Ginzburg-Landau equation and vortex state in d+s superconductivity

The a-b anisotropy in the superconducting state of some of the high Tc cuprates with orthorhombic distortion is in part due to d+s wave superconductivity and in part due to the normal state anisotropy in N-0(phi) (the density of states) and v(F)(phi) (the Fermi velocity) within the a-b plane, where phi is the angle the quasiparticle momentum makes from the a axis. Therefore it is of great interest to sort out the origin of the a-b anisotropy and determine the relevant parameters from existing experiments. In particular we predict : a) a large a-b anisotropy in the upper critical field when (B) over right arrow parallel to a-b and b) the stability of the vortex lattice which is very close to the regular hexagonal one in the vicinity ot T = Tc.