Anomalous Flux Pinning in ?-Pyrochlore Oxide Superconductor KOs2O6

The superconducting transition of the ?-pyrochlore oxide KOs2O6 with Tc = 9.60 K is studied by resistivity measurements under various magnetic fields using a high-quality single crystal. The reentrant behavior of superconductivity is observed near Tc in low magnetic fields below 2 T. The recovered resistance probably due to flux flow exhibits an anomalous angle dependence, indicating that flux pinning is enhanced in magnetic fields along certain crystallographic directions such as [110], [001] and [112]. It is suggested that there is an intrinsic pinning mechanism coming from the specific crystal structure of the ?-pyrochlore oxide.