Optically Pumped Nuclear Magnetic Resonance Measurements of the Electron Spin Polarization in GaAs Quantum Wells near Landau Level Filling Factorν=13

The Knight shift of the ${}^{71}\mathrm{Ga}$ 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, $P(\nu ,T)\equiv \frac{〈S_z(\nu ,T)〉}{\max 〈S_z〉}$, near $\nu =\frac{1}{3}$. The $P\left(T\right)$ data at $\nu =\frac{1}{3}$ probe the neutral spin-flip excitations of a fractional quantum Hall ferromagnet. In addition, the saturated $P\left(\nu \right)$ drops on either side of $\nu =\frac{1}{3}$, even in a $B_{\mathrm{tot}}=12\mathrm{}\mathrm{T}$ field. The observed depolarization is quite small, consistent with an average of $\sim 0.1$ 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 $\nu =\frac{1}{3}$.