Thermal Conductivity of the Pyrochlore Superconductor KOs_2O_6: Strong Electron Correlations and Fully Gapped Superconductivity

摘要

To elucidate the nature of the superconducting ground state of the geometrically frustrated pyrochlore KOs_2O_6 (T_c = 9.6 K), the thermal conductivity was measured down to low temperatures (～T_c/100). We found that the quasiparticle mean free path is strikingly enhanced below a transition at T_p = 7.8 K, indicating enormous electron inelastic scattering in the normal state. In magnetic fields, the conduction at T → 0 K is nearly constant up to ～0.4H_(c2), in contrast with the rapid growth expected for superconductors with an anisotropic gap. This unambiguously indicates a fully gapped superconductivity, in contrast with previous studies. These results highlight that KOs_2O_6 is unique among superconductors with strong electron correlations.