Electronic properties of epitaxial La1−xSrxRhO3 thin films

We report on the synthesis and electronic properties of epitaxial perovskite ${\mathrm{La}}_{1\text{−}x}{\mathrm{Sr}}_x\mathrm{Rh}{\mathrm{O}}_3$ thin films. Thin films with a Sr content ranging from $x=0$ to 0.5 have been grown using molecular beam epitaxy. Transport and x-ray photoemission spectroscopy data reveal an insulator-metal-insulator transition, accompanied by a $p$- to $n$-type carrier change observed in Hall measurements. Combined with theoretical calculations, we find that the addition of Sr does not directly dope carriers into the conduction band, but rather induces localized Rh $4d$ states within the $\mathrm{La}\mathrm{Rh}{\mathrm{O}}_3$ band gap. The bandwidth of the impurity band increases with Sr content, eventually causing the valence band (VB) and the localized Rh $4d$ band to overlap, which explains the first insulator-to-metal transition occurring at $x=0.35$. For Sr content $x>0.4$, possible cation ordering results in an increase of the gap between the VB and the Rh $4d$ band, leading to the second metal-to-insulator transition. We map out the electronic phase diagram of the Sr-doped $\mathrm{La}\mathrm{Rh}{\mathrm{O}}_3$ system and suggest strategies to engineer the electronic states in rhodate systems via delocalizing the ${\mathrm{Rh}}^{3+}$ states.