Optically Pumped Nuclear Magnetic Resonance Research Articles

Describing angular momentum conventions in circularly polarized optically pumped NMR in GaAs and CdTe

The physical phenomena governing hyperpolarization through optical pumping of conduction electrons continue to be explored in multiple semiconductor systems. One early finding has been the asymmetry between the optically pumped nuclear magnetic resonance (OPNMR) signals when generated by different circular polarizations (i.e., light helicities). Because these resonances are asymmetric, the midpoint between the signals prepared with each of the two circular polarizations is either a positive or negative value, termed an “offset” that is representative of an optical Overhauser enhancement. Both negative offsets (in GaAs) and positive offsets (in CdTe) have been observed. The origins of these offsets in semiconductors are believed to arise from thermalized electrons; however, to the best of our knowledge, no study has systematically tested this hypothesis. To that end, we have adopted two configurations for OPNMR experiments—one in which the Poynting vector of the laser light and magnetic field are parallel, and one in which they are antiparallel, while other experimental conditions are kept the same. We find that the OPNMR signal response to a fixed helicity of light depends on the experimental configuration, and this configuration needs to be accounted for in order to properly describe the OPNMR results. Further, studying the offsets as a function of field strength shows that the optical Overhauser enhancement (the offset) increases in magnitude with field strength. Finally, by describing all angular momentum and phasing conventions unambiguously, we are able to determine that the absorptively-phased appearance of 113Cd (and 125Te) OPNMR in CdTe is a consequence of the sign of the nuclear gyromagnetic ratios for these isotopes.

Characterization of elastic interactions in GaAs/Si composites by optically pumped nuclear magnetic resonance

Elastic interactions in GaAs/Si bilayer composite structures were studied by optically pumped nuclear magnetic resonance (OPNMR). The composites were fabricated by epoxy bonding of a single crystal of GaAs to a single crystal of Si at 373 K followed by selective chemical etching of the GaAs at room temperature to obtain a series of samples with GaAs thickness varying from 37 μm to 635 μm, while the Si support thickness remained fixed at 650 μm. Upon cooling to below 10 K, a biaxial tensile stress developed in the GaAs film due to differential thermal contraction. The strain perpendicular to the plane of the bilayer and localized near the surface of the GaAs was deduced from the quadrupolar splitting of the Gallium-71 OPNMR resonance. Strain relaxation by bowing of the composite was observed to an extent that depended on the relative thickness of the GaAs and Si layers. The variation of the strain with GaAs layer thickness was found to be in good agreement with a general analytical model for the elastic relationships in composite media.

Resistively detected NMR spectra of the crystal states of the two-dimensional electron gas in a quantizing magnetic field

Transport experiments on the two-dimensional electron gas (2DEG) confined into a semiconductor quantum well and subjected to a quantizing magnetic field have uncovered a rich variety of uniform and nonuniform phases such as the Laughlin liquids, the Wigner, bubble and Skyrme crystals and the quantum Hall stripe state. Optically pumped nuclear magnetic resonance (OP-NMR) has also been extremely useful in studying the magnetization and dynamics of electron solids with exotic spin textures such as the Skyrme crystal. Recently, it has been demonstrated that a related technique, resistively-detected nuclear magnetic resonance (RD-NMR), could be a good tool to study the topography of the electron solids in the fractional and integer quantum Hall regimes. In this work, we compute theoretically the RD-NMR line shapes of various crystal phases of the 2DEG and study the relation between their spin density and texture and their NMR spectra. This allows us to evaluate the ability of the RD-NMR to discriminate between the various types of crystal states.

Effects of strain and quantum confinement in optically pumped nuclear magnetic resonance in GaAs: Interpretation guided by spin-dependent band structure calculations

A combined experimental-theoretical study of optically pumped nuclear magnetic resonance (OPNMR) has been performed in a $\mathrm{GaAs}\text{/}\mathrm{A}{\mathrm{l}}_{0.1}\mathrm{G}{\mathrm{a}}_{0.9}\mathrm{As}$ quantum well film epoxy bonded to a Si substrate with thermally induced biaxial strain. The photon energy dependence of the Ga OPNMR signal was recorded at magnetic fields of 4.9 and 9.4 T at a temperature of 4.8--5.4 K. The data were compared to the nuclear spin polarization calculated from the electronic structure and differential absorption to spin-up and spin-down states of the electron conduction band using a modified $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model based on the Pidgeon-Brown model. Comparison of theory with experiment facilitated the assignment of features in the OPNMR energy dependence to specific interband Landau level transitions. The results provide insight into how effects of strain and quantum confinement are manifested in optical nuclear polarization in semiconductors.

