Stretched exponential spin relaxation in organic superconductors

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 $\kappa-(ET)_2Cu[N(CN)_2]Br$.