Department of Physics, Ben-Gurion University, Beer-Sheva 84105, Israel
Abstract:
Data on multilayered superconductors such as YBa2Cu3O7PrBa2Cu3O7 show a strong dependence of Tc on anisotropy. In particular, the ratio Tcτ, 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 Ec of vortices perpendicular to the layers. It is shown that if Ec is large, such that the anisotropy is larger than exp(-EcTc), Tc is near a fluxon transition which is described by fluctuations of flux loops parallel to the layers. In the latter case Tcτ can vary by more than 2.4, accounting for the data.