Thermal conductivity and annealing effects in the iron-based superconductor FeSe_0.3Te_0.7

Thermal conductivity measurements in magnetic fields have been performed for FeSe_0.3Te_0.7 single crystals as-grown and annealed at 400 °C for 100 h in vacuum (∼10^-4 Pa). It has been found that the thermal conductivity in the ab-plane, κab, of the annealed crystal shows an enhancement at low temperatures just below the superconducting transition temperature, T_c, owing to the thermal conductivity due to quasiparticles, while the κab of the as-grown crystal does not. This suggests that FeSe1-Tex is a strongly correlated electron system. It has also been found that both the degree of the enhancement of κab just below T_c and the behavior of the suppression of κab by the application of magnetic field for the annealed crystal depend on the thickness of the crystal. These results indicate that bulk superconductivity is absent in the as-grown crystal, appears only in the surface area in the annealed-thick crystal and appears not only in the surface area but also inside of the annealed-thin crystal. It has been concluded that excess Fe included in the as-grown crystal is removed from the surface by annealing.