2,3]. A classical simulation of the Stern-Gerlach effect produced a “donut” shape after the particles passed through the non-uniform magnetic field. It results in a hole in the middle of the particle distribution, with the surrounding annulus growing in density as we move towards the outer edge of the shape.  

 

[1] R. P. Feynman, R. B. Leighton, and M. Sands, The Feynman Lectures on Physics (Addison-Wesley, New York, 1964), Vol II, p. 35-3.  

[2] P. Alstrom, P. Hjorth, and R. Mattuck, Am. J. Phys. 50, 697 (1982)  

[3] S. Singh and N. K. Sharma, Am. J. Phys. 52, 274 (1984)     

Exploring Exotic Microphases in Simulated Spin-Orbit Coupled Boson Systems

We assess the robustness of a novel quantum microemulsion phase in Rashba spin-orbit coupled (SOC) Bose-Einstein condensates using finite-temperature, field-theoretic simulations. Our goal is to motivate experimental progress in realizing Rashba SOC in ultracold gases—an engineered 2D isotropic interplay between spin and momentum that has yet to be successfully realized experimentally—by clarifying the microemulsion's broad viability. Our group's previous research suggests that the microemulsion phase only exists under immiscible conditions, which may limit realistic experiments. Contrary to our expectations, we find that similar to the immiscible stripe superfluid phase, the miscible plane-wave phase undergoes a Kosterlitz Thouless-like transition into the microemulsion, losing superfluidity at elevated temperature. We conclude with a phase diagram that demonstrates the existence of the microemulsion, regardless of miscibility.   

Molecular Dynamics Simulation of Photovoltaic Exciton Dissociation

The dynamics of exciton particles in photovoltaic systems plays a key role in determining the electrical efficiency of such systems. Excitons consist of electron-hole pairs that generate electric currents through dissociation, and a quantitative understanding of exciton dynamics is relevant to the design of efficient photovoltaics. This rate of dissociation is known to depend on the degree of orbital energy disorder prevailing over the molecular lattice of the photovoltaic, and this dependence has previously been investigated using kinetic monte carlo techniques that employ pre-assigned lattice site transition rate constants and static lattice sites.

In this study, we simulate this dissociation process using accelerated molecular dynamics (MD), which accommodates lattice vibrations and allows lattice site transitions based on particle dynamics. Treating the electron and hole as separate particles, we investigate the time-averaged separation between these particles over the course of multiple trial simulations and calculate their fractional dissociation yield. The energetic disorder of the lattice is varied and the corresponding fractional dissociation yield of the exciton is determined and compared with experimental results.   

Quadrupolar-dipolar excitonic transition in a tunnel-coupled van der Waals heterotrilayer

Strongly bound excitons determine light-matter interactions in van der Waals (vdW) heterostructures of two-dimensional semiconductors. Unlike fundamental particles, quasiparticles in condensed matter, such as excitons, can be tailored to alter their interactions and realize emergent quantum phases. Here, using a WS2/WSe2/WS2 heterotrilayer, we create a quantum superposition of oppositely oriented dipolar excitons – a quadrupolar exciton – wherein an electron is layer-hybridized in WS2 layers while the hole localizes in WSe2. In contrast to dipolar excitons, symmetric quadrupolar excitons only redshift in an out-of-plane electric field. At higher densities and finite electric field, the nonlinear Stark shift of quadrupolar excitons becomes linear, signalling a transition to dipolar excitons resulting from exciton-exciton interactions, while at vanishing electric field, reduced exchange interaction suggests antiferroelectric correlations between dipolar excitons. Our results present vdW heterotrilayers as a field-tunable platform to engineer light-matter interactions and explore quantum phase transitions between spontaneously ordered many-exciton phases.   

Spin Relaxation in Strained Graphene-TMD Heterostructures using Ab Initio Methods

While graphene is a compelling candidate for spintronics due to weak spin-orbit coupling (SOC), adding strain and transition metal dichalcogenide (TMD) (MoSe₂, WS₂) monolayers allows for preferentially modulated SOC in graphene-based spintronics devices. Previous work includes ab initio studies of SOC in twisted graphene/TMD layers along with studies of spin relaxation in intrinsic, strained graphene structures. However, systematic studies of SOC in strained graphene-TMD nanostructures are currently lacking. Additionally, SOC combined with momentum relaxation models is crucial to quantifying spin relaxation times in these nanostructures. In this work, we use ab initio calculations to obtain spin relaxation times in strained graphene-TMD nanoribbons and elucidate the role of strain/TMD-induced band structure features and scattering mechanisms on spin lifetimes. Density Functional Theory (DFT) calculations were performed with strained graphene/TMD heterostructure supercells. SOC constants were then extracted by fitting DFT band structure calculations to a strained Hamiltonian model with orbital and spin-orbital components. DFT band structures were also used to determine momentum relaxation times based on phonon, coulombic impurity, and defect scattering mechanisms. We then present spin relaxation times computed via Eliot-Yafet and D’yakonov Perel spin relaxation mechanisms in strained, graphene-TMD nanoribbons.    

