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 ⁷¹Ga 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(ν, T) = ⟨S_z(ν, T)⟩/max ⟨S_z⟩, near ν = 1/3. The P(T) data at ν = 1/3 probe the neutral spin-flip excitations of a fractional quantum Hall ferromagnet. In addition, the saturated P(ν) drops on either side of ν = 1/3, even in a B_tot = 12T field. The observed depolarization is quite small, consistent with an average of ∼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 ν = 1/3.