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) is proportional to T(0.6) for 0.1 K≤T≤10 K. Further, we investigate nuclear spin inhomogeneities.