Group 2: Spectroscopy

Task of WG 2: Improve the theoretical description of spectroscopy

WG leader: Barbara Brena (SE), Deputy: Mebarek Alouani (FR)

The ongoing refinement of established spectroscopic experiments as well as the introduction of new ones continuously increases the demands to the accompanying theory. An example for the first one is the description of spin-resolved photo emission for circularly polarized radiation. This is a central issue when dealing with the spin-polarized surface of topological insulators that are a new promising material class in spintronics. The ongoing development of new and more efficient spin detectors will clearly enhance the importance of the related issues. Standard angle-resolved photo emission has recently been extended to its resonant version combining element-specific and wave vector resolved information. 

First steps for an adequate description of the new experimental developments have been made, but further work is urgently needed and  is a central issue of this COST action. Apart from refining and extending the theoretical description of spectroscopic experiments there is a strong need to incorporate dynamical correlations in an experiment-specific way to lead to a complete and quantitative theory. This important issue is strongly linked to those of WG 1, but includes schemes that go much beyond the treatment of ground state properties. For example, one can make use of the Bethe-Salpeter equation and time-dependent DFT as a formal platform. Obviously, this provides a natural link to WG 3.

In order to monitor improvements and differences in the various spectroscopy codes the WG 2 will define and install a common set of test cases for a wide range of spectroscopic calculations. Cross checks of the programs will reveal in particular improvements achieved by full-potential techniques or the impact of spin-orbit coupling that gives rise to many important dichroic effects. Here comparative studies will monitor the differences resulting from a treatment via perturbation theory and via the Dirac equation. In a similar way the various levels to incorporate correlation effects will be compared with respect to their impact on spectroscopic properties. Another main activity of the WG is to promote schemes that account for dynamical correlations in spectroscopy that go well beyond the simple final state or Z+1 models. Again the exchange of experience and program modules together with the supply of benchmark data sets will accelerate corresponding program developments. This will be used for example when implementing the solution of the Bethe-Salpeter equation for disordered systems using multiple scattering theory. Finally, the rapid exchange of new ideas will be used to develop in the future the adequate theoretical description of new experiments on demand. An example for this will be the extension of a recent theory for resonant photo emission to the angle-resolved case.