Four-fold anisotropy of the parallel upper critical magnetic field in a 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 which
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 Q2D d-wave
superconductor at T=0, where it has the highest possible value.
Our results are applicable to Q2D d-wave high-Tc and organic
superconductors.