Assignments of transitions in optically-pumped NMR of GaAs/AlGaAs quantum wells on a bulk GaAs substrate

We investigate optically-pumped nuclear magnetic resonance (OPNMR) in GaAs/AlGaAs multiple quantum wells. The photon energy dependence of the ${}^{69}\text{Ga}$ OPNMR shows a sign change in the signal at the light-hole transition for both ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}}$ circular polarized irradiation. The energy for this transition and those of other Landau levels can be predicted by an eight-band $\stackrel{P\vec}{k}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{P\vec}{p}$ theory used to calculate the conduction band spin polarization. Comparison of the spin polarization with the OPNMR measurements shows the origin of many features. In addition, we find features associated with the bulk GaAs substrate. In an external magnetic field, there are regions of low quantum well absorptivity between the Landau levels and in this region, OPNMR signals arising from the bulk GaAs substrate can be observed. The discrete nature of transitions between Landau levels in two-dimensional electron systems compared to the Landau levels in three-dimensional systems leads to the change in the phase of the OPNMR signal at the light-hole transitions.

Helicity independent optically-pumped nuclear magnetic resonance in gallium arsenide

We present new phenomenology for optically-pumped nuclear magnetic resonance (OPNMR) of gallium arsenide. When pumping at low irradiation intensity, the OPNMR signal becomes independent of light helicity. The results are consistent with a mechanism in which the bulk signal represents competition between nuclear quadrupolar and electron-nuclear hyperfine relaxation. This mechanism is further supported by the scaling behavior of OPNMR for isotopes with varying hyperfine and quadrupolar interactions. These results indicate the magnitude and sign of nuclear polarization in the sample may be controlled as a function of depth by tuning photon energy and laser intensity, portending submicron scale patterning of nuclear magnetization.

Goldstone Mode Induced by Skyrmions in Quantum Hall Systems

The properties of two-dimensional electron system exhibit rich physics in the presence of strong magnetic field. Electron-electron interactions add more interesting many-body phenomena to the systems, such as the frractional quantum Hall effect. From field theory method [1,2], a new type of topological excitation, Skyrmion, is predicted to occur around filling factor, 1/m, with odd integer m. The optically pumped nuclear magnetic resonance (OPNMR) measurements [3] support the existence of Skyrmions or spin texture excitations. Recently the present author has proposed that in quasi-2 + 1 dimensional quantum antiferromagnet the doped hole induces magnetic-disordered hedgehog-like soliton, which is composed of the doped hole and the cloud of SU(2) Yang-Mills fields with the spin disorder around the hole, by using the gauge-invariant effective Lagrangian with spontaneous symmetry breaking [4-7]. In this study, extending the theoretical formula, we introduce the gaugeinvariant effective Lagrangian density for the skyrmions around the filling factor = 1, and present the massless gauge field (Goldstone mode) induced by the Skyrmion. The massless gauge field introduces the long-range interaction, which will play an important role in the Skyrme crystal [8]. In addition, we suggest the presence of the skyrmion glass.

Nuclear Magnetic Resonance of Laser‐Polarized Noble Gases in Molecules, Materials, and Organisms

AbstractFor Abstract see ChemInform Abstract in Full Text.

OPNMR — a local probe of spin physics

In the quest to understand (and to make use of) spin effects in mesoscopic systems, the development of new experimental techniques will play a key role. In this review, we will describe some recent work using Optically Pumped Nuclear Magnetic Resonance (OPNMR), a novel spectroscopy that has proven to be an useful probe of electron spin physics in the quantum Hall regimes. We will show: (1) the OPNMR signal from a single GaAs quantum well, (2) the ‘motionally narrowed’ lineshape reflects the variation of the electron density across the ∼300 Å well, and (3) the dramatic changes in the lineshape which provide evidence for Skyrmion localization along the plane of the well.

Ultraslow electron spin dynamics in the fractional quantum Hall regime

Recent optically pumped nuclear magnetic resonance (OPNMR) measurements in two different electron-doped multiple quantum well samples near the fractional quantum Hall ground state ν=13 are reviewed. Below 0.5 K, the spectra provide evidence that the spin-reversed charged excitations of this ground state are localized over the NMR time scale of ∼40 μs. By varying NMR pulse parameters, the electron spin temperature (as measured by the Knight shift) could be driven above the lattice temperature. For Tlattice<0.5 K, these non-equilibrium measurements imply that the value of the electron spin-lattice relaxation time τ1s lies between 100 μs and 500 ms at ν=13.

Ultraslow electron spin dynamics in GaAs quantum wells probed by optically pumped NMR

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.

Optical Pumping in Solid State Nuclear Magnetic Resonance

An important current trend in solid state nuclear magnetic resonance (NMR) is the growing exploitation of optical pumping of nuclear spin polarizations as a means of enhancing and localizing NMR signals. Recent work has been concentrated in two areas, namely optically pumped NMR in semiconductors and optical pumping of noble gases. Progress in these two areas, including technical developments and new applications in physical chemistry, condensed matter physics, and biomedical sciences, is reviewed. Likely directions for future developments are suggested.