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

The character of temperature dependences of the electric conductivity of MgB2 granular BCS superconductors at temperatures of ~35–45 K in external magnetic fields Hext of up to ~2 kOe is studied. An increase in the superconducting transition width ΔTc 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.