Control of electron spin dynamics in a wide GaAs quantum well by a lateral confining potential

The spin dynamics of electrons laterally confined in a wide GaAs quantum well with the use of a special mosaic electrode deposited onto the sample plane has been investigated. Comparative measurements with a semitransparent electrode have been simultaneously carried out to distinguish changes in electron spin dynamics due to the band bending from those due to the lateral confinement controlled by applying an external bias. The electron spin lifetimes in the traps has been found to increase strongly with the applied bias. The measured values of the electron g-factor in the quantum well plane and the magnetic-field dependence of the electron spin lifetimes indicate the emergence of strong three-dimensional confinement in the center of an orifice in the mosaic electrode. The examined electron spin relaxation anisotropy is caused by the anisotropy of the confining potential.     

Nuclear spin-lattice relaxation mechanisms in kaolinite confirmed by magic-angle spinning

Spin-lattice relaxation mechanisms in kaolinite have been reinvestigated by magic-angle spinning (MAS) of the sample. MAS is useful to distinguish between relaxation mechanisms: the direct relaxation rate caused by the dipole-dipole interaction with electron spins is not affected by spinning while the spin diffusion-assisted relaxation rate is. Spin diffusion plays a dominant role in ¹H relaxation. MAS causes only a slight change in the relaxation behavior, because the dipolar coupling between ¹H spins is strong. ²⁹Si relaxes directly through the dipole-dipole interaction with electron spins under spinning conditions higher than 2 kHz. A spin diffusion effect has been clearly observed in the ²⁹Si relaxation of relatively pure samples under static and slow-spinning conditions. ²⁷Al relaxes through three mechanisms: phonon-coupled quadrupole interaction, spin diffusion and dipole-dipole interaction with electron spins. The first mechanism is dominant, while the last is negligibly small. Spin diffusion between ²⁷Al spins is suppressed completely at a spinning rate of 2.5 kHz. We have analyzed the relaxation behavior theoretically and discussed quantitatively. Concentrations of paramagnetic impurities, electron spin-lattice relaxation times and spin diffusion rates have been estimated.     

Ab initio GW calculations of the time and length of spin relaxation of excited electrons in metals with inclusion of the spin-orbit coupling

An ab initio method has been proposed for calculating the spin relaxation time of excited electrons in metals in the framework of the GW method with inclusion of the spin-orbit coupling. The time and length of spin relaxation in Al, Cu, Au, Nb, and Ta have been calculated. The concept of the spin-flip phase space has been introduced. It has been demonstrated that the ratio between the spin relaxation time and the lifetime of the excited electron is well explained within this concept. The time and length of spin relaxation in Nb appear to be considerably shorter than those in Al, Cu, and Au. These quantities in Ta are especially small in accordance with the strong spin-orbit coupling. A comparison of the results with the previous data on the time and length of spin relaxation due to the interaction with impurities and phonons shows that, at an excited electron energy of the order of 1 eV, the inelastic electron-electron scattering in the presence of spin-orbit coupling is a dominant mechanism of spin relaxation.     

Spin relaxation of two-dimensional electrons in a hall ferromagnet

Electron spin resonance in a system of two-dimensional electrons with a high electron mobility has been investigated and the position, width, intensity, and line shape of the resonance microwave absorption have been studied as functions of the filling factor and temperature. It has been shown that the ESR linewidth in high-electron-mobility GaAs/AlGaAs quantum wells may reach 30 MHz, which corresponds to a spin relaxation time of the two-dimensional electrons of 10 ns. The experimental data on the linewidth of the spin resonance at a filling factor of 1 are compared with the theoretical results for various spin relaxation mechanisms. It has been shown that the dominant mechanism of spin relaxation at a filling factor of 1 and a temperature of 1.5–4 K is the mutual scattering of spin excitons.     

<Superscript>29</Superscript>Si nuclear spin relaxation in microcrystals of plastically deformed Si: B samples

Single crystals and microcrystals Si: B enriched with ²⁹Si isotopes have been studied using nuclear magnetic resonance and electron paramagnetic resonance (EPR) in the temperature range from 300 to 800 K. It has been found that an increase in the temperature from 300 to 500 K leads to a change in the kinetics of the relaxation of the saturated nuclear spin system. At 300 K, the relaxation kinetics corresponds to direct electron–nuclear interaction with inhomogeneously distributed paramagnetic centers introduced by the plastic deformation of the crystals. At 500 K, the spin relaxation occurs through the nuclear spin diffusion and electron–nuclear interaction with an acceptor impurity. It has been revealed that the plastic deformation affects the EPR spectra at 9 K.     

