Spin polarization of two-dimensional electrons in GaAs quantum wells around Landau level filling (formula presented) from NMR measurements of gallium nuclei

A nuclear magnetic resonance (NMR) study has been carried out on a (formula presented) multiple-quantum-well sample, containing two-dimensional (2D) electron layers, in the quantum Hall regime. The spin polarization (formula presented) of 2D electrons is determined from the hyperfine shift of (formula presented) nuclei, measured by standard pulsed NMR. We have used the tilted-magnetic field technique to investigate the effect of an increasing Zeeman energy on (formula presented) around Landau level filling factor (formula presented) Mostly on the basis of measurements performed at (formula presented) and (formula presented) we conjecture that in the high-B and low-T limit, (formula presented) follows the predictions of the noninteracting-electron model. The low-temperature (formula presented) 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 (formula presented) and the reason why remains unclear; simple explanation in terms of the nonuniform electron density does not seem satisfactory.