Two-bandgap magneto-thermal conductivity of polycrystalline MgB2

The thermal conductivity κ(B,T) of polycrystalline superconducting MgB2 has been studied as a function of temperature (from 4 up to 50 K) and (up to a rather large 14 T) applied magnetic field. No anomaly in the thermal conductivity κ(B,T) is observed around the superconducting transition in the absence or presence of magnetic fields up to 14 T; at that field value the superconductivity of MgB2 persisted. The thermal conductivity in zero field shows a T-linear increase up to 50 K. The thermal conductivity is found to increase with increasing field at high fields. We interpret the findings as if there are two subsystems of quasiparticles with different field-dependent characters in a two- (L- and S-) band superconductor reacting differently with the vortex structure. The unusual enhancement of κ(B,T) at low temperature but higher than a (Bc2S ∼3 T) critical field is interpreted as a result of the overlap of the low energy states outside the vortex cores in the S band. An order of magnitude of the Bc2S temperature dependence is given, in agreement with recent results by other authors under different experimental conditions