The entropic corrections to the flux-line energy of extreme type-II superconductors are computed using a schematic dual Villain model description of the flux quanta. We find that the temperature profile of the lower critical field vanishes polynomially at the transition with an exponent ν≃2/3 in the isotropic case, while it exhibits an inflection point for the case of weakly coupled layers in parallel magnetic field. It is argued that vestiges of these effects have already been observed in high-temperature superconductors. © 1996 The American Physical Society.

• Received 29 January 1996

DOI:https://doi.org/10.1103/PhysRevB.54.497