MAGNETIC-FIELD DEPENDENCE OF THE OPTICAL OVERHAUSER EFFECT IN GAAS

When nuclear spin order is induced by optical excitation near the band gap of a semiconductor such as GaAs, the effect is referred to as opt ical pumping. This paper presents measurements of the optical pumping rate in semi-insulating GaAs over the magnetic field range of 0-24 T a t temperatures of 1.5 K and 4.2 K. The enhanced nuclear polarization w as sampled by radio wave detected NMR. The data were recorded using Bi tter-type magnets which permitted rapid ramping between the pumping an d sampling fields in a time short compared to the nuclear spin lattice relaxation time in the dark. The field dependence has been fitted to a relaxation model which includes spin diffusion and dark relaxation t erms. Fits were obtained by fixing the g factor to its literature valu e. The fitted parameters include the correlation time for electron spi n-density fluctuations, the average hyperfine field, and the nuclear s pin diffusion coefficient.