Restoration of Superconductivity in High Parallel Magnetic Fields in Layered Superconductors

Abstract

We derive an equation determining the upper critical field ${H}_{c2}^{\ensuremath{\Vert}}(T)$ parallel to conducting planes of a layered superconductor from the BCS theory. It extends the descriptions of ${H}_{c2}^{\ensuremath{\Vert}}(T)$ within the Ginzburg-Landau-Abrikosov-Gor'kov theory and the Lawrence-Doniach model to the case of strong magnetic fields. From this equation, it follows that orbital effects of an electron motion along an open Fermi surface in a magnetic field start to restore superconductivity at magnetic fields higher than the quasiclassical upper critical field and result in the appearance of a reentrant phase with ${T}_{c}(H)\ensuremath{\simeq}{T}_{c}(0)$. A stability of the reentrant phase against fluctuations is discussed.