RETRACTED: Unprecedented superconductivity in β-pyrochlore osmate KOs2O6 (Retracted Article)

Superconductivity in the potassium osmium oxide KOs2O6 crystallizing in the beta-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 alpha-type pyrochlore oxide Cd2Re2O7 which is believed to be a conventional s-wave superconductor. The superconducting transition temperature T-c of KOs2O6 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.