Sean Barrett

OPTICALLY PUMPED NUCLEAR MAGNETIC RESONANCE MEASUREMENTS OF THE ELECTRON SPIN POLARIZATION IN GAAS QUANTUM WELLS NEAR LANDAU LEVEL FILLING FACTOR NU = 1/3

The Knight shift of Ga-71 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(nu,T)= /max , near nu=1/3. The P(T) data at nu=1/3 probe the neutral spin-flip excitations of a fractional quantum Hall ferromagnet. In addition, the saturated P(nu) drops on either side of nu=1/3, even in a Btot=12 Tesla field. The observed depolarization is quite small, consistent with an average of about 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 nu=1/3.

Optically Pumped NMR Studies of Electron Spin Polarization and Dynamics: New Constraints on the Composite Fermion Description ofν=1/2

The Knight shift and the nuclear spin-lattice relaxation rate 1/T1 of Ga-71 nuclei are measured in GaAs quantum wells at Landau level filling factor nu = 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 nu = 1/2.

Phosphorus-31 MRI of hard and soft solids using quadratic echo line-narrowing

Magnetic resonance imaging (MRI) of solids is rarely attempted. One of the main reasons is that the broader MR linewidths, compared to the narrow resonance of the hydrogen (1H) in free water, limit both the attainable spatial resolution and the signal-to-noise ratio. Basic physics research, stimulated by the quest to build a quantum computer, gave rise to a unique MR pulse sequence that offers a solution to this long-standing problem. The “quadratic echo” significantly narrows the broad MR spectrum of solids. Applying field gradients in sync with this line-narrowing sequence offers a fresh approach to carry out MRI of hard and soft solids with high spatial resolution and with a wide range of potential uses. Here we demonstrate that this method can be used to carry out three-dimensional MRI of the phosphorus (31P) in ex vivo bone and soft tissue samples.

The Intrinsic Origin of Spin Echoes in Dipolar Solids Generated by Strong Pulses

In spectroscopy, it is conventional to treat pulses much stronger than the linewidth as delta functions. In NMR, this assumption leads to the prediction that

Anomalies in the NMR of silicon: Unexpected spin echoes in a dilute dipolar solid

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Simulating Quantum Fields with Cavity QED

pulses do not refocus the dipolar coupling. However, NMR spin echo measurements in dipolar solids defy these conventional expectations when more than one

Generating Unexpected Spin Echoes in Dipolar Solids with π Pulses

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Controlling coherence using the internal structure of hard pi pulses.

pulse is used. Observed effects include a long tail in the CPMG echo train for short delays between

Optically pumped NMR in the quantum Hall regimes

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Ultraslow electron spin dynamics in the fractional quantum Hall regime

pulses, an even-odd asymmetry in the echo amplitudes for long delays, an unusual fingerprint pattern for intermediate delays, and a strong sensitivity to