Thermodynamic and magnetic properties of superconductors with anisotropic energy spectrum, MgB2

The behavior of the superconducting transition temperature T (c) and that of the jump of electron heat capacity (C-S -C-N )/C-N of the compound MgB2 at T=T-c with substitution of boron and magnesium atoms by other atoms from the periodic table of the elements, corresponding to introduction of additional electrons or holes in this compound are researched. The microscopic superconductivity theory in MgB2 systems in the magnetic field parallel to the crystallographic axis (H parallel to c) is built. The magnitude of the upper critical field H-c2 is determined and its temperature dependence in two-band systems with different and identical topologies of Fermi surface cavities of the corresponding bands is studied. The obtained results and their comparisons with the experimental data demonstrate that all kinds of anomalies of the physical properties of the compound MgB2 are effectively described by the two-band model.