Electron-phonon interactions (EPIs) are ubiquitous in condensed matter and materials physics. For example EPIs play a central role in the electrical resistivity of metals, the carrier mobility of semiconductors, the pairing mechanism of conventional superconductors, and the optical properties of indirect-gap semiconductors and insulators. More fundamentally, the EPI is the simplest realization of the interaction between fermion and boson fields, arguably one of the pillars of many-particle physics and quantum electrodynamics. The EPI has been studied for almost a century, however only during the last two decades predictive, non-empirical calculations have become possible. In this talk I will outline the theoretical and computational framework underlying modern electron-phonon calculations from first principles, and illustrate recent progress in this area by discussing representative work from our group. In particular I will touch upon our recent investigations of polarons in the angle-resolved photoelectron spectra of transition metal oxides [1,2], the superconducting pairing mechanism in transition metal dichalcogenides , and nonadiabatic Kohn anomalies in the inelastic X-ray scattering spectra of doped semiconductors . I will conclude by discussing opportunities for future work, and the key challenges for advancing theoretical and computational research on electron-phonon physics .
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This event, organized by UT's SIAM chapter, aims to introduce graduate students and senior undergraduates to research outside an academic setting. The focus will be on broad research themes that are relevant to the industry and national labs. It should be noted that while there will be some technical content, this will not be the main thrust of the talk.
Dr. Hagberg will give an overview of Los Alamos National Laboratory history and research followed by some examples of current research directions and challenges. He will describe some projects where applied mathematicians and computational scientists are solving national security science problems. Finally, he will discuss research opportunities at Los Alamos.
Aric Hagberg is the Deputy Division Leader of the Computer, Computational, and Statistical Sciences Division at Los Alamos National Laboratory. He received his Ph.D. in Applied Mathematics from the University of Arizona in 1994, joined Los Alamos as a Director's Funded Postdoctoral Fellow at the Center for Nonlinear Studies, and became a Staff Scientist in the Theoretical Division in 1997. Aric is active in the applied mathematics academic community. He has published more than 60 research articles in the fields of mathematics, computer science, and physics and has led projects in nonlinear dynamics, network modeling, and optimization of complex systems. He is a founding author of NetworkX, the widely-used open-source software package for analysis and modeling of complex networked systems.
This talk describes a method for extending the classical Irving-Kirkwood procedure used in statistical mechanics for extracting local fluxes to the problem of continuum-on-continuum thermomechanical multiscale modeling. Expressions for stress and heat flux derived here are contrasted to those obtained using the standard Hill-Mandel approach. The polar nature of the macroscopic solid and the role of multiscale invariance are also addressed in the context of this method. Applications are explored within the finite element-based homogenization of solids.
Panos Papadopoulos is Professor of Mechanical Engineering at the University of California, Berkeley. He received his Diploma from the Aristotle University of Thessaloniki, Greece and his MS and PhD from the University of California, Berkeley.
Photoacoustic computed tomography (PACT) is an emerging soft-tissue imaging modality that has great potential for a wide range of preclinical and clinical imaging applications. It can be viewed as a hybrid imaging modality in the sense that it utilizes an optical contrast mechanism combined with ultrasonic detection principles, thereby combining the advantages of optical and ultrasonic imaging while circumventing their primary limitations. In this talk, we review our recent advancements in image reconstruction approaches for PACT in acoustically heterogeneous media. Such advancements include physics-based models of the measurement process for both fluid and elastic media and associated optimization-based inversion methods. Applications of PACT to transcranial brain imaging will be presented. Open challenges related to the joint reconstruction of optical and acoustic parameters in PACT will also be presented.
Professor Anastasio received is Ph.D. from the University of Chicago in 2001, and is the Director of the Computational Bioimaging Laboratory at Washington University of St. Louis. He is an internationally recognized expert on tomographic image reconstruction, imaging physics, and the development of novel computed biomedical imaging systems. He has conducted pioneering research in the fields of photoacoustic computed tomography, diffraction tomography and X-ray phase-contrast imaging. He received an NSF CAREER award in 2006 for research related to image reconstruction topics. He is on the editorial boards of the Journal of Biomedical Optics and Medical Physics, and is on the organizing committee for the SPIE Photonics West Photon Plus Ultrasound Conference and serves on the OSA FiO program committee.
Anastasio’s current research interests include the development of biomedical imaging methods, image reconstruction, and inverse problems in imaging and theoretical image science. His current research projects include the development of advanced X-ray, optical, and acoustical imaging systems that are based on wave physics and can provide important structural and physiological tissue information. These projects include photoacoustic and thermoacoustic imaging, X-ray phase-contrast imaging, optical and acoustical tomography and holography, and improvement of existing clinical imaging methods.
With the end of Moore's law and increasing concerns about operating costs, high performance computing (HPC) systems are undergoing drastic changes which are making traditional parallelization strategies suboptimal. In this seminar, we aim to discuss some of the broad design constraints and trends for these modern HPC systems. Our talk will specifically focus on addressing the growth in concurrency using the C++ runtime library, High Performance ParallelX (HPX). We will present scaling results comparing HPX's performance to a MPI parallelization for a discontinuous Galerkin Kernel. Results will be shown on NERSC's Cori, with a more traditional Haswell architecture and TACC's Stampede2, with the Knights Landing architecture.
I will present the results of a theoretical investigation into the dynamics of interacting flapping swimmers. Our study is motivated by ongoing experiments in the Applied Math Lab at the Courant Institute, in which freely-translating, heaving hydrofoils interact hydrodynamically to choose their relative positions and velocities. We develop a discrete dynamical system in which flapping swimmers shed point vortices during each flapping cycle, which in turn exert forces on the swimmers. We present a framework for finding exact solutions to the evolution equations and for assessing their stability, giving physical insight into the preference for certain observed "schooling states". The model may be extended to arrays of flapping swimmers, and to configurations in which the swimmers' flapping frequencies are incommensurate. Generally, our results indicate how hydrodynamics may mediate schooling and flocking behavior in biological contexts.