Temperature dependence of the in-plane resistivity in underdoped Bi₂Sr₂CaCu₂O_y single crystals

A number of underdoped Bi2Sr2CaCu2Oy single crystals have been obtained by two different ways: (1) substitution of Y for Ca, and (2) removing oxygen from crystals by vacuum annealing. The in-plane resistivity (rho(ab)) measurements reveal different transport properties between these crystals. For the Y-doped crystals, rho(ab)(T) exhibits metallic behavior until the crystal reaches a doping level where the metal-insulator transition should occur. An S-like T-dependent rho(ab)(T) in the underdoped samples, similar to the observations in underdoped YBa2Cu3O7-y and La2-xSrxCuO4, was observed. However, for vacuum-annealed crystals, rho(ab)(T) becomes nonmetallic. We analyze the data and argue that the disorder introduced by vacuum annealing is much greater than that by Y doping as long as the crystals have the same T-c