The microscopic theory of the upper critical field Hc2 in two-band systems in the magnetic field directed parallel and perpendicular to the plane (ab). Application to MgB2

Abstract

The microscopic superconductivity theory in 2 systems in the magnetic fields directed parallel and perpendicular to the plane (ab) is built. The two energy bands of different dimensions overlapping on the Fermi surface (two-dimensional σ - band and three- dimensional π -band) are taken into account. The theory is built taking into account the weak dispersion of the electron energy of σ -band in z direction. The systems of equations for the superconductive transition temperature c T and the upper critical field ()0 2 ab c H are obtained. The analytic solutions of the equations for ()0 2 ab c H are found in the two temperature areas: nearby the zero temperature 0 c TT