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Science
Goals
The Synchrotron-related Theory Group connects to the
experimental programs at the Advanced Photon Source by the development
of theoretical methodologies that provide new pathways for the study
of the electronic, magnetic, and structural properties of materials and
creating a link between experimental results and theoretical models.
Research Overview
X-ray spectroscopy
The preceding decades have seen a large increase in
the use of x-ray spectroscopic tools. The applicability of these techniques
was advanced rapidly by new theoretical insights into the use of the
polarization of the x-rays. This enabled the link between integrated
intensities of the spectral function and ground-state properties.
This opened up the field of X-ray Magnetic Dichroism (XMD) which allows
the study of magnetic materials with polarized x-rays. Recently, second
order spectroscopies have come into focus. Examples are Resonant Inelastic
X-ray Scattering (RIXS) and polarized and spin-resolved resonant photoemission.
The theoretical study of x-ray spectroscopy includes:
- Relating the various techniques to fundamental materials
properties by the derivation of sum rules and theoretical analysis
of the cross section. This includes the effects of the polarization
of the x-rays.
- Obtaining a more detailed understanding of second-order
spectroscopies. These techniques are often complicated by the presence
of the intermediate-state propagator.
Strong electron correlations and magnetism
As a result of direct transitions into the valence shell,
x-ray spectroscopy offers a direct probe into the microscopic behavior
of strongly correlated and magnetic systems. Although a successful interpretation
of spectroscopy can lead to deeper insights into these materials, it
often requires detailed modeling of complex materials and phenomena.
Our studies in this area include:
- The calculations of spectral lineshapes with various numerical
tools. Our group has a special expertise in the exact diagonalization
of small clusters including the full-multiplet Coulomb interaction
and spin-orbit coupling. These calculations are often important
for the interpretation of transition-metal and rare-earth compounds.
- Providing
a link between the results of x-ray spectroscopy and materials
research and theory. Theoretical interpretation is essential
to relate the spectral to charge and magnetic excitations, dynamic
structure factors, spin polarization, etc.
Relations to other programs
The Synchrotron-related Theory Group has connections
with the Condensed
Matter Theory Group (MSD), XOR, CMC, BESSRC,
IXS, Brookhaven
National Lab, and Sergey Stepanov who develops X-Ray
Server for online
analysis of X-ray scattering data. In addition, we contribute to the Argonne
Theory Institute.
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