Bandwidth dependence of insulator-metal transitions in perovskite cobalt oxides

Abstract

Insulator-metal transitions and related electronic structures have been investigated for single crystals of perovskite-type cobalt oxides $R\mathrm{Co}{\mathrm{O}}_{3}(R=\mathrm{L}\mathrm{a},\phantom{\rule{0ex}{0ex}}\mathrm{P}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{N}\mathrm{d},\phantom{\rule{0ex}{0ex}}\mathrm{S}\mathrm{m},\phantom{\rule{0ex}{0ex}}\mathrm{E}\mathrm{u},\phantom{\rule{0ex}{0ex}}\mathrm{a}\mathrm{n}\mathrm{d}\phantom{\rule{0ex}{0ex}}\mathrm{G}\mathrm{d})$ with $R$-dependent variation of one-electron bandwidth ($W$). All the compounds exhibit the insulating ground state but undergo gradual insulatormetal ($I\ensuremath{-}M$) transitions at 500-700 K. Systematic variation of the $I\ensuremath{-}M$ transition temperatures (${T}_{\mathrm{IM}}$) as well as the thermal and optical charge gaps in the insulating phase have been observed with change of $R$ species (or equivalently $W$). Anomalously low ${T}_{\mathrm{IM}}$ as compared with the charge gap in each $R\mathrm{Co}{\mathrm{O}}_{3}$ suggests that the $I\ensuremath{-}M$ transition should be viewed as a thermally induced Mott transition.