Anisotropic Resistivities of Precisely Oxygen Controlled Single-CrystalBi2Sr2CaCu2O8+δ: Systematic Study on ``Spin Gap'' Effect

Abstract

The in-plane resistivity ${\ensuremath{\rho}}_{a}(T)$ and the out-of-plane resistivity ${\ensuremath{\rho}}_{c}(T)$ have been systematically measured for ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ 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 ${\ensuremath{\rho}}_{c}(T)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(a/T)\mathrm{exp}(\ensuremath{\Delta}/T)+c$. In contrast to the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ system, we find that the onset of the semiconducting ${\ensuremath{\rho}}_{c}(T)$ does not coincide with the opening of the spin gap seen in the ${\ensuremath{\rho}}_{a}(T)$ in this ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ system.