Stretched exponential spin relaxation in organic superconductors

Abstract

Proton NMR measurements on the organic superconductor $\kappa -(ET{)}_{2}Cu[N(CN{)}_2]Br$ ($T_C=11.6$ 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 $\kappa -(ET{)}_{2}Cu[(NCS{)}_2]Br$ ($T_C=10.5$ 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 $\kappa -(ET{)}_{2}X$ conductors, including changes in conduction electron spin resonance, electronic phase separation and the onset of antiferromagnetic order. Analogous behavior in $L{a}_{2-x}S{r}_{x}Cu{O}_4$ suggests that a density wave may develop in $\kappa -(ET{)}_{2}Cu[N(CN{)}_2]Br$.

12 pages, 6 figures

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