NMR Study on the Vortex Slush Phase in Organic Superconductor \kappa-(BEDT-TTF)2Cu(NCS)2

TL;DR

This study uses 1H-NMR to investigate the vortex states in an organic superconductor, revealing a vortex glass-liquid transition and a quantum vortex slush state near the upper critical field.

Contribution

It provides the first NMR evidence of the vortex glass-liquid transition and quantum vortex slush state in -(BEDT-TTF)2Cu(NCS)2, highlighting the role of vortex fluctuations.

Findings

Vortex glass-liquid transition observed at ~0.75 T.

Divergence in nuclear spin relaxation rate near transition.

Change in relaxation curve curvature indicating quantum vortex slush.

Abstract

The vortex state in a single crystal of the layered organic superconductor \kappa-(BEDT-TTF2Cu(NCS)2, where BEDT-TTF (or ET) is bis(ethylenedithio)tetrathiafulvalene, was studied by 1H-NMR. Under a low field region around 0.75 T, the vortex glass-liquid transition was demonstrated by a diverging of the longitudinal nuclear spin relaxation rate and peak-broadening in spectra. Under a high field region near the upper critical field Hc2(0)~7 T, the curvature of nuclear spin relaxation curves showed a drastic change at the temperature where the emergence of the quantum vortex slush state was reported. The mechanism in this curvature change was discussed in terms of the fluctuating field produced by fragments of vortex glass.