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 RCoO(3) (R=La, Pr, Nd, Sm, Eu, and Gd) with R-dependent variation of one-electron bandwidth (W). All the compounds exhibit the insulating ground state but undergo gradual insulator-metal (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 RCoO(3) suggests that the I-Ari transition should be viewed as a thermally induced Mott transition.