Temperature-Dependent Soft X-ray Photoemission and Absorption Studies of Charge Disproportionation in La_1-<i>x</i>Sr_<i>x</i>FeO₃

Abstract

We have measured the temperature dependence of the photoemission and x-ray\nabsorption spectra of La$_{1-x}$Sr$_x$FeO$_3$ (LSFO) epitaxial thin films with\n$x=0.67$, where charge disproportionation ($3{Fe}^{3.67+}\\to 2{Fe}^{3+}+\n{Fe}^{5+}$) resulting in long-range spin and charge ordering is known to occur\nbelow $T_{CD}=190$ K. With decreasing temperature we observed gradual changes\nof the spectra with spectral weight transfer over a wide energy range of $\\sim\n5$ eV. Above $T_{CD}$ the intensity at the Fermi level ($E_F$) was relatively\nhigh compared to that below $T_{CD}$ but still much lower than that in\nconventional metals. We also found a similar temperature dependence for\n$x=0.4$, and to a lesser extent for $x=0.2$. These observations suggest that a\nlocal charge disproportionation occurs not only in the $x=0.67$ sample below\n$T_{CD}$ but also over a wider temperature and composition range in LSFO. This\nimplies that the tendency toward charge disproportionation may be the origin of\nthe unusually wide insulating region of the LSFO phase diagram.\n