Frank de Groot (SIMES Friday seminar)

Date(s) - Apr 5 2013
11:00 AM - 12:30 PM

Shasta Room, Bldg 40, Room 361


 Towards an ab-initio description of X-ray absorption spectraof transition metal systems

 Frank de Groot

Debye Institute of Nanomaterials Science

Utrecht University, Netherlands


The interpretation of x-ray absorption spectra is at present divided into a number of theoretical approaches that are adapted to specific systems, elements or x-ray absorption edges. For example, band structure based methods are used for solids and molecular density functional and quantum chemical approaches for molecules. Real space multiple scattering codes such as FEFF can be used for any system.

I will limit myself to the x-ray absorption spectra of transition metal systems, including both solids, nanoparticles and molecular complexes. Many issues are still open concerning the interpretation of both the transition metal K (pre)-edges and L edges. Transition metal K edges have traditionally been interpreted as a combination of a quadrupole pre-edge plus a dipole edge, with the additional mixing of dipole character into the pre-edge region when inversion symmetry is broken. In the interpretation of K edges of transition metal oxides and other extended systems, one encounters the complication that in addition to local quadrupole transitions additional pre-peaks are visible due to transitions to p-character that hybridizes with the 3d-character of the oxygen-bridged transition metal neighbours [1].

The L edges of transition metal oxides can be considered as a well recognised set of spectra where ab-initio approaches are still not able to capture all effects that determine the spectral shape. In particular the final state overlap between the core hole wave function and the relatively localised 3d wave functions, the multiplet effect, is still not fully incorporated into most ab-initio calculations [2,3]. A number of ab-initio routes are being developed. BSE approaches seem to work well for 3d0 systems. The DFT-CI code from Ikeno et al. can very well describe molecular and solid state complexes as far as the main ligand field multiplet structure is concerned [4]. The last step into the ab-initio description of L edges is the balanced treatment of (1) multiplet effects, (2) charge transfer effects and (3) translation symmetry. Recently we have organised a workshop on this topic and I will give a brief overview of the recent developments as presented at this meeting [5]. I will give my view on ideas and expectations on the development of such theory.

[1]   A. Juhin, F. de Groot, G.Vankó, M.Calandra and C.Brouder, Phys. Rev. B. 81, 115115 (2010)

[2]   Core Level Spectroscopy of Solids, Frank de Groot and Akio Kotani (Taylor & Francis, 2008)

[3]   E. Stavitski and F.M.F. de Groot, Micron 41, 687 (2010).

[4]   H. Ikeno, F. de Groot, E. Stavitski and I. Tanaka, J. Phys. Cond. Matt. 21, 104208 (2009).

[5]; References to recent first principle multiplet calculations can be found at the meeting website.