Four-Fold Anisotropy of the Parallel Upper Critical Magnetic Field in a Pure Layered d-Wave Superconductor at T = 0

It is well known that a four-fold symmetry of the parallel upper critical magnetic field disappears in the Ginzburg–Landau (GL) region in quasi-two-dimensional (Q2D) d-wave superconductors. Therefore, it has been accurately calculated so far as a correction to the GL results, which is valid close to superconducting transition temperature and is expected to be stronger at low temperatures. As to the case T = 0, some approximated methods have been used, which are good only for closed electron orbits and inappropriate for the open orbits that exist in a parallel magnetic field in Q2D superconductors. For the first time, we accurately calculate the four-fold anisotropy of the parallel upper critical magnetic field in a pure Q2D d-wave superconductor at T = 0, where it has the highest possible value. Our results are applicable to Q2D d-wave high-T_c and organic superconductors.