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

We have measured the electrical resistivity of Bi2.2Sr2Ca0.8Cu2O8+δ in the vicinity of Tc for various angles between the [CuO2] double layers of the crystal and the magnetic field. Defining Tc at the transition midpoints, we have measured values for -dHc2/dT=45 T/K and -dHc2/dT=0.75 T/K for the magnetic field parallel and perpendicular to the [CuO2] planes, respectively. This results in an anisotropy of a factor 60. The numerical results are sensitive to the definition of Tc and larger values for the anisotropy cannot be excluded. The results are compared with the anisotropic three-dimensional Ginzburg-Landau theory.