Spin polarization of two-dimensional electrons in GaAs quantum wells around Landau level filling ν=1 from NMR measurements of gallium nuclei -: art. no. 085327

A nuclear magnetic resonance (NMR) study has been carried out con a GaAs/Al0.3Ga0.7As multiple-quantum-well sample, containing two-dimensional (2D) electron layers, in the quantum Hall regime. The spin polarization To of 2D electrons is determined from the hyperfine shift of Ga-69,Ga-71 nuclei, measured by standard pulsed NMR. We have used the tilted-magnetic field technique to investigate the effect of an increasing Zeeman energy on To around Landau level filling factor nu =1. Mostly on the basis of measurements performed at B = 17 T and T = 1.5 K, we conjecture that in the high-B and low-T limit, P(nu) follows the predictions of the noninteracting-electron model. The low-temperature P(nu =1) is found to be far below the expected full polarization, and the observed nuclear spin-lattice relaxation rate is not reduced, in contrast to the observations by optically pumped NMR. The two techniques obtain different results only close to nu =1, and the reason why remains unclear; simple explanation in terms of the nonuniform electron density does not seem satisfactory.