Angular dependence of the upper critical field of Bi2.2Sr2Ca0.8Cu2O8+δ

Abstract

We have measured the electrical resistivity of ${\mathrm{Bi}}_{2.2}$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_{0.8}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ in the vicinity of $T_c$ for various angles between the [${\mathrm{CuO}}_2$ ${]}_{\mathrm{\infty }}$ double layers of the crystal and the magnetic field. Defining $T_c$ at the transition midpoints, we have measured values for -${\mathit{dH}}_{\mathit{c}2}^{\mathrm{\parallel }}$/dT=45 T/K and -${\mathit{dH}}_{\mathit{c}2}^{\mathrm{\perp }}$/dT=0.75 T/K for the magnetic field parallel and perpendicular to the [${\mathrm{CuO}}_2$ ${]}_{\mathrm{\infty }}$ planes, respectively. This results in an anisotropy of a factor 60. The numerical results are sensitive to the definition of $T_c$ and larger values for the anisotropy cannot be excluded. The results are compared with the anisotropic three-dimensional Ginzburg-Landau theory.