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 $RCo{O}₃(R=La,{{0ex}{0ex}}Pr,{{0ex}{0ex}}Nd,{{0ex}{0ex}}Sm,{{0ex}{0ex}}Eu,{{0ex}{0ex}}and{{0ex}{0ex}}Gd)$ 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 (${T}IM$) 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 ${T}IM$ as compared with the charge gap in each $RCo{O}₃$ suggests that the $I{-}M$ transition should be viewed as a thermally induced Mott transition.