OPTICALLY PUMPED NUCLEAR-MAGNETIC-RESONANCE MEASUREMENTS OF THE ELECTRON-SPIN POLARIZATION IN GAAS QUANTUM-WELLS NEAR LANDAU-LEVEL FILLING FACTOR NU=1 3/

The Knight shift of the Ga-71 nuclei is measured in two different elec tron-doped multiple quantum well samples using optically pumped NMR. T hese data are the first direct measurements of the electron spin polar ization, P(nu, T) equivalent to [S-z(nu,T)]/max[S-z] , near nu = 1/3. The P(T) data at nu = 1/ 3 probe the neutral spin-flip excitations of a fractional quantum Hall ferromagnet. In addition, the saturated P(nu ) drops on either side of nu = 1/3 even in a B-tot = 12 T field. The o bserved depolarization is quite small, consistent with an average of s imilar to 0.1 spin flips per quasihole (or quasiparticle), a value whi ch does not appear to be explicable by the current theoretical underst anding of the fractional quantum Hall effect near nu = 1/3.