Many-body effects in x-ray absorption and magnetic circular dichroism spectra within the LSDA+DMFT framework

TL;DR

This study evaluates the LSDA+DMFT method's effectiveness in describing x-ray absorption and magnetic circular dichroism spectra of transition metals, showing improvements over LSDA but highlighting remaining discrepancies and the need for advanced core hole treatment.

Contribution

It demonstrates the application of LSDA+DMFT to XANES and XMCD spectra, revealing its advantages and limitations compared to LSDA, and emphasizes the importance of core hole effects beyond the final state approximation.

Findings

LSDA+DMFT improves the asymmetry of the L3 white line in spectra.

Discrepancies remain in the L3/L2 intensity ratio compared to experiments.

Accounting for the core hole within the final state approximation is insufficient.

Abstract

The theoretical description of photoemission spectra of transition metals was greatly improved recently by accounting for the correlations between the d electrons within the local spin density approximation (LSDA) plus dynamical mean field theory (DMFT). We assess the improvement of the LSDA+DMFT over the plain LSDA in x-ray absorption spectroscopy, which --- unlike the photoemission spectroscopy --- is probing unocccupied electronic states. By investigating the L2,3-edge x-ray absorption near-edge structure (XANES) and x-ray magnetic circular dichroism (XMCD) of Fe, Co, and Ni, we find that the LSDA+DMFT improves the LSDA results, in particular concerning the asymmetry of the L3 white line. Differences with respect to the experiment, nevertheless, remain --- particularly concerning the ratio of the intensities of the L3 and L2 peaks. The changes in the XMCD peak intensities invoked by the use of the LSDA+DMFT are a consequence of the improved description of the orbital polarization and are consistent with the XMCD sum rules. Accounting for the core hole within the final state approximation does not generally improve the results. This indicates that to get more accurate L2,3-edge XANES and XMCD spectra, one has to treat the core hole beyond the final state approximation.