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

A nuclear magnetic resonance (NMR) study has been carried out con a GaAs/Al 0.3Ga0.7As multiple-quantum-well sample, containing two-dimensional (2D) el ectron layers, in the quantum Hall regime. The spin polarization To of 2D e lectrons is determined from the hyperfine shift of Ga-69,Ga-71 nuclei, meas ured by standard pulsed NMR. We have used the tilted-magnetic field techniq ue to investigate the effect of an increasing Zeeman energy on To around La ndau level filling factor nu =1. Mostly on the basis of measurements perfor med at B = 17 T and T = 1.5 K, we conjecture that in the high-B and low-T l imit, P(nu) follows the predictions of the noninteracting-electron model. T he low-temperature P(nu =1) is found to be far below the expected full pola rization, and the observed nuclear spin-lattice relaxation rate is not redu ced, in contrast to the observations by optically pumped NMR. The two techn iques obtain different results only close to nu =1, and the reason why rema ins unclear; simple explanation in terms of the nonuniform electron density does not seem satisfactory.