Nuclear Spin Populations Research Articles

Local dynamic nuclear polarization using quantum point contacts.

We have used quantum point contacts (QPCs) to locally create and probe dynamic nuclear polarization (DNP) in GaAs heterostructures in the quantum Hall regime. DNP is created via scattering between spin-polarized Landau level electrons and the Ga and As nuclear spins, and it leads to hysteresis in the dc transport characteristics. The nuclear origin of this hysteresis is demonstrated by nuclear magnetic resonance (NMR). Our results show that QPCs can be used to create and probe local nuclear spin populations, opening up new possibilities for mesoscopic NMR experiments.

Spin Hamiltonian spectroscopy and spin-spin cross relaxation by optical pumping of Pr +3: LaF 3

The Zeeman-quadrupole Hamiltonian spectrum of hyperfine optical ground states of Pr +3: LaF 3 is mapped out by monitoring resonant laser transmission through a single crystal over a wide range of magnetic fields. Magnetic field sweep or rf resonance excitation applied during optical pumping produces variations in hole burning which indicate nuclear spin population transfer between Pr +3 states. Spin-spin cross relaxation between Pr +3 and F spin reservoirs and its influence on the Pr +3 NMR spectrum is detected.

NMR imaging of paramagnetic centers in solids via dynamic nuclear polarization

During the past few years the scope and importance of NMR imaging techniques, especially in medicine, have grown enormously. Recent developments have extended NMR imaging into the realm of solid materials (Z-d), and include a technique reported from this laboratory that takes advantage of the dilute spin behavior of 13C in natural abundance (1). O f course, the sensitivity limitations encountered with 13C in natural abundance in typical solids can be severe, and techniques that can enhance 13C spin polarization are therefore of interest. One method that has been shown to provide major enhancements of nuclear spin polarization in paramagnetic samples with favorable relaxation properties is dynamic nuclear polarization, DNP (5-8). Hence, it appeared worthwhile to combine the DNP technique for generating nuclear spin polarization with the dilute-spin 13C technique for NMR imaging introduced earlier (I). This approach, which is the subject of this paper, has the further potential of providing a method for obtaining a spatial image of the paramagnetic centers responsible for the DNP phenomenon. In dynamic nuclear polarization (5) the electron spin resonance of a paramagnetic center is irradiated (ideally saturated) at the resonance microwave frequency. The resulting perturbation of the electron spin polarization is transmitted via electron spin/nuclear spin interactions to the nuclear spin populations. When NMR signals are detected the magnitude of the observed NMR signal intensity is enhanced by a factor up to the ratio of the magnitudes of the electron spin and nuclear spin magnetic moments. The actual value of the enhancement depends upon the relative contributions of the three competing types of DNP mechanisms (Overhauser, solid-state effect, thermal mixing effect), the microwave frequency employed and the details of the dynamics of pertinent spin transition probabilities (i.e., relaxation times and the rate of microwave-stimulated electron spin transitions). Wind and co-workers (5-8) have observed DNP enhancement factors in the ‘H and r3C spectra of both static solids and solids rotated rapidly at the magic angle. In the work presented here, &, is 1.4 T, and the frequencies of the ‘H, ‘%Z, and ESR radiation were 60.0 MHz, 15.1 MHz, and 39.4 GHz, respectively. The DNP system is based upon a CP/MAS spectrometer that employs a Varian 1.4 T electro-

Iterative schemes for broad-band and narrow-band population inversion in NMR

We derive iterative schemes for generating rf pulse sequences that invert nuclear spin populations over either broad or narrow ranges of resonance offsets and rf amplitudes. The schemes employ only phase shifting and concatenation operations. Iterative application of a scheme produces a series of pulse sequences with successive improvements in population inversion performance.

Anomalous NMR Line Shapes in Solid Ordered Ortho-Hydrogen and Para-Deuterium

Some highly anomalous features have been observed in the low-temperature NMR spectrum of high-purity (> 99% $J=1$), solid para-deuterium. The intensity of the low-frequency side of the Pake doublet is enhanced and that of the high-frequency side reduced, sometimes becoming negative, corresponding to an emission of power. The anomaly is particularly pronounced and of the opposite sign for the $I=2$ resonance of the $J=0$ impurity molecules which exhibit enhancements as large as 50. The effect, which has also been observed in high-purity ortho-hydrogen but with smaller enhancements, is ascribed to a pumping process resulting from ortho-para conversion. In the ordered phase, conversion takes place from only one of the $J=1$ substates since the excited states, the librons, are not thermally populated. This results in a disturbance of the nuclear-spin populations away from thermal equilibrium, and the observed intensity depends on a balance between the ortho-para pumping mechanism and the normal spin-lattice relaxation processes. The results are compared to the theory of ortho-para conversion given in a companion paper. Measurements of the temperature dependence of the spin-lattice relaxation time ${T}_{1}$ in ordered para-${\mathrm{D}}_{2}$ are also presented. At high temperatures there is a fair agreement with theory based on the inelastic scattering of librons, but at lower temperatures ${T}_{1}$ is much shorter than predicted and depends strongly on the $J=0$ impurity concentration. The two Pake doublets predicted by Harris for the $J=0$ ${\mathrm{D}}_{2}$ molecules have been resolved and an accurate determination made of the widths. Finally, some measurements of the order parameter via the width of the $J=1$ Pake doublet are presented and compared to current theories and previous experiments.