Anisotropic Resistivities of Precisely Oxygen Controlled Single-Crystal Bi₂Sr₂CaCu₂O_8+δ: Systematic Study on “Spin Gap” Effect

The in-plane resistivity ρ_a(T) and the out-of-plane resistivity ρ_c(T) have been systematically measured for Bi₂Sr₂CaCu₂O_8+δ single crystals with their oxygen contents precisely controlled. In the underdoped region, deviation from T-linear in-plane resistivity, which evidences the opening of the “spin gap,” is clearly observed, while the out-of-plane resistivity is well reproduced by the activation-type phenomenological formula ρ_c(T) = (a/T) exp(ऴ/T) + c. In contrast to the YBa₂Cu₃O_7-δ system, we find that the onset of the semiconducting ρ_c(T) does not coincide with the opening of the spin gap seen in the ρ_a(T) in this Bi₂Sr₂CaCu₂O_8+δ system.