Nuclear spin relaxation mediated by Fermi-edge electrons in $n$-type GaAs

A method based on the optical orientation technique was developed to measure the nuclear-spin lattice relaxation time $T_1$ in semiconductors. It was applied to bulk $n$-type GaAs, where $T_1$ was measured after switching off the optical excitation in magnetic fields from $400$ to $1200$ G at low ($<30\,$K) temperatures. The spin-lattice relaxation of nuclei in the studied sample with $n_D = 9 \cdot 10^{16}$ cm$^{-3}$ was found to be determined by hyperfine scattering of itinerant electrons (Korringa mechanism) which predicts invariability of $T_1$ with the change of magnetic field and linear dependence of the relaxation rate on temperature. This result extends the experimentally verified applicability of the Korringa relaxation law in degenerate semiconductors, previously studied in strong magnetic fields (several Tesla), to the moderate field range.