Optically pumped nuclear magnetic resonance measurements of the electron spin polarization in GaAs quantum wells near Landau level filling factor ν=1/3

The Knight shift of the Ga-71 nuclei is measured in two different electron-doped multiple quantum well samples using optically pumped NMR. These data are the first direct measurements of the electron spin polarization, 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 observed depolarization is quite small, consistent with an average of similar to 0.1 spin flips per quasihole (or quasiparticle), a value which does not appear to be explicable by the current theoretical understanding of the fractional quantum Hall effect near nu = 1/3.