Evidence forO2pHole-Driven Conductivity inLa1−xSrxMnO3(0≤x≤0.7)andLa0.7Sr0.3MnOzThin Films

Oxygen $K$-edge electron-energy-loss spectra have been measured for ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ $\left(0\le x\le 0.7\right)$ and ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_z$ thin films as a function of $x$ and $z$. The spectra show a prepeak at the Fermi level, corresponding to transitions to empty states in the ${\mathrm{O}}_{2p}$ band, at the threshold of the $K$ edge around 529 eV. This prepeak systematically increases with an increase in conductivity through divalent doping $\left(x\right)$ or oxygen content $\left(z\right)$. This confirms that these materials are charge-transfer-type oxides with carriers having significant oxygen $2p$ hole character. We argue that, the double exchange mechanism has to include the role of oxygen hole density to satisfactorily describe their transport properties.