Phase transitions in a MgB2 granular BCS superconductor in weak magnetic fields

Abstract

The character of temperature dependences of the electric conductivity of MgB₂ granular BCS superconductors at temperatures of ~35–45 K in external magnetic fields H_ext of up to ~2 kOe is studied. An increase in the superconducting transition width ΔT_c with an increase in Hext is found. The presence of a system of weak links in MgB₂-based granular superconductors is established. On the basis of experimental data, MgB₂ granular superconductor is assigned to two-level superconducting systems and the H–T phase diagram is constructed.