Torque theory of anisotropic superconductors with no phenomenological parameter in determining vortex core size

Abstract

The contribution of vortex core has been taken into account properly in constructing a torque theory for multiband superconductors. We employ the prescription of describing internal magnetic field in the vortex lattice by Hao et al. and by Yaouanc et al. to derive a torque formula as a natural extension of a preceding London theory. In marked contrast with the preceding model, our novel formula does not contain a phenomenological parameter η, which prevents us from obtaining a true upper critical field H c2 by analyzing an experimental torque curve. The parameter η was originally introduced to take care of the uncertainty in determining the vortex core size ξ v . Furthermore, we reveal that the η value is universally scaled by anisotropy γ, magnetic field B, and H c2 due to field dependence of ξ v . This may revitalize the single-band Kogan model in combination with a universal function η(γ, B, H c2 ) instead of a constant η.