From low to high-temperature superconductivity: A dimensional crossover phenomenon? A finite size effect?

Abstract

We explore the Ginzburg-Landau version of a model for layered superconductors, in which superconducting material consisting of M unit cells is separated by insulating layers that are N unit cells thick. Invoking the recently proposed interlayer coupling-which enhances pairing within the superconducting sheets-the order parameter coupling turns out to be of Heisenberg type, analogous to layered magnets. The mean field result for Tc (M, N) is found to be in remarkable agreement with experiment for ultrathin NbSe2 crystals and Sn-SiO, Al-AlO2, YBCO/PrBCO superlattices. The fact that Tc (M, N) increases with increasing M is traced back to a finite size effect, corresponding to a crossover from nearly two to three-dimensional superconductivity. This change from low to high-temperature superconductivity is found to be independent of the sign of the interlayer interaction. An electrostatic model for the interaction on YBCO/PrBCO superlattices is in reasonable agreement with experiment. The orbital upper critical field Hc2{norm of matrix} turns out to be unlimited ited while Hc2⊥ is proportional to Tc (M, N). © 1991 Springer-Verlag.