Doping and temperature dependence of nuclear spin relaxation in n-type GaAs

We investigate the strong field nuclear spin relaxation rate in n-type GaAs for doping densities from the quasi-insulating over the metal-to-insulator up to the quasimetallic regime. The rate measured at 6.5 K increases in the quasi-insulating regime with doping density due to nuclear spin diffusion to the donor electrons and shows a distinct maximum at the critical density of the Mott metal-to-insulator transition. The density dependence of the nuclear spin relaxation rate can be quantitatively calculated over the whole density regime taking into account the effective number of localized electrons and the interaction of free electrons via the Korringa mechanism. Only the nuclear spin relaxation rate of the very lowest doped sample shows a significant deviation from these calculations. Temperature-dependent measurements suggest in this case an additional nuclear spin relaxation channel which is negligible at higher doping densities and is linked to the electron spin relaxation time.