Temperature-dependent soft x-ray photoemission and absorption studies of charge disproportionation in La$_{1-x}$Sr$_x$FeO$_3$

TL;DR

This study investigates how temperature affects the electronic structure of La$_{1-x}$Sr$_x$FeO$_3$ thin films, revealing gradual spectral changes and charge disproportionation phenomena that influence their insulating behavior.

Contribution

It provides new insights into the temperature-dependent charge disproportionation and electronic structure evolution in LSFO across different compositions.

Findings

Spectral weight transfer over ~5 eV with temperature change.

Charge disproportionation occurs not only below $T_{CD}$ but also at higher temperatures.

Charge disproportionation may explain the wide insulating phase in LSFO.

Abstract

We have measured the temperature dependence of the photoemission and x-ray absorption spectra of La$_{1-x}$Sr$_x$FeO$_3$ (LSFO) epitaxial thin films with $x=0.67$, where charge disproportionation ($3{Fe}^{3.67+}\to 2{Fe}^{3+}+ {Fe}^{5+}$) resulting in long-range spin and charge ordering is known to occur below $T_{CD}=190$ K. With decreasing temperature we observed gradual changes of the spectra with spectral weight transfer over a wide energy range of $\sim 5$ eV. Above $T_{CD}$ the intensity at the Fermi level ($E_F$) was relatively high compared to that below $T_{CD}$ but still much lower than that in conventional metals. We also found a similar temperature dependence for $x=0.4$, and to a lesser extent for $x=0.2$. These observations suggest that a local charge disproportionation occurs not only in the $x=0.67$ sample below $T_{CD}$ but also over a wider temperature and composition range in LSFO. This implies that the tendency toward charge disproportionation may be the origin of the unusually wide insulating region of the LSFO phase diagram.