We report record high 29Si spin polarization obtained using dynamic nuclear polarization in microcrystalline silicon powder. Unpaired electrons in this silicon powder are due to dangling bonds in the amorphous region of this intrinsically heterogeneous sample. 29Si nuclei in the amorphous region become polarized by forced electron-nuclear spin flips driven by off-resonant microwave radiation while nuclei in the crystalline region are polarized by spin diffusion across crystalline boundaries. Hyperpolarized silicon microparticles have long T1 relaxation times and could be used as tracers for magnetic resonance imaging.

NMR spin echo measurements of 29 Si in Silicon powders have uncovered a variety of surprising phenomena that appear to be independent of doping. These surprises include long tails and even-odd asymmetry in Carr-Purcell-Meiboom-Gill (CPMG) echo trains, and anomalous stimulated echoes with several peculiar characteristics. Given the simplicity of this spin system, these results, which to date defy explanation, present a new and interesting puzzle in solid state NMR.PACS numbers: 03.65. Yz, 03.67.Lx, 76.20.+q, 76.60.Lz In order to implement quantum computation (QC) based upon spins in semiconductors [1,2,3,4,5], a detailed understanding of spin dynamics in these materials is required. To this end, we carried out a series of NMR measurements that were motivated by a simple question: what is the 29 Si decoherence time (T 2 ) in Silicon? Earlier NMR studies in Silicon addressed other questions [6,7,8].We find that it is possible to detect the 29 Si (4.67% natural abundance (n.a.), spin-1 2 ) NMR signals out to much longer times than was previously thought possible, and so far, we have been unable to explain these results in terms of well-known NMR theory [9]. Surprises in such a simple spin system appear brand new to NMR, and understanding their origin is of fundamental importance. In this paper, we describe the phenomena and recount tests we have made to explore possible explanations.Two standard experiments that measure T 2 are reported. First, using the Hahn echo sequence (HE: 90 X -TE 2 -180 Y -TE 2 -ECHO [10]), the measured decay, with T 2HE ≈ 5.6 msec, is in quantitative agreement with that expected for the static 29 Si-29 Si dipolar interaction. This decay mechanism is commonly encountered in solids, and a number of ingenious pulse sequences have been invented to manipulate the interaction Hamiltonian, pushing echoes out to times well beyond T 2HE [9,11,12,13,14,15,16,17]. A common thread running through those sequences is the use of multiple 90 • pulses, and pulses applied frequently compared to T 2HE , which refocus the homonuclear dipolar coupling. The same cannot be said about the second sequence that we used to measure T 2 , the Carr-Purcell-Meiboom-Gill sequence (CPMG: 90 X -TE 2 -180 Y -TE 2 -ECHO repeat n−times [18]). Specifically, the CPMG sequence is not expected to excite echoes beyond T 2HE , since 180 • pulses should not affect the bilinear homonuclear interaction. This statement is exact in two important limits: either for unlike spins or for magnetically-equivalent spins.Therefore, we were surprised to find that CPMG echoes are detectable long after T 2HE , and the echo peaks appear nearly identical in Silicon samples with very different dopings. This CPMG "tail" appears to be even larger at low temperatures. In addition, as the interpulse

In spectroscopy, it is conventional to treat pulses much stronger than the linewidth as deltafunctions. In NMR, this assumption leads to the prediction that π pulses do not refocus the dipolar coupling. However, NMR spin echo measurements in dipolar solids defy these conventional expectations when more than one π pulse is used. Observed effects include a long tail in the CPMG echo train for short delays between π pulses, an even-odd asymmetry in the echo amplitudes for long delays, an unusual fingerprint pattern for intermediate delays, and a strong sensitivity to π-pulse phase. Experiments that set limits on possible extrinsic causes for the phenomena are reported. We find that the action of the system's internal Hamiltonian during any real pulse is sufficient to cause the effects. Exact numerical calculations, combined with average Hamiltonian theory, identify novel terms that are sensitive to parameters such as pulse phase, dipolar coupling, and system size. Visualization of the entire density matrix shows a unique flow of quantum coherence from non-observable to observable channels when applying repeated π pulses.

NMR spin echo measurements of 13C in C60, 89Y in Y2O3, and 29Si in silicon are shown to defy conventional expectations when more than one pi pulse is used. Multiple pi-pulse echo trains may either freeze out or accelerate the decay of the signal, depending on the pi-pulse phase. Average Hamiltonian theory, combined with exact quantum calculations, reveals an intrinsic cause for these coherent phenomena: the dipolar coupling has a many-body effect during any real, finite pulse.

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