Transport properties of Bi 2Sr 2- xLa xCaCu 2O 8+ δ single crystals grown by a floating-zone method

A parent insulator of the Bi-based high- T_c superconductor Bi ₂Sr _{2- x}La _xCaCu ₂O _{8+ δ} was successfully grown by a traveling solvent floating-zone method. The in-plane resistivity ρ_ab shows metallic behavior (d ρ_ab/d T>0) near room temperature, although the magnitude of the resistivity is far above the Mott limit for metallic conduction. The Hall mobility of the parent insulator does not so much differ from that of an optimally doped sample. These results suggest that the system is not a simple insulator with a well-defined gap, and the hole in the parent insulator is essentially itinerant, as soon as it is doped.