Physics Colloquium - Spring 2019

Speaker: Katherine Davis, Princeton University

Abstract: Metalloenzymes are metal-containing proteins that catalyze a broad range of biological transformations important for life. A detailed understanding of the mechanisms underlying their reactivity requires a confluence of methods to elucidate both local electronic changes to the metal center as well as global structural motions. Although x-ray spectroscopic methods are incredibly powerful, they are nonetheless limited in scope. X-ray crystallography, in contrast, can provide key insights into global structure. Here, I will explore the application of these x-ray techniques to diverse enzymatic systems. For example, to investigate the mechanism by which Nature splits water during photosynthesis, we performed time-resolved laser pump, x-ray probe experiments to characterize sub-millisecond reactive intermediates of photosystem II, the metalloenzyme complex responsible for the oxidation of water in plants and cyanobacteria. This approach probed the catalytic Mn4CaO5 cluster and yielded the first ever x-ray emission data on the water splitting step. Intriguingly, no additional oxidation past the S3-intermediate state was observed, and evidence of significant Mn reduction within 50 μs of the final laser flash led to the proposal of a new mechanistic model of natural water oxidation.

Speaker: L. Thompson, University of Louisville, Department of Chemistry

Abstract: Computational simulation is vital to understand and corroborate mechanistic and spectroscopic experiments of excited states processes. However, methodologies for simulating excited states are limited, either due to inability to describe important effects away from the vertical excitation region (e.g. response based approaches), or due to poor computational scaling (e.g. configuration-interation based approaches). In this talk, I will discuss our developments towards a hierarchy of efficient computational approaches that can simulate excited state reaction paths without limitation as to the nature of electronic states involved. I will show our first steps in constructing models that behave variationally with regard to any electronic state and yield orthogonal solutions consisting of nonlinearly optimized basis solutions which form the set of first-order approximations to different electronic states. In addition, I discuss our approaches for rapidly obtaining sets of nonlinearly optimized solutions that approximate different electronic states for use as initial basis states in the models. Applications that demonstrate the utility of the method to the study of photocatalysis will be discussed.

Speaker: Zbigniew Was

Abstract: For many years the best methods for phenomenology was to design one dimensional distributions of the best sensitivity to particular physics quantity, such as in case of tau leptons, LHC measurements can be sin² theta_W or Higgs boson parity. Some aspects of such studies will be presented. At present, techniques based on Machine Learning reach popularity, but the previously developed approach of optimal variables remain important as essential result benchmark.

Speaker: Jeff Hey, RDI Technologies

Abstract: This talk will focus on how basic astronomy research at the University of Louisville’s Physics & Astronomy Department spawned a high growth startup, RDI Technologies, that now employs 20+ people, including 3 Ph.D. physicists. RDI’s products are currently being used in many Fortune 100 companies including Google and NASA. The talk will include a discussion on the research that occurred at UofL, as well as the current technology, applications and ongoing research at RDI Technologies. Similarities between physics research at the University and that within a company, especially a startup, will be covered, as well as how the research experience laid the foundation for entrepreneurship.

Speaker: C. Bolech, University of Cincinnati Department of Physics

Abstract: Cold atomic gases have emerged as very versatile laboratory realizations for strongly correlated quantum systems in highly tunable setups. Computationally, it is desirable to study the systems directly in the continuum, as their natural description calls for. This is to avoid artifacts coming from lattice discretization, which affect the results of otherwise reliable methods like the density-matrix renormalization group (DMRG). In the last few years, work to reformulate these methods directly for low-dimensional quantum field theories (QFTs) using a continuum version of the quantum-information concept of matrix-product states (MPS) has been producing very encouraging results. I will provide a progress report of these efforts as seen from the fermionic front line. Interacting imbalanced Fermi gases constitute a nice playground for theories and experiments alike. Coincidentally, their physics is particularly rich in one dimension, where in the attractive case they can display algebraic superfluid order with an unusual pair-density wave character that is not found in higher dimensions. This is the one-dimensional analog of the elusive Fulde-Ferrell Larkin-Ovchinnikov (FFLO) pairing mechanism. Computational approaches are extremely valuable to study these systems beyond what rigorous analytic approaches can offer and thus make better contact with the experimental scenarios. In turn, this kind of theory-experiment dialogue is ideally suited to inform and validate the development of continuum matrix-product states (cMPS) as a specially tailored numerical framework for the optimally compressed description of low-dimensional QFTs.

Speaker: Benjamin Frandsen, BYU Physics and Astronomy

Abstract: The origin and implications of nematicity in iron-based superconductors remain among the most pressing questions surrounding these fascinating materials. Recent efforts to address this topic have focused not only on the nematic phase itself, but also on the nematic fluctuations that exist outside the region of static nematicity. Pair distribution function (PDF) analysis of x-ray and neutron total scattering data is a proven method of studying local, short-range structural correlations that deviate from the average structure, such as the orthorhombic distortions associated with nematic fluctuations in the high-temperature tetragonal phase of iron-based superconductors. Focusing primarily on the representative hole-doped system (Sr,Na)Fe₂As₂, I will present PDF analysis that reveals a remarkably large region of nanometer-scale local orthorhombic distortions in temperature-composition space, reaching up to approximately 500 K for the parent compound and extending to high doping levels near optimal superconductivity. These results offer a rich and detailed view of nematic fluctuations in iron-based superconductors, helping guide future experimental and theoretical work.