Thermodynamics of vortices in layered superconductors and the dependence of Tc on anisotropy

Abstract

Data on multilayered superconductors such as YBa 2Cu 3O 7⧸PrBa 2Cu 3O 7 show a strong dependence of T c on anisotropy. In particular, the ratio T c⧸τ, where τ is the effective XY coupling constant, is found to vary by much more than the theoretical XY limit of 2.4. Layered superconductors allow for an additional anisotropy due to the core energy E c of vortices perpendicular to the layers. It is shown that if E c is large, such that the anisotropy is larger than exp(- E c⧸ T c), T c is near a fluxon transition which is described by fluctuations of flux loops parallel to the layers. In the latter case T c⧸τ can vary by more than 2.4, accounting for the data.