Optically pumped nuclear magnetic resonance (OPNMR) measurements were performed in two different electron-doped multiple quantum well samples near the fractional quantum Hall effect ground state nu = 13. Below 0.5 kelvin, the spectra provide evidence that spin-reversed charged excitations of the nu = 13 ground state are localized over the NMR time scale of about 40 microseconds. Furthermore, by varying NMR pulse parameters, the electron spin temperature (as measured by the Knight shift) could be driven above the lattice temperature, which shows that the value of the electron spin-lattice relaxation time tau1s is between 100 microseconds and 500 milliseconds at nu = 13.

The Knight shift of 71 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(ν, T ) ≡ Sz (ν,T ) max Sz , near ν= 1 3. The P(T ) data at ν= 1 3 probe the neutral spinflip excitations of a fractional quantum Hall ferromagnet. In addition, the saturated P(ν) drops on either side of ν= 1 3, even in a Btot=12 Tesla field. The observed depolarization is quite small, 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 ν= The electron spin played no role in the earliest theory [1] of the fractional quantum Hall effect (FQHE) [2], where the Zeeman energy E Z ≡ g * µ e B tot was assumed to be infinite. However, for a two-dimensional electron system (2DES) in GaAs, E Z is only ∼ 1 70 of the electron-electron Coulomb energy E C ≡ e 2 /ǫ l B ∼ 160 K at 10 Tesla, raising the possibility that interactions can lead to quantum Hall states with non-trivial spin configurations [3]. This idea underlies the recent theoretical predictions [4,5] that the charged excitations of the ν=1 integer quantum Hall ground state are novel spin-textures called skyrmions, with experimentally observable consequences [6-9] (Here ν≡n/n B , where n is the number of electrons per unit area, and n B = eB/hc ≡ 1/2πl 2

Optically pumped nuclear magnetic resonance measurements of 71 Ga spectra were carried out in an n-doped GaAs/Al0.1Ga0.9As multiple quantum well sample near the integer quantum Hall ground state ν=1. As the temperature is lowered (down to T ≈ 0.3 K), a "tilted plateau" emerges in the Knight shift data, which is a novel experimental signature of quasiparticle localization. The dependence of the spectra on both T and ν suggests that the localization is a collective process. The frozen limit spectra appear to rule out a 2D lattice of conventional Skyrmions.One of the most surprising twists in the recent history of the quantum Hall effects [1] was the prediction [2] that novel spin textures called Skyrmions can be the charged quasiparticles introduced by small deviations (|δν|) from ferromagnetic quantum Hall ground states [3] (e.g., at Landau level filling factor ν=1 or 1 3 ). A Skyrmion has an effective number of spin reversals K and "size" λ that are determined by the competition between the Coulomb energy (which increases both) and the Zeeman energy (which reduces both). Qualitatively, this cylindrically symmetric spin texture has a down spin at r=0 and a smooth radial transition to up spins at r=∞. In between, the nonzero XY spin components have a vortical configuration [2,4]. The addition of Skyrmions to the ν=1 ground state was predicted to result in a rapid drop in the electron spin polarization, as |δν| is increased [5]. Several experiments are consistent with this [6][7][8] In this Letter, we report the first spectroscopic evidence for Skyrmion localization. The multiple quantum well sample used in this work was previously studied at higher temperatures [6,10]. The new data presented here were obtained by extending the optically pumped nuclear magnetic resonance (OPNMR) technique [22] to lower temperatures (T ≈ 0.3 K) as described elsewhere [23,24]. Figure 1 shows some OPNMR spectra for ν close to one. Nuclei within the quantum wells are coupled to the spins of the two-dimensional electron system via the isotropic Fermi contact interaction [25], which shifts the corresponding well resonance (labeled "W " on

The Knight shift and the nuclear spin-lattice relaxation rate 1/T1 of 71 Ga nuclei are measured in GaAs quantum wells at Landau level filling factor ν = 1/2 using optically pumped NMR. The temperature dependences of these data are compared with predictions of a weakly-interacting composite fermion model. Our 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 ν = 1/2.

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