Ginzburg-Landau theory of a d-wave superconductor with mass anisotropy -- Pairing symmetry and vortex structures

YBa2Cu3O7 (YBCO) exhibits a large anisotropy between the a (or y) and b (or x) axes in the CuO2 planes. This anisotropy can be modeled by introducing an anisotropic mass parameter λ = mx/my. Assuming a d-wave pairing interaction together with a repulsive on-site Coulomb interaction, the authors developed a Ginzburg-Landau theory for a d-wave superconductor with mass anisotropy in the presence of a magnetic field. They show that the order parameter always has s + d symmetry. The vortex structures for λ = 1 and λ > 1 have been numerically studied. For high Tc cuprates with tetragonal structure (λ = 1), the vortex shows a four-fold symmetry and the vortex lattice may have oblique or triangular structure depending on the strength of the applied magnetic field, temperature, and the other parameters. For YBCO they choose λ = 2, the single vortex has an elliptic shape, and the vortex lattice always shows an oblique structure. All these results are in good agreement with experimental measurements