Thermal conductivity of single-crystalline MgB2

The ab-plane thermal conductivity κ of single-crystalline hexagonal MgB₂ has been measured as a function of magnetic field H with orientations both parallel and perpendicular to the c axis and at temperatures between 0.5 and 300 K. In the mixed state, κ(H) measured at constant temperatures reveals features that are not typical for common type-II superconductors. The observed behavior may be associated with the field-induced reduction of two superconducting energy gaps, significantly different in magnitude. A nonlinear temperature dependence of the electronic thermal conductivity is observed in the field-induced normal state at low temperatures. This behavior is at variance with the law of Wiedemann and Franz, and suggests an unexpected instability of the electronic subsystem in the normal state at T~1 K.