Normal-state transport properties of Bi2+xSr2−yCuO6+δ crystals

Abstract

Transport anisotropies of ${\rho }_c$.${\rho }_{\mathrm{ab}}$≃${10}^4$ to ${10}^5$ were measured for superconducting and nonsuperconducting ${\mathrm{Bi}}_{2+\mathrm{x}}$ ${\mathrm{Sr}}_{2\mathrm{-}\mathrm{y}}$ ${\mathrm{CuO}}_{6\pm \mathrm{\delta }}$ crystals. In superconducting samples ${\rho }_{\mathrm{ab}}$ increases linearly with temperature from just above $T_c$≃7 to 700 K. The implication of the ${\rho }_{\mathrm{ab}}$ results is that classical electron-phonon scattering mechanisms are inadequate. The anisotropy and $T_c$ for various layered superconductor systems are compared. In all crystals studied ${\rho }_c$ is nonmetallic, varying as a power law $T^{\mathrm{-}\alpha }$, α≃0.5–1.