UPPER CRITICAL-FIELD OF S-WAVE AND D-WAVE SUPERCONDUCTORS WITH ANISOTROPIC EFFECTIVE MASS

We have derived strong-coupling equations for the upper critical field Bc2(T) of a spin-fluctuation-mediated d-wave superconductor. Two possible choices for the d-wave gap function are examined. Even for an isotropic Fermi surface they lead to different reduced fields bc2 when the magnetic field is applied in the z direction or in the xy plane. They also cause a temperature-dependent Bc2 anisotropy. We include in the analysis the full angular dependence of the upper critical field near Tc and compare it with the angular dependence caused by an s-wave superconductor with different masses (m) in the xy plane and (M) in the z direction. As the ratio of the d wave to constant part of the magnetic susceptibility g is reduced below 1, the upper critical field is strongly suppressed as is the critical temperature, compared with g=1, which corresponds to the conventional electron-phonon case. Strong-coupling effects are also considered explicitly. © 1990 The American Physical Society.