Possible anisotropic order parameter in pyrochlore superconductor KOs ₂O ₆ probed by muon spin rotation

Magnetic penetration depth (λeff) in a pyrochlore superconductor KOs ₂O ₆ was studied by means of muon spin rotation (μSR). The temperature dependence of the transverse spin relaxation rate, which is proportional to λ _eff ^-2, exhibits a considerable deviation from that expected for the empirical two-fluid model (∝ T ⁴), obeying a T ² tendency. Furthermore, λ _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 KOs ₂O ₆ is anisotropic or multigapped with a small energy gap.