Stretched exponential spin relaxation in organic superconductors

Proton NMR measurements on the organic superconductor Kappa -(ET) sub(2) Cu[N(CN) sub(2)]Br [where ET represents bis(ethylenedithio)tetrathiafulvalene] (T sub(C) = 11.6 K) exhibit stretched exponential spin-lattice relaxation below T [asymptotically =] 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) sub(2)Cu(NCS) sub(2) (T sub(C) = 10.5 K) shows single exponential spin-lattice relaxation and a conventional Nernst effect. The temperature range of T sub(C) < T < 30 K encompasses several phenomena in the Kappa -(ET) sub(2)X conductors, including changes in conduction-electron-spin resonance, electronic phase separation, and the onset of antiferromagnetic order. Analogous behavior in La sub(2-x) Sr sub(x)CuO sub(4) suggests that a spin glass or density wave may develop in Kappa -(ET) sub(2) Cu[N(CN) sub(2)]Br.