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Register a new userKnight shift and nuclear spin relaxation in Fe/n-GaAs heterostructures
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We investigate the dynamically polarized nuclear-spin system in Fe/emph{n}-GaAs heterostructures using the response of the electron-spin system to nuclear magnetic resonance (NMR) in lateral spin-valve devices. The hyperfine interaction is known to act more strongly on donor-bound electron states than on those in the conduction band. We provide a quantitative model of the temperature dependence of the occupation of donor sites. With this model we calculate the ratios of the hyperfine and quadrupolar nuclear relaxation rates of each isotope. For all temperatures measured, quadrupolar relaxation limits the spatial extent of nuclear spin-polarization to within a Bohr radius of the donor sites and is directly responsible for the isotope dependence of the measured NMR signal amplitude. The hyperfine interaction is also responsible for the $2text{ kHz}$ Knight shift of the nuclear resonance frequency that is measured as a function of the electron spin accumulation. The Knight shift is shown to provide a measurement of the electron spin-polarization that agrees qualitatively with standard spin transport measurements.
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We study the nuclear magnetic relaxation rate and Knight shift in the presence of the orbital and quadrupole interactions for three-dimensional Dirac electron systems (e.g., bismuth-antimony alloys). By using recent results of the dynamic magnetic susceptibility and permittivity, we obtain rigorous results of the relaxation rates $(1/T_1)_{rm orb}$ and $(1/T_1)_{rm Q}$, which are due to the orbital and quadrupole interactions, respectively, and show that $(1/T_1)_{rm Q}$ gives a negligible contribution compared with $(1/T_1)_{rm orb}$. It is found that $(1/T_1)_{rm orb}$ exhibits anomalous dependences on temperature $T$ and chemical potential $mu$. When $mu$ is inside the band gap, $(1/T_1)_{rm orb} sim T ^3 log (2 T/omega_0)$ for temperatures above the band gap, where $omega_0$ is the nuclear Larmor frequency. When $mu$ lies in the conduction or valence bands, $(1/T_1)_{rm orb} propto T k_{rm F}^2 log (2 |v_{rm F}| k_{rm F}/omega_0)$ for low temperatures, where $k_{rm F}$ and $v_{rm F}$ are the Fermi momentum and Fermi velocity, respectively. The Knight shift $K_{rm orb}$ due to the orbital interaction also shows anomalous dependences on $T$ and $mu$. It is shown that $K_{rm orb}$ is negative and its magnitude significantly increases with decreasing temperature when $mu$ is located in the band gap. Because the anomalous dependences in $K_{rm orb}$ is caused by the interband particle-hole excitations across the small band gap while $left( 1/T_1 right)_{rm orb}$ is governed by the intraband excitations, the Korringa relation does not hold in the Dirac electron systems.
We demonstrate a scheme for optically patterning nuclear spin polarization in semiconductor/ferromagnet heterostructures. A scanning time-resolved Kerr rotation microscope is used to image the nuclear spin polarization that results when GaAs/MnAs epilayers are illuminated with a focused laser having a Gaussian profile. Rather than tracking the intensity profile of the laser spot, these images reveal that the nuclear polarization forms an annular lateral structure having circular symmetry with a dip rather than a peak at its center.
Based on a Monte Carlo method, we investigate the influence of transport conditions on the electron spin relaxation in GaAs. The decay of initial electron spin polarization is calculated as a function of distance under the presence of moderate drift fields and/or non-zero injection energies. For relatively low fields (a couple of kV/cm), a substantial amount of spin polarization is preserved for several microns at 300 K. However, it is also found that the spin relaxation rate increases rapidly with the drift field, scaling as the square of the electron wavevector in the direction of the field. When the electrons are injected with a high energy, a pronounced decrease is observed in the spin relaxation length due to an initial increase in the spin precession frequency. Hence, high-field or high-energy transport conditions may not be desirable for spin-based devices.
Spin-lattice relaxation of the nuclear spin system in p-type GaAs is studied using a three-stage experimental protocol including optical pumping and measuring the difference of the nuclear spin polarization before and after a dark interval of variable length. This method allows us to measure the spin-lattice relaxation time $T_1$ of optically pumped nuclei in the dark, that is, in the absence of illumination. The measured $T_1$ values fall into the sub-second time range, being three orders of magnitude shorter than in earlier studied n-type GaAs. The drastic difference is further emphasized by magnetic-field and temperature dependences of $T_1$ in p-GaAs, showing no similarity to those in n-GaAs. This unexpected behavior is explained within a developed theoretical model involving quadrupole relaxation of nuclear spins, which is induced by electric fields within closely spaced donor-acceptor pairs.
We have studied hyperfine interactions between spin-polarized electrons and lattice nuclei in Al_0.1Ga_0.9As/GaAs quantum well (QW) heterostructures. The spin-polarized electrons are electrically injected into the semiconductor heterostructure from a metallic ferromagnet across a Schottky tunnel barrier. The spin-polarized electron current dynamically polarizes the nuclei in the QW, and the polarized nuclei in turn alter the electron spin dynamics. The steady-state electron spin is detected via the circular polarization of the emitted electroluminescence. The nuclear polarization and electron spin dynamics are accurately modeled using the formalism of optical orientation in GaAs. The nuclear spin polarization in the QW is found to depend strongly on the electron spin polarization in the QW, but only weakly on the electron density in the QW. We are able to observe nuclear magnetic resonance (NMR) at low applied magnetic fields on the order of a few hundred Oe by electrically modulating the spin injected into the QW. The electrically driven NMR demonstrates explicitly the existence of a Knight field felt by the nuclei due to the electron spin.
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