Possible unconventional superconductivity and weak magnetism in NaxCoO2·yH2O probed by μSR

The superconducting property of sodium cobalt oxyhydrate, Na0.35CoO2·1.3H2O, has been studied by means of muon spin relaxation (μSR) down to 2 K. It was found that the zero-field muon spin relaxation rate is independent of temperature, indicating that no static magnetism appears in this compound above 2 K. The result also provides evidence against the breakdown of time-reversal symmetry for the superconducting order parameter. In water excess sample Na0.334CoO2·1.32H2O, which shows no superconductivity, the weak magnetism was observed below 6 K. This fact suggests that the magnetic interaction plays an important role for the appearance of the unconventional superconductivity.