Real-time Approaches for Dynamic Structure and Electronic Excitations in Condensed Matter

Real-time approaches are becoming increasingly important in understanding dynamical properties and excitations in condensed matter due to the increasing availability of high performance computational facilities.Here we discuss two such real-time approaches.

I. The first approach is based on real-time simulations of the dynamic structure of nano-scale materials base on finite-temperature density functional theory/molecular dynamics and x-ray spectroscopy theory. This approach is illustrated for the case of nanoclusters of Pt metal supported on gamma-alumina which are used as catalysts. The simulations reveal a complex dynamical structure on multiple-time scales including, hindered Brownian-like librational motion of the center of mass and fluctuating bonding, which explain many of their unusual properties.

II. The second is a real-time, time-dependent density functional theory (RT-TDDFT) approach for ab-initio calculations of frequency-dependent linear and non-linear optical response. This method is an extension to hyperpolarizabilities of an approach based on calculations of the time-evolution operator using the electronic structure program SIESTA. Instead of calculating excited quantum states, which can be a bottleneck in frequency-space calculations, the response of large molecular systems to time-varying electric fields is calculated in real-time. To speed the calculations of non-linear response, our approach uses Gaussian enveloped quasi-monochromatic external fields. With this approach we obtain frequency dependent second harmonic generation (SHG), DC nonlinear rectification, and the electro-optic effect (EOE), which are important in photonics. The method is illustrated with calculations for nano-scale, photonic nonlinear optical (NLO) molecules, and yields results in good agreement with experiment.

Speaker Details

Rehr received a Ph.D. in Theoretical Condensed Matter Physics from Cornell University in 1972 working with Prof. N. D. Mermin. A NATO postdoctoral fellowship at King’s College London (1972-73), and a postdoctoral appointment with Prof. W. Kohn at University of California, San Diego, (1973-1975) followed. In 1975, Rehr joined the Department of Physics at the University of Washington. Rehr held visiting appointments at Cornell University (1986-88) working with Prof. K. Wilson and at Lund University (spring 1994) with Prof. Lars Hedin Rehr is also a Consulting Professor at Stanford Synchrotron Radiation Laboratory and a Co-coordinator of the DOE Computational Materials Science Network. Rehr’s research specialties are in condensed matter theory, with major interests in excited state electronic structure and the theory of x-ray and electron spectra. Over the past ten years Rehr’s group has made a number of significant achievements and is recognized as a leader in the field of theoretical x-ray and electron spectroscopies. A major accomplishment was the solution to the EXAFS problem, and subsequently theories of x-ray and electron spectroscopy, for which our codes are in use world- wide. These accomplishments were recognized by the International XAFS Society Outstanding Achievement Award in 2006. More recently we have extended our approach to treat x-ray magnetic circular dichroism, inelastic x-ray spectra and both linear and non-linear optical spectra. Our group was the first in the Department to introduce high performance parallel computation and currently operates two linux clusters. Over the past ten years we have published over 100 papers in refereed journals, and our citation record exceeds 11,000. Over the past several years our research has been supported by large grants from the DOE, the DOE Computational Materials Science Network and by additional grants from the NSF, NIST, and NIH/SSRL. Currently our grants support 4 graduate students and two postdocs, and over the past 10 years we have graduated 6 Ph.D.s.

Date:
Speakers:
J. J. Rehr
Affiliation:
Dept. of Physics, Univ. of Washington