Photo-induced Vortex States in the Transition Metal Dichalcogenides

The response of the low-energy quasiparticles in transition metal dichalcogenides (TMDs), such as MoS₂, to circularly polarized light leads to valley-dependent optical phenomena such as valley-selective dichroism [1]. In addition to the spin angular momentum resulting from light polarization, optical vortex beams carry orbital angular momentum resulting from the space modulation of their wavefronts. Our work explores the effects of the interaction between low-energy electrons in TMDs and light vortex beams. Using the Floquet formalism, we determine the light polarizations that conserve the total angular momentum and derive the orbital, valley, spin, and pseudo-spin dependence of electron-photon states. Within the one-photon approximation, we show that the reduced Floquet Hamiltonian is equivalent to a model for an s-wave superconductor with multiple vortices. The mapping allows us to determine the spectra, number, and valley dependence of the low-energy vortex states in the irradiated system. We also numerically diagonalize the total Floquet Hamiltonian using Bessel decomposition methods that reveal the emerging photon-dressed electronic vortex states. This approach fully characterizes each state's valley-spin dependence and its real-space distribution.

[1] Di Xiao, et al., PRL 108, 196802 (2012)   

Combining Light Scattering and Small Angle X-ray Scattering for Particle Characterization

Aqueous solutions of polystyrene microspheres (PS) of varying diameter (25nm – 490nm) were used as a test system of known size distribution to master a combined approach to particle characterization using Static Light Scattering (SLS) and Small Angle X-ray Scattering (SAXS), before applying the combined SLS/SAXS approach to studying polymeric microgels of similar sizes. SAXS is a scattering technique that can be used to determine the average size, shape, and internal structure of nanoparticles in solution. SLS can also be used to determine the size and shape distribution of particles, but SAXS probes systems over a wider range of scattering vectors (q). The higher q-range allows the examination of smaller particles, as well as portions of large particles, and elements of internal structure. Also, unlike light scattering, SAXS yields information on the internal structure of particles as X-rays penetrate many non-transparent materials and have enough resolution to probe small internal structural elements. By utilizing both methods we can learn more about the system of interest such as polymeric microgels and develop a more complete understanding of its internal structure. This presentation will highlight our successful use of combined SLS and SAXS (done with Xenocs Xeuss 3.0) in studying the test system of polystyrene microspheres. The preliminary SAXS results on polysaccharide microgels will also be presented together with an attempt to reconcile these SAXS results with the earlier multiangle SLS measurements on the same system.   

FPGA based laser feedback interferometry using heterodyne and phase shifting measurements of large and nanometer displacements in the optical path.

Using a broadband electro-optic phase modulator with a custom designed laser feedback interferometer enables the implementation of heterodyning or phase-shifting approaches for phase measurements. An FPGA based lock-in amplifier (Moku, Liquid Instruments) with a combined linear feedback controller can be used for rapid measurement and control of the optical path length (OPL). For large changes in the OPL, the interferometric response (i.e., the intensity) sweeps through a series of fringes and it is necessary to use phase unwrapping techniques. As the OPL varies, however, the rate of change of the sensitivity of the interferometer also varies. Provided the interferometer is primarily used to measure nanometer changes, the OPL can be maintained about a position of highest sensitivity using feedback control. By contrast, when the interferometer is used to measure nanometer variations in the OPL that are superimposed upon large changes in the OPL, it is critical to account for the rate of change of sensitivity. This effect is readily illustrated by measuring a series of discrete, equally sized nm steps of a piezoelectric device that is mounted on translator which introduces large changes in the OPL. Analysis of these combined changes requires methods that compensate for the variation in sensitivity.   

Scanning Interferometer Aimed at Characterizing Laser Coherence Lengths

Interference fringes generated by superposed beams of light offer one of the most striking and useful applications of wave mechanics in modern society. Utility is limited, however, by the fact that fringes, as generated by overlapping a given laser beam upon itself, can only be observed on length scales shorter than the laser beam’s coherence length. Unfortunately, the coherence lengths of many commonly available light sources, like inexpensive diode lasers and laser pointers, are often not reported. Here we summarize progress on an experiment aimed at characterizing these properties systematically. Results are expected to be relevant to the construction of low-cost interferometric devices [1] that may in turn be of use in pedagogical contexts and industry.

[1] T. M. Melody, et al., Am. J. Phys. 91, 132-141 (2023)   

Investigation of Pulse Jitter in a Periodic Laser Pulse Train

Pulse jitter, a consequence of quantum and other systemic noise, affects the periodicity of pulses. To view this, periodic pulse trains are generated using an optically pumped Nd:YAG laser incorporating an intracavity saturable absorber as a passive Q-switch. After this was built, the characterization of this laser was performed, deepening our understanding of this system. Stable periodic pulse train generation was achieved by varying the pump power, cavity length, and absorber location inside the cavity. After this, the dependence of the pulse jitter and rise time on pump power was investigated.   

Design and testing of photon-based hardware random number generator.

Hardware random number generators (HRNG) are widely used in the computer world for the security purposes as well as in the science world as a source of the high quality randomness for the models and simulations. Currently existing HRNG are either costly or very slow and of questionable quality. This work proposes a simple design of the HRNG based on the low-number photon absorption by a detector (a photo-multiplyer tube of a silicon-based one) that can provide a large volume of high quality random numbers. The different options of processing and the testing of quality of the generator output are presented.