Pseudogap formation and superconductivity of Bi₁.₇₄Pb₀.₃₈Sr₁.₈₈CuO₆₊δ by studies of out-of-plane resistivity in magnetic fields

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We report the out-of-plane resistivity in the systematically oxygen-controlled single crystals Bi₁.₇₄Pb₀.₃₈Sr₁.₈₈CuO₆₊δ in magnetic fields parallel to the c-axis. Two characteristic temperatures T* and T** are found at which a semiconductor-like upturn starts to develop and the magnetoresistance changes its sign from positive to negative at lower temperature, respectively. The obtained phase diagram strongly suggests that the pseudogap temperature correlating to the superconducting gap is not T* but T**. The gap opened at T* may relate to another phenomenon with the different energy scale such as the antiferromagnetic excitation due to the exchange interaction between Cu²⁺ spins.