Stretched exponential spin relaxation in organic superconductors

Proton NMR measurements on the organic superconductor kappa-(ET)(2)Cu[N(CN)(2)]Br [where ET represents bis(ethylenedithio)tetrathiafulvalene] (T-C = 11.6 K) exhibit stretched exponential spin-lattice relaxation below T approximate to 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) (T-C = 10.5 K) shows single exponential spin-lattice relaxation and a conventional Nernst effect. The temperature range of 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 La2-xSrxCuO4 suggests that a spin glass or density wave may develop in kappa-(ET)(2)Cu[N(CN)(2)]Br.