Optically pumped NMR in the quantum Hall regimes

We review our recent measurements of the ⁷¹Ga Knight shift and linewidth 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, ℘(v, T) ≡ 〈S_z(v, T)〉/max〈S_z〉, near v = 1/3. The ℘(T) data at v = 1/3 probe the neutral spin-flip excitations of a fractional quantum Hall ferromagnet. In addition, the drop in the saturated ℘(v) on either side of v = 1/3 is 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 FQHE near v = 1/3. Below 0.5 K, the spectra also provide evidence that spin-reversed charged excitations of the v = 1/3 ground state are localized over the NMR time scale of ∼40 ms.