Phase transitions in a Mg[B.sub.2] granular BCS superconductor in weak magnetic fields

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