Bandwidth dependence of insulator-metal transitions in perovskite cobalt oxides

Insulator-metal transitions and related electronic structures have been investigated for single crystals of perovskite-type cobalt oxides $R\mathrm{Co}{\mathrm{O}}_3(R=\mathrm{L}\mathrm{a},\mathrm{P}\mathrm{r},\mathrm{N}\mathrm{d},\mathrm{S}\mathrm{m},\mathrm{E}\mathrm{u},\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{G}\mathrm{d})$ with $R$-dependent variation of one-electron bandwidth ($W$). All the compounds exhibit the insulating ground state but undergo gradual insulatormetal ($I-M$) transitions at 500-700 K. Systematic variation of the $I-M$ 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 $I-M$ transition should be viewed as a thermally induced Mott transition.