Orbital localization and the role of the Fe and As $4p$ orbitals in BaFe$_{2}$As$_{2}$ probed by XANES

TL;DR

This study uses polarization-dependent XANES to investigate the unoccupied electronic states in BaFe₂As₂, revealing the roles of Fe and As orbitals and how Co and Mn substitutions alter electronic anisotropy and orbital mixing.

Contribution

It provides new insights into the orbital contributions and effects of transition metal substitutions on the electronic structure of BaFe₂As₂ using combined experimental and theoretical approaches.

Findings

States with p_z-orbital character dominate near the Fermi level.

Co substitution decreases spectral anisotropy, promoting p_z orbital states.

Mn substitution increases p_z anisotropy, indicating enhanced Fe 3d-4p mixing.

Abstract

The polarization dependence of the near edge x-ray absorption spectroscopy (XANES) is an element specific probe to the real-space distribution of the density of unoccupied states in solid-state materials. In this paper, we present Fe and As $K$-edge experiments of Ba(Fe$_{1-x}$$M_{x}$)$_{2}$As$_{2}$ ($M=$ Mn, Co and $x=0.0$ and $0.08$). The experiments reveal a strong polarization dependence of the probed XANES spectra, which concerns mainly an increase of the intensity of electronic transitions when the beam polarization is set out of the sample's $ab$ crystallographic plane. The results show that states with $p_{z}$-orbital character dominate the density of unoccupied states close to the Fermi level. Partial substitution of Fe by Co is shown to decrease the intensity anisotropy, suggesting that Co promotes electronic transfer preferentially to states with $p_{z}$-orbital character. On the other hand, Mn substitution causes the increase of the spectra $p_{z}$-orbital anisotropy, which is proposed to take place by means of an enhanced local Fe $3d4p$ mixing, unveiling the role of Fe $4p$ states in the localization of the Fe $3d$ orbitals. Moreover, by comparing our results to previous experiments, we identify the relative mixing between Fe and the pnictide $4p_{x,y,z}$ orbitals as a clear divide between the electronic properties of iron arsenides and selenides. Our conclusions are supported by multiple-scattering theory calculations of the XANES spectra and by quantum chemistry calculations of Fe coordination electronic structure.