The Theory and Software 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.
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.
- Time-dependent and nonequilibrium spectroscopy. In recent years, there has been an increased emphasis in measuring x-ray spectra on materials excited by visible or infrared radiation. Our group is developing methodologies to understand time-dependent x-ray spectroscopy on materials away from equilibrium.
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.
Energy conversion and energy storage
A major focus of the laboratory is to find new materials that allow the conversion between different types of energies, such as from visible to chemical energy. In addition, renewable energy sources bring with them the problem of energy storage. The Group is performing research in the following areas:
- Optimization and new battery designs. Our research focuses on the development of new analysis tools than can aid in the optimization of battery materials such as lithium doped transition-metal oxides. Integrating the interpretation of results from powder diffraction, EXAFS, etc. can lead to a more detailed insight in these materials.
- Understanding ultrafast dynamics of photoexcited systems. After photoexcitation, a material will generally undergo a cascade to a lower-energy state. Understanding x-ray spectroscopy on these materials will increase our knowledge of, for example, energy conversion processes occuring in solar cells and photosynthesis.
Crystallographic structure determination provides a detailed three-dimensional model of how materials are constructed from their constituent atoms. The software team supports the widely used GSAS and EXPGUI structure refinement packages as well as the high-throughput automation for the 11-BM powder diffractometer and the CMPR data reduction/visualization package. A new code, GSAS-II is being developed jointly with the Structural Science group to develop data reduction, refinement and visualization capabilities in a modern and open framework that will allow model fitting from techniques beyond diffraction.
A new project is underway in collaboration with the Magnetic Materials group for simulation and fitting of magnetic scattering from x-ray and neutron reflectometry data.