Unexpected metal-insulator transition in thick Ca_1-xSr_xVO₃ film on SrTiO₃ (100) single crystal

Epitaxial Ca_1-xSr_xVO₃ (0 ≦ x ≦ 1) thin films were grown on (100)-oriented SrTiO₃ substrates by using the pulsed laser deposition technique. In contrast to the previous report that metal-insulator transition (MIT) in Ca_1-xSr_xVO₃ (CSVO) was achieved only for extremely thin films (several nm thick), MIT was observed at 39, 72, and 113 K for films with a thickness of 50 nm. The electronic structure was investigated by hard and soft X-ray photoemission spectroscopy (HX-PES and SX-PES). The difference between these PES results was significant due to the variation in an escape depth of photoelectrons of PES. While HX-PES showed that the V 2p_3/2 spectra consisted of four peaks (V⁵⁺, V⁴⁺, V³⁺, and V^2+/1+), SX-PES showed only three peaks (V⁵⁺, V⁴⁺, and V³⁺). This difference can be caused by a strain from the substrate, which leads to the chemical disorder (V⁵⁺, V⁴⁺, V³⁺, and V^2+/1+). The thin film near the substrate is affected by the strain. The positive magnetoresistance is attributed to the effect of electron-electron interactions in the disorder system. Therefore, the emergence of MIT can be explained by the electron-electron interactions from the chemical disorder due to the strain.