Upper critical field in layered superconductors

The theoretical statements about a restoration of a superconductivity at ma gnetic fields higher than the quasiclassical upper critical field and a ree ntrance of superconductivity at temperatures T-c(H) approximate to T-c(0) i n the superconductors with open Fermi surfaces are reinvestigated taking in to account a scattering of quasiparticles on the impurities. The system of integral equations for determination of the upper critical field parallel t o the conducting planes in a layered conventional and unconventional superc onductors with impurities are derived. The H-c2(T) values for the "clean" c ase in the Ginzburg-Landau regime and at any temperature in the "dirty" cas e are found analytically. The upper limit of the superconductor purity when the upper critical field definately has a finite value is established.