Optically pumped NMR studies of electron spin polarization and dynamics: New constraints on the composite fermion description of nu=1/2

The Knight shift and the nuclear spin-lattice relaxation rate 1/T-1 of Ga-7 1 nuclei are measured in GaAs quantum wells at Landau level filling factor nu = 1/2 using optically Dumped NMR. The data are compared with predictions of a weakly interacting composite fermion model. Our thermodynamic measure ments of the electron spin polarization and spin dynamics near the transiti on between fully and partially spin-polarized ground states provide new con straints on the theoretical description of the nu = 1/2 state.