Electron spin resonance and low-field microwave absorption of alkali-metal-doped fullerene C60 superconductors

Abstract

Alkali-metal-doped C[sub 60] superconductors formed by alkali-metal azide decomposition show a time evolution for the development of the superconducting phase after initial sample preparation which extends over about 15 h for storage at room temperature. This is studied quantitatively for K[sub 3]C[sub 60] and Rb[sub 3]C[sub 60], and differences are seen between these two systems which are consistent with redistribution of the alkali metal in the C[sub 60] matrix accounting for the time evolution observed. This is supported by separate experiments in which the storage temperature after sample preparation is decreased to 77 K. It is interesting that electron spin resonance (ESR) signals are seen in all of these samples in both the superconducting and nonsuperconducting temperature regions and that they show time evolutions after sample preparation that are qualitatively similar to the low-field microwave absorption (LFMA) signals which characterize the development of the superconducting phase. However, a detailed comparison shows differences between the ESR signal changes with time and the development of the superconducting phase which suggests that the origins of the ESR and LFMA signals are independent. It is interesting that similar ESR changes with storage time are observed in nonsuperconducting Cs[sub 3]C[sub 60] as are observed inmore » superconducting K[sub 3]C[sub 60] and Rb[sub 3]C[sub 60]. 10 refs., 10 figs.« less