Muon spin rotation and SQUID investigation of superconductivity in (NH₃)_xNaK₂C₆₀(x∼0.7)

The family of superconducting fullerides (NH₃)_xNaK₂C₆₀ shows an anomalous correlation between T_c and lattice parameter. To better understand the origin of this anomaly we have studied a representative x=0.72 compound using superconducting quantum interference device (SQUID) magnetometry and muon spin rotation spectroscopy. The lower critical field H_c1, measured by the trapped magnetization method, is less than 1 G, a very small value as compared with that of other fullerides. Muon spin depolarization in the superconducting phase shows also quite small local-field inhomogeneities, of the order of those arising from nuclear dipolar fields. On the other hand, the 40-T value for H_c2, as extracted from magnetometry data, is comparable to that of other fullerides. We show that these observations cannot be rationalized within the framework of the Ginzburg-Landau theory of superconductivity. Instead, the anomalous magnetic properties could be interpreted taking into account the role played by polaronic instabilities in this material.