室温下(Ga,Mn)As中载流子的自旋弛豫特性

运用飞秒时间分辨抽运-探测克尔光谱技术,研究了室温下退火及未退火(Ga,Mn)As的载流子自旋弛豫的激发能量密度依赖性,发现电子自旋弛豫时间随激发能量密度增加而增大,而在同一激发能量密度下,退火样品比未退火样品具有更短的载流子复合时间、电子自旋弛豫时间和更大的克尔转角,显示DP机理是室温下(Ga,Mn)As的电子自旋弛豫的主导机理.退火(Ga,Mn)As的超快克尔增强效应显示其在超高速全光自旋开关方面的潜在应用价值,也为(Ga,Mn)As铁磁性起源的p-d交换机理提供了证据. 关键词： (Ga Mn)As稀磁半导体 时间分辨克尔光谱 电子自旋弛豫 DP机理     

Electrical detection of spin accumulation in a p-type GaAs quantum well

We report on experiments in which a spin-polarized current is injected from a GaMnAs ferromagnetic electrode into a GaAs layer through an AlAs barrier. The resulting spin polarization in GaAs is detected by measuring how the tunneling current, to a second GaMnAs ferromagnetic electrode, depends on the orientation of its magnetization. Our results can be accounted for by sequential tunneling with the nonrelaxed spin splitting of the chemical potential, that is, spin accumulation, in GaAs. We discuss the conditions on the hole spin relaxation time in GaAs that are required to obtain the large effects we observe.     

Electrical readout of the local nuclear polarization in the quantum Hall effect: a hyperfine battery

It is demonstrated that the now well-established "flip-flop" mechanism of spin exchange between electrons and nuclei in the quantum Hall effect can be reversed. We use a sample geometry which utilizes separately contacted edge states to establish a local nuclear spin polarization--close to the maximum value achievable--by driving a current between electron states of different spin orientation. When the externally applied current is switched off, the sample exhibits an output voltage of up to a few tenths of a mV, which decays with a time constant typical for the nuclear spin relaxation. The surprising fact that a sample with a local nuclear spin polarization can act as a source of energy and that this energy is well above the nuclear Zeeman splitting is explained by a simple model which takes into account the effect of a local Overhauser shift on the edge state reconstruction.     

Hyperfine switching triggered by resonant tunneling for the detection of a single nuclear spin qubit

A novel detection mechanism and a robust control of a single nuclear spin-flip by hyperfine interactions between the nuclear spin and tunneling electron spin are proposed on the basis of ab initio non-equilibrium Green's function calculations. The calculated relaxation times of the nuclear spin of proton in a nano-contact system, Pd(electrode)-H₂-Pd(electrode), show that ON/OFF switching of hyperfine interactions is effectively triggered by resonant tunneling mediated through the d-orbitals of Pd. The relaxation times at ON-resonance are ∼¹⁰³ times faster than those at OFF-resonance, indicating that ON-resonance is suitable for the detection (read-out) of nuclear spin states. In addition, the effectiveness of bias voltage applications at OFF-resonance for selective operations on the proton qubit is demonstrated in the calculations of the resonant frequencies of proton using the gauge-invariant atomic orbital method.     

Effect of electrode material on impedance spectra of metal-polyethylene structures with carbon nanotubes

The effect of an electrode material on electrical properties of a composite material based on super-high-molecular polyethylene (SHMPE) filled with carbon nanotubes has been studied using impedance spectroscopy. Using the method of replacing the sample by an equivalent electric circuit, it has been found that, depending on the electrode material, a blocking barrier with high active resistance and a space charge region adjacent to it arise in the interface region. It has been shown that the barrier height is controlled by surface electronic states of SHMPE and weakly depends on the electron work function of metal electrodes (Bardeen barrier). The characteristic times of electrical relaxation characterizing bulk and interface regions of the composite under study have been determined.     

Negative circular polarization of InP QD luminescence: Mechanism of formation and main regularities

The spectrum and kinetics of the circular polarization of InP quantum dot (QD) photoluminescence have been experimentally investigated under different conditions of optical excitation and at different bias voltages applied to the sample. It is established that, at a bias of about ?0.1 V, the degree of photoluminescence polarization is negative and reaches ?50% in limiting cases. It is concluded that the negative polarization is formed in QDs containing one recident electron per dot and is mainly caused by the optical orientation of the electron spin. It is shown that all experimentally observed regularities are well described in the framework of the model assuming the energy relaxation of photogenerated electron-hole pairs accompanied by the electron- hole spin flip-flop process.     

Modulation of the distance dependence of paramagnetic relaxation enhancements by CSA × DSA cross-correlation

Paramagnetic metal ions