Stretched exponential spin relaxation in organic superconductors

TL;DR

This study reveals inhomogeneous magnetic phases in organic superconductor $-(ET)_2Cu[N(CN)_2]Br$ through stretched exponential NMR relaxation, indicating complex magnetic behavior coexisting with superconductivity, unlike the related compound which shows conventional relaxation.

Contribution

It demonstrates the presence of inhomogeneous magnetic phases in a specific organic superconductor using proton NMR, highlighting differences with related compounds and suggesting density wave development.

Findings

Stretched exponential relaxation indicates inhomogeneous magnetic phases.

Inhomogeneous phase onset coincides with a large Nernst signal.

Contrasts with related superconductor showing single exponential relaxation.

Abstract

Proton NMR measurements on the organic superconductor $\kappa-(ET)_2Cu[N(CN)_2]Br$ ($T_C = 11.6$ K) exhibit stretched exponential spin-lattice relaxation below $T\approx 25$ K, suggestive of an inhomogeneous magnetic phase that develops in the normal state and coexists with superconductivity. The onset of this phase coincides approximately with a large normal state Nernst signal reported previously. By contrast, the closely related superconductor $\kappa-(ET)_2Cu[(NCS)_2]Br$ ($T_C = 10.5$ K) shows single exponential spin-lattice relaxation and a conventional Nernst effect. The temperature range $T_C < T < 30$ K encompasses several phenomena in the $\kappa-(ET)_2X$ conductors, including changes in conduction electron spin resonance, electronic phase separation and the onset of antiferromagnetic order. Analogous behavior in $La_{2-x}Sr_xCuO_4$ suggests that a density wave may develop in $\kappa-(ET)_2Cu[N(CN)_2]Br$.