Nuclear Spin-Lattice Relaxation Induced by Thermally Fluctuating Flux Lines in Type-II Superconductors

TL;DR

This paper presents a theoretical model explaining how thermally fluctuating flux lines in type-II superconductors cause nuclear spin-lattice relaxation, with results showing temperature and field-dependent relaxation rates.

Contribution

It introduces a harmonic fluctuation model to describe nuclear spin relaxation due to flux line dynamics in clean type-II superconductors, highlighting vibrational modes' role.

Findings

Relaxation rate $T_1^{-1}$ is greatly enhanced below $T_c$ at low fields.

$T_1^{-1}$ shows a peak at intermediate fields as a function of temperature.

Vibrational modes along flux lines are crucial in the relaxation process.

Abstract

Thermal motion of the flux lines (FL) gives rise to fluctuating magnetic fields. These dynamic fields couple to the nuclei in the sample and relax the nuclear spins. Based on a model of harmonic fluctuations, we provide a theoretical description of the nuclear spin-lattice relaxation (NSLR) process due to the fluctuating FLs in clean type-II superconductors. At low fields, the calculated longitudinal relaxation rate $T_1^{-1}$ is enormously enhanced at temperatures just below $T_c$. At intermediate fields, the resulting $T_1^{-1}$ exhibits a peak structure as a function of temperature, which is eventually suppressed as the field is increased. The vibrational modes which have components propagating along the FLs play an essential role in the $T_1^{-1}$ process.