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 (MgB2) in the vicinity of Tc and at lower temperatures. It was established that MgB2, like oxide-based high-temperature superconductors, exhibits a negative thermal expansion coefficient at low temperatures. The anomaly of α(T) in MgB2 is significantly affected by the magnetic field. It was established that, in addition to the well-known superconducting transition at Tc≈40 K, MgB2 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 MgB2 becomes negative. The low-temperature anomalies are related to the presence of a second group of charge carriers in MgB2 and to an increase in the density of the Bose condensate corresponding to these carriers at Tc2≈10–12 K.