Solid-state $^3\mathrm{He}$ NMR of the superconducting rubidium endofulleride $\mathrm{Rb_3(^3He@C_{60})}$

TL;DR

This study uses solid-state $^3$He NMR to investigate a new superconducting fulleride with encapsulated helium, revealing complex vortex phases and unusual relaxation behavior that persists above the transition temperature.

Contribution

It introduces a novel $^3$He-encapsulated superconducting fulleride and demonstrates the use of NMR to probe vortex phases and relaxation dynamics in this material.

Findings

Evidence for co-existing vortex liquid and solid phases.

Frequency-dependent spin-lattice relaxation times.

Persistence of relaxation phenomena above $T_c$.

Abstract

A new variant of the superconducting fulleride $\mathrm{Rb_{3}C_{60}}$ is presented, with $\mathrm{^{3}He}$ atoms encapsulated in the $\mathrm{C_{60}}$ cages. The $\mathrm{^{3}He}$ nuclei act as sensitive NMR probes embedded in the material. The superconducting and normal states are characterised by $\mathrm{^{3}He}$ NMR. Evidence is found for co-existing 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 utilising an inverse Laplace transform. Surprisingly, this phenomenon persists, in attenuated form, at temperatures well above the superconducting transition.