  Superconductivity in the potassium osmium oxide KOs2O6 crystallizing in the
beta-pyrochlore structure is studied by means of electrical resistivity,
magnetic susceptibility and specific heat. It is the second superconductor in
the family of pyrochlore oxides, following the alfa-type pyrochlore oxide
Cd2Re2O7 which is believed to be a conventional s-wave superconductor. The
superconducting transition temperature Tc of KOs2O6 is 9.6 K, almost one order
higher than the Tc = 1.0 K of Cd2Re2O7. Moreover, the superconductivity of
KOs2O6 is remarkably robust under high magnetic fields, with a large upper
critical magnetic field Hc2 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 Hc2 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.
