Unprecedented superconductivity in β-pyrochlore osmate KOs ₂O₆

Superconductivity in the potassium osmium oxide KOs₂O ₆ crystallizing in the β-pyrochlore structure is studied by electrical resistivity, magnetic susceptibility and specific heat measurements. It is the second superconductor in the family of pyrochlore oxides, following the α-type pyrochlore oxide Cd₂Re₂O₇ which is believed to be a conventional s-wave superconductor. The superconducting transition temperature T_c of KOs₂O ₆ is 9.6 K, almost one order higher than the T_c -1.0 K of Cd₂Re₂O₇. Moreover, the superconductivity of KOs₂O₆ is markedly robust under high magnetic fields, with a large upper critical magnetic field H_c2 of about 38 T, which seems to exceed Pauli's limit expected for conventional superconductivity. This is also in contrast to the case of Cd₂Re₂O₇, in which the H_C2 is 0.29 T, much smaller than the corresponding Pauli's limit. These distinct contrasts strongly suggest that the mechanism of superconductivity is essentially different between the two pyrochlore oxides.