Torque formula as sum in reciprocal lattice for anisotropic superconductors

Abstract

The magnetic torque theory proposed by Kogan has frequently been used to explain the anisotropy in various superconductors. It introduces a phenomenological parameter η to account for the uncertainty in defining the vortex core size in anisotropic superconductors. The model treats η as a factor of the order of unity, and its physical meaning remains to be clarified. Our new theory follows the prescription given by Yaouanc et al. and does not need an ambiguous parameter η to evaluate the torque. The free energy can be obtained by summing up partial contributions in the reciprocal lattice space. It is meaningful to trace the behavior of η in the Kogan model with a change in the anisotropy parameter γ under the constant field B. We find that the parameter η changes gradually with γ under the constant field.