Temperature Dependence of the Magnetic Penetration Depth in the Vortex State of the Pyrochlore Superconductor, [Formula presented]

We report transverse-field and zero-field muon spin rotation and relaxation studies of the superconducting rhenium oxide pyrochlore, [Formula presented]. Transverse-field measurements ([Formula presented]) show line broadening below [Formula presented], which is characteristic of a vortex state, demonstrating conclusively the type-II nature of this superconductor. The penetration depth is seen to level off below about 400 mK [Formula presented], with a rather large value of [Formula presented]. The temperature independent behavior below [Formula presented] is consistent with a nodeless superconducting energy gap. Zero-field measurements indicate no static magnetic fields developing below the transition temperature.