Ginzburg-Landau equation and the upper critical field in anisotropic (d+s)-wave superconductivity

We have proposed earlier a (d+s)-wave superconductivity model to account for the high-T-c cuprates that exhibit an orthorhombic distortion. Within this model, we derive here the Ginzburg-Landau equation microscopically. Included are the a-b anisotropy terms which already exist in the normal phase, using an anisotropic mass model. We calculate the upper critical field H-c2 in the cases where the applied magnetic field B is parallel to the c axis or in the a-b plane. In this last case, we obtain a simple expression for H-c2 which is a function of cos(2 theta) where theta is the angle B makes from the a axis. In particular, for YBCO(123) we predict H-c2(b)/H-c2(a)=3.15, which is much larger than the one reported in a recent experiment.