Graduate student Debadarshini Mishra, Department of Physics, University of Connecticut
Photo-Induced Ultrafast Dynamics in Molecules
Imaging electronic and molecular dynamics at ultrafast timescales is crucial for understanding the mechanisms of chemical reactions, which are of fundamental importance in fields ranging from materials science to biochemistry. Furthermore, gaining insights into these processes at the atomic and molecular levels can enable precise control over reaction dynamics, leading to significant technological advancements through the development of efficient catalysts, innovative materials, and targeted drugs. In this dissertation talk, I will present my work on imaging time-resolved dynamics in molecular systems, using various light sources and ultrafast spectroscopy techniques. First, I will discuss a method for the direct visualization of neutral fragments in roaming reactions, which involve an unconventional dissociation process, using coincident Coulomb explosion imaging. Next, I will explore ultrafast electron diffraction as a different yet complementary imaging technique to identify the competing non-radiative relaxation pathways for a UV-excited molecule. Finally, I will briefly discuss our recent work on relaxation and fragmentation dynamics in large molecules, particularly C60, and isomerization and excited-state dynamics in small molecules.
Graduate student Mitchell Bredice, Department of Physics, University of Connecticut
Kinetics, Nucleation, and Relaxation Dynamics of Ion-Seeded Nanoparticles
The recent interest in studying the adsorption and emission spectra of the hazy atmospheres of exoplanets stimulates the interest in clusters, small aggregates of atoms or molecules. The nucleation and dynamics of nanoparticles in the Earth’s atmosphere and their impact on the global climate and environment is another important area of research stimulating investigations of nucleation processes. However, how these small aggregates form is not wholly understood. Traditionally, nucleation of clusters or other phases is described through Classical Nucleation Theory. Although this theory has many discrepancies in describing the nucleation of submicron particles. In this work, we have performed molecular dynamics simulations of the nucleation of ion-seeded nanoparticles, specifically ArnH+ clusters, to investigate the microscopic mechanisms of nucleation from a gas or liquid phase. From these simulations, we have studied the stages of the nonequilibrium and equilibrium growth of ArnH+ clusters and analyzed the size distribution and internal energy relaxation of nascent clusters during different stages of their growth. The fundamental impact of the internal energy relaxation on the nonequilibrium nucleation of small ArnH+ clusters has been demonstrated. This analysis has generally been avoided in previous investigations due to assumptions of the equilibrium nature of the nucleation process. The results of our simulations showed that nanoparticles are formed in highly excited states, thus the cluster growth and relaxation are concurrent processes, and that relaxation of the cluster internal energy can delay cluster growth processes. To further investigate the internal energy relaxation, an ensemble of molecular dynamics simulations was performed for the detailed analysis of the average time evolution of kinetic, potential, and total energies of small ArnH+ clusters, and their kinetic energy relaxation. The results of the performed simulations have been explained through the use of a collisional Boltzmann equation describing the energy relaxation processes. Lastly, the general relationship between nonequilibrium growth and internal energy relaxation is discussed.
Graduate student Dharma Basaula, Department of Physics, University of Connecticut
This dissertation is focused on formulating, testing and validating a finite element method based computational framework for the evaluation and prediction of thermoelectric properties and performance of polycrystalline nanostructured materials and composites at mesoscale. The developed framework includes capabilities for building geometrical models of complex interfacial structures and, with the availability of appropriate input parameters, can be used predictively, providing new avenues for improvement of operational efficiency of nanoengineered thermo- electric materials and composites. The following benchmark problems were investigated on the first stage of this project, progressing from simple to more advanced cases: (a) effective Seebeck effect in a thermocouple; (b) Peltier heating and cooling at a single interface between two materials with different Seebeck coefficients; (c) coupled heat and electrical current transport through an anisotropic polycrystalline material. Excellent agreement with prior experimental or computational results was observed for the cases where such information was available. On the second stage, ‘digital twins’ for the experimental measurements of thermal and electrical conductivities, and Seebeck coefficient in a material sample were developed within the same computational framework, allowing one to evaluate its thermoelectric figure of merit ZT(T). This approach was tested on three popular nanocrystalline thermoelectric systems: n-type Si, n-type Si0.80Ge0.20, and p-type BiSbTe, providing excellent agreement with previously measured values of ZT(T) and highlighting the importance of interfacial properties for making accurate predictions of the material thermoelectric performance and efficiency. Finally, on the third stage, the sensitivity of sample thermoelectric properties and the resulting ZT(T) to variations in the system microstructure, morphology and input material parameters was elucidated.
Graduate student Bochao Xu, DEpartment of Physics, University of Connecticut
Scanning SQUID Investigation of Time-reversal Symmetry Breaking in Exotic Quantum Materials
Spontaneous breaking of time-reversal symmetry in condensed matter systems arises from correlated electronic arrangements leading to various quantum phenomena, such as ferromagnetism, unconventional superconductivity, and topological states of matter. However, these underlying electronic orders are often difficult to detect experimentally if the magnetism associated with the time reversal symmetry breaking is weak. In such cases, the subtle magnetization and complex domain structure call for investigation by experimental techniques with both high magnetic sensitivity and high spatial resolution. In this dissertation talk, I will discuss my exploration of time-reversal symmetry breaking in two systems: a magnetic Weyl semimetal and a Kagome material detected using scanning superconducting quantum interference device (SQUID) microscopy. Both materials exhibit intriguing magnetic structures which were not detectable by the bulk measurements. We show that the Weyl semimetal hosts a tunable heterogeneous domain structure that is likely linked to its unconventional electronic properties. Additionally, our local probe reveals a ferromagnetic-like state in the Kagome material system, contributing evidence to the highly controversial problem within the community regarding the existence of time-reversal symmetry breaking and its underlying mechanism in this material. These results highlight the significance of quantum sensing in advancing the frontier of new correlated materials, and showcase these materials as an ideal playground for studying the magnetism-electrons interplay.