No abstract

We calculate the upper critical magnetic field Hc2 in the framework of a microscopic superconductivity theory with two energy bands of different dimensions on the Fermi surface with the cavity topology typical of the compound MgB2 taken into account (an anisotropic system). We assume an external magnetic field parallel to the crystallographic z axis. We obtain analytic formulas in the low-temperature range (T /Tc 1) and also near the critical temperature`(T − Tc)/Tc 1´. We compare the temperature dependence of Hc2 for a two-band anisotropic system with that of H 0 c2 corresponding to a two-band isotropic system (with Fermi-surface cavities of the same topology). We determine the role of the bandstructure anisotropy, the positive curvature of the upper critical field near the critical temperature, and the important role of the ratio v1/v2 of the velocities on the Fermi surface in determining Hc2. We also obtain the values of the parameters ∆1 and ∆2 along the line of the critical magnetic field.

The theory of thermodynamic properties of two-band superconductor with reduced density charge carriers is developed on the base of phonon superconducting mechanism with strong electron-phonon interaction.This theory is adapted to describe the behavior of critical temperature T c , energy gaps ∆ 1 , ∆ 2 , and the relative jump of electron specific heat (C S − C N )/C N in the point T = T c along with the variation of charge carrier density in the compound MgB 2 when substitutional impurities with different valence are introduced into the system. It is shown, that according to the filling mechanism of energy bands which overlap on Fermi surface, the quantities T c , ∆ 1 , ∆ 2 decrease when this compound is doped with electrons and remain constant or weakly change when the system is doped with holes. The theory qualitatively agrees with the experimental data.Also is shown that the consideration of inter-and intraband scattering of electrons on impurity potential improves this agreement.