Possible anisotropic order parameter in pyrochlore superconductor KOs2O6 probed by muon spin rotation

Magnetic penetration depth (lambda(eff)) in a pyrochlore superconductor KOs2O6 was studied by means of muon spin rotation (mu SR). The temperature dependence of the transverse spin relaxation rate, which is proportional to lambda(-2)(eff), exhibits a considerable deviation from that expected for the empirical two-fluid model (proportional to T-4), obeying a T-2 tendency. Furthermore, lambda(eff) apparently increases with increasing applied field, suggesting the presence of nonlinear and nonlocal effects. The present result strongly suggests that the superconducting order parameter in KOsO6 is anisotropic or multigapped with a small energy gap.