Optically pumped NMR study of electron spin dynamics in the fractional quantum Hall regime

A new optically pumped nuclear magnetic resonance (OPNMR) laboratory has been built at Yale to extend the study of spin physics in the quantum Hall regimes to lower temperatures (T ∼ 300 mK) and higher magnetic fields (B ∼ 12 T).; This facility was used to measure the electron spin polarization P as a function of temperature and the Landau level filling factor nu around nu = 1/3 in two different electron-doped multiple quantum well samples. In addition, our OPNMR measurements provided the first evidence that below T ≈ 500 mK, the spin-reversed charged excitations of the nu = 1/3 ground state become spatially localized over the NMR time scale of about 40 microseconds.; The two-dimensional electron spin system at nu = 1/3 could also be driven out of equilibrium by varying NMR pulse parameters, which led to the establishment of electron spin temperature different from the crystalline lattice temperature. Observation of this effect implies that for T < 500 mK the electron spin-lattice relaxation time t1s is between 100 microseconds and 500 milliseconds at nu = 1/3.; We also report a preliminary attempt to use OPNMR as a probe of material properties in heterostructures made of GaN---a novel semiconductor with a band gap in the blue region of visible spectrum. Our first NMR measurements of the quadrupole shift and the linewidth at 1.5 K < T < 300 K provide evidence that the local electric field gradients have an angular scatter of +/-2°, exceeding the spread in the local crystalline lattice orientations by a factor of ∼20.