Optically Pumped NMR Measurements of the Electron Spin Polarization in   GaAs Quantum Wells near Landau Level Filling Factor nu=1/3
  The Knight shift of 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)=<Sz(nu,T)>/max<Sz>, 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 Btot=12 Tesla field. The observed depolarization is quite small, consistent with an average of about 0.1 spin-flips per quasihole (or quasiparticle), a value which does not appear to be explicable by the current theoretical understanding of the FQHE near nu=1/3. 
P. Khandelwal (1), N. N. Kuzma (1), S. E. Barrett (1), L. N. Pfeiffer   (2) and K. W. West (2) ((1) Dept. of Physics, Yale U...