MuSR and SQUID Investigation of Superconductivity in (NH_3)_0.75NaK_2C_60

TL;DR

This study investigates the superconducting properties of (NH_3)_0.75NaK_2C_60 using SQUID and MuSR techniques, revealing unusual magnetic behavior likely linked to polaronic effects rather than conventional theories.

Contribution

It provides new insights into the magnetic properties of (NH_3)_0.75NaK_2C_60, highlighting the role of polaronic instabilities in its superconductivity, which challenges traditional Ginzburg-Landau theory.

Findings

Very small lower critical field H_c1 (<1 G)

Small local field inhomogeneities from MuSR

High upper critical field H_c2 (~40 T)

Abstract

The family of superconducting fullerides (NH_3)_xNaK_2C_60 shows an anomalous correlation between T_c and lattice parameter. To better understand the origin of this anomaly we have studied a representative x=0.75 compound using SQUID magnetometry and MuSR 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.