Calculated NMR T_2 relaxation due to vortex vibrations in cuprate superconductors

TL;DR

This paper models the NMR T_2 relaxation caused by vortex vibrations in cuprate superconductors, finding qualitative agreement with experiments but discrepancies in relaxation rates and time dependence.

Contribution

It provides a theoretical calculation of vortex-induced T_2 relaxation in YBa_2Cu_3O_7, challenging previous assumptions about vortex motion's role in T_1 relaxation.

Findings

Relaxation variation matches experimental trends

Calculated rates are two orders of magnitude slower than observed

Time dependence differs from experimental measurements

Abstract

We calculate the rate of transverse relaxation arising from vortex motion in the mixed state of YBa_2Cu_3O_7 with the static field applied along the c axis. The vortex dynamics are described by an overdamped Langevin equation with a harmonic elastic free energy. We find that the variation of the relaxation with temperature, average magnetic field, and local field is consistent with experiments; however, the calculated time dependence is different from what has been measured and the value of the rates calculated is roughly two orders of magnitude slower than what is observed. Combined with the strong experimental evidence pointing to vortex motion as the dominant mechanism for T_2 relaxation, these results call into question a prior conclusion that vortex motion is not significant in T_1 measurements in the vortex state.