Optically pumped NMR: Revealing spin-dependent Landau level transitions in GaAs

Ramaswamy, Kannan; Mui, Stacy; Crooker, S. A.; Pan, Xingyuan; Sanders, G. D.; Stanton, C. J.; Hayes, Sophia E.

doi:10.1103/physrevb.82.085209

Cited by 15 publications

(5 citation statements)

References 25 publications

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“…At high fields, the effects of the creation of these sub-bands on the NMR photon energy spectrum is twofold: the optical band gap changes, thus shifting the spectrum, and there appears a complex semi-oscillatory structure with super-bandgap irradiation 33,34,36,37 . An example of such structure can be seen in the 9.39 T data 33 shown in …”

Section: A Energy Shifts With Magnetic Fieldmentioning

confidence: 99%

Optically pumped InP: Nuclear polarization from NMR frequency shifts

et al. 2011

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There are a number of mechanisms that can produce frequency shifts in the NMR spectra of optically pumped semiconductors, including the hyperfine interaction, nuclear dipolar fields, and indirect-or J-couplings. Using optically pumped Fe-doped InP, we explore how to experimentally distinguish the shift mechanisms from one another, and then exploit the shifts to measure the absolute nuclear polarization. Furthermore, we optically pump, using circularly polarized light, at a much lower field (2.35 T) than previous work, permitting us to explore the field-dependence of the nuclear polarization rate, the spin-lattice relaxation time, and the NMR photon energy spectrum. We measure similar polarizations as obtained at higher fields, but with a significantly faster nuclear polarization rate, making operation at lower fields attractive for optically pumped InP as a source of nuclear spin polarization.

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Section: A Energy Shifts With Magnetic Fieldmentioning

confidence: 99%

Optically pumped InP: Nuclear polarization from NMR frequency shifts

et al. 2011

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“…of Florida), which enabled us to assign features observed in the oscillations of the OPNMR signal to specific Landau level transitions. 23 A representative calculation of optical magnetoabsorption is shown in Figure 3a (the dotted line) for transitions expected with σ − optical pumping of GaAs in a 4.7T external magnetic field at a temperature of 6K. What is evident, is that the "total spin-polarized optical magnetoabsorption" expression (α ↑ +α ↓ )-which is the sum producing both "spin-up" and "spin-down" electrons-is not the best match to the OPNMR data.…”

Section: Resultsmentioning

confidence: 97%

Experimental measurements of optically-pumped NMR (OPNMR) and spin polarization in bulk GaAs and GaAs/AlGaAs quantum wells

et al. 2014

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Optically-pumped nuclear magnetic resonance (OPNMR) is a measurement scheme that utilizes optical pumping of conduction electrons within a semiconductor to polarize systems of nuclear spins to which they are coupled. The spectroscopic power of NMR techniques is brought to bear on these rare spin systems through enhancement of the nuclear spin polarization, here in direct-gap semiconductors such as bulk semi-insulating GaAs and GaAs/AlGaAs quantum wells. The nuclear spins act as reporters of the electron spins that are oriented during optical pumping with circularly polarized laser light, at specific photon energies. The effects of penetration depth of the laser in the sample can be understood when irradiating at energies less than the bandgap energy, as well as details of coupling to interband transitions originating from Landau levels at photon energies greater than the bandgap energy. We show that OPNMR is particularly sensitive to the sign of magnetization that results from light hole-to-conduction band transitions because the sign of magnetization is reversed when the light hole states in the valence band are accessed.

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“…1 shows that the 31 P signal persists to above 50 K above the bandgap (1.428 eV) at 2.35 T. Presumably, this is a consequence of the difference in band structure at the two fields. 23 It suggests that, in addition to the observed faster cross relaxation rate 7 , lower magnetic fields may provide a more favorable temperature dependence for optical pumping in the photon-energy regime desired for high surface polarization.…”

Section: A Temperature Dependencementioning

confidence: 99%

Temporal and spatial evolution of nuclear polarization in optically pumped InP

et al. 2015

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The electron-nuclear interaction in optically-pumped NMR of semiconductors manifests itself through changes in spectral features (resonance shifts, line widths, signal amplitudes) and through the magnitude of the nuclear spin polarization. We show that these spectral features can provide a measure of the parameters that govern the optical pumping process: electron-nuclear cross relaxation rate, Bohr radius and fractional occupancy of the optically relevant defect (ORD), and electron polarization at the ORD. Applying a model of the spatial and temporal evolution of the nuclear spins under optical pumping to 31 P in semi-insulating InP we find an ORD Bohr radius of 6 nm, independent of the electron polarization used to fit the data, confirming the ORD is a shallow donor. For an electron polarization of -0.15, the ORD fractional occupancy is 0.02, leading to an electron-nuclear cross relaxation time of 0.20 s and a hyperfine frequency shift of 8.1 kHz for superbandgap irradiation. Allowing the electron polarization to vary in the model constrained to the hyperfine shift data, we find the fractional occupancy and electron-nuclear cross relaxation rate to be approximately inversely proportional to the electron polarization. From the long time evolution of the nuclear polarization we calculate an ORD density of 5 × 10 15 cm −3 .

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Optically pumped NMR: Revealing spin-dependent Landau level transitions in GaAs

Cited by 15 publications

References 25 publications

Optically pumped InP: Nuclear polarization from NMR frequency shifts

Optically pumped InP: Nuclear polarization from NMR frequency shifts

Experimental measurements of optically-pumped NMR (OPNMR) and spin polarization in bulk GaAs and GaAs/AlGaAs quantum wells

Temporal and spatial evolution of nuclear polarization in optically pumped InP

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