The Knight shift and the nuclear spin-lattice relaxation rate ${1/T}_1$ of ${}^{71}\mathrm{Ga}$ nuclei are measured in GaAs quantum wells at Landau level filling factor $\nu =\frac{1}{2}$ using optically pumped NMR. The data are compared with predictions of a weakly interacting composite fermion model. Our thermodynamic measurements of the electron spin polarization and spin dynamics near the transition between fully and partially spin-polarized ground states provide new constraints on the theoretical description of the $\nu =\frac{1}{2}$ state.

• Received 19 July 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.5074