The origin of anisotropy and high density of states in the electronic structure of Cr$_2$GeC by means of polarized soft X-ray spectroscopy and ab initio calculations

TL;DR

This study combines polarized soft X-ray spectroscopy and ab initio calculations to reveal the anisotropic electronic structure and high density of states at the Fermi level in Cr₂GeC, highlighting the role of orbital hybridization.

Contribution

It provides the first detailed experimental and theoretical analysis of the anisotropy and density of states in Cr₂GeC using combined spectroscopy and DFT methods.

Findings

Demonstrates anisotropy in in-plane and out-of-plane bonding.

Reveals significantly higher Ge 4s state intensity at the Fermi level than predicted by DFT-GGA.

Shows hybridization causes redistribution of spectral intensity, affecting electronic properties.

Abstract

The anisotropy in the electronic structure of the inherently nanolaminated ternary phase Cr$_{2}$GeC is investigated by bulk-sensitive and element selective soft x-ray absorption/emission spectroscopy. The angle-resolved absorption/emission measurements reveal differences between the in-plane and out-of-plane bonding at the (0001) interfaces of Cr$_{2}$GeC. The Cr $L_{2,3}$, C $K$, and Ge $M_{1}$, $M_{2,3}$ emission spectra are interpreted with first-principles density-functional theory (DFT) including core-to-valence dipole transition matrix elements. For the Ge $4s$ states, the x-ray emission measurements reveal two orders of magnitude higher intensity at the Fermi level than DFT within the General Gradient Approximation (GGA) predicts. We provide direct evidence of anisotropy in the electronic structure and the orbital occupation that should affect the thermal expansion coefficient and transport properties. As shown in this work, hybridization and redistribution of intensity from the shallow $3d$ core levels to the $4s$ valence band explain the large Ge density of states at the Fermi level.