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Muon spin rotation and SQUID investigation of superconductivity in (NH3)xNaK2C60 (x∼0.7)

The family of superconducting fullerides (NH 3 ) x NaK 2 C 6 0 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 c 1 , 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 c 2 , 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.