Anomalous low-temperature behavior of the thermal characteristics of MgB2

Abstract

We have studied the behavior of the thermal expansion coefficient α( T ) (in a zero magnetic field and at H ≈4 T), the heat capacity C ( T ), and the thermal conductivity κ( T ) of magnesium boride (MgB 2 ) in the vicinity of T c and at lower temperatures. It was established that MgB 2 , like oxide-based high-temperature superconductors, exhibits a negative thermal expansion coefficient at low temperatures. The anomaly of α( T ) in MgB 2 is significantly affected by the magnetic field. It was established that, in addition to the well-known superconducting transition at T c ≈40 K, MgB 2 exhibits an anomalous behavior of both heat capacity and thermal conductivity in the region of T ≈10–12 K. The anomalies of C ( T ) and κ( T ) take place in the same temperature interval where the thermal expansion coefficient of MgB 2 becomes negative. The low-temperature anomalies are related to the presence of a second group of charge carriers in MgB 2 and to an increase in the density of the Bose condensate corresponding to these carriers at T c2 ≈10–12 K.