Breakdown of the Korringa Law of Nuclear Spin Relaxation in Metallic GaAs

We present nuclear spin relaxation measurements in GaAs epilayers using a new pump-probe technique in all-electrical, lateral spin-valve devices. The measured T-1 times agree very well with NMR data available for T > 1 K. However, the nuclear spin relaxation rate clearly deviates from the well-established Korringa law expected in metallic samples and follows a sublinear temperature dependence T-1(-1) proportional to T-0.6 for 0.1 K <= T <= 10 K. Further, we investigate nuclear spin inhomogeneities.