About possible existence of intermediate and mixed states in the two-band superconductors (on example of magnesium diboride)

Abstract

A review of papers devoted to the definition of the type of two-band superconductors (such as magnesium diboride), where the Abrikosov criterion is violated was done. Within Josephson model «Superconductor of 1-type — Insulator — Superconductor of 2-type» was shown that type of such superconductors depends on the values of the phenomenological coefficients |a1|and |a2|  temperature. In this context, the concept of a new type of superconductors in magnesium diboride of «Type of 1,5 superconductivity» seems premature. In our opinion, the observed effects in these superconductors is due to the coexistence of intermediate and mixed states under the influence of an external weak magnetic field.