Superconducting properties of Na-doped C60 prepared from sodium azide

The Na-doped C60 superconductor prepared from sodium azide (NaN3) were systematically studied. Superconducting (SC) transition temperature (Tc) was evaluated to be 12–17 K from the low-field signal (LFS) of microwave absorption (MA) measurement. Several parameters such as critical field of the Josephson junction, the bulk Hc1(0) and Hc2(0), the London penetration depth and coherence length were estimated from LFS and SQUID experiments. The Pauli contribution increases with increasing SC volume fraction, but Curie spins are still detected at the low temperature even in the best sample with 38% of SC volume fraction.