Electronic states in gallium arsenide quantum wells probed by optically pumped NMR

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Science  09 Jun 1995:
Vol. 268, Issue 5216, pp. 1460-1463
DOI: 10.1126/science.7539550


An optical pumping technique was used to enhance and localizenuclear magnetic resonance (NMR) signals from an n-dopedGaAs/Al0.1Ga0.9As multiple quantum well structure, permitting directradio-frequency measurements of gallium-71 NMR spectra and nuclearspin-lattice relaxation rates (1/T1) as functions of temperature (1.6K < T < 4.2 K) and the Landau level filling factor (0.66 < v< 1.76). The measurements reveal effects of electron-electroninteractions on the energy levels and spin states of thetwo-dimensional electron system confined in the GaAs wells. Minima in1/T1 at v approximately 1 and v approximately 2/3 indicate energy gapsfor electronic excitations in both integer and fractional quantum Hallstates. Rapid, temperature-independent relaxation at intermediate vvalues indicates a manifold of low-lying electronic states with mixedspin polarizations.