Solid-State ³He NMR of the Superconducting Rubidium Endofulleride Rb₃(³He@C₆₀)

A new variant of the superconducting fulleride Rb3C60 is presented, with He-3 atoms encapsulated in the C-60 cages. The 3He nuclei act as sensitive NMR probes embedded in the material. The superconducting and normal states are characterized by He-3 NMR. Evidence is found for coexisting vortex liquid and vortex solid phases below the superconducting transition temperature. A strong dependence of the spin-lattice relaxation time constant on spectral frequency is observed in the superconducting state, as revealed by two-dimensional NMR utilizing an inverse Laplace transform. Surprisingly, this phenomenon persists, in attenuated form, at temperatures well above the superconducting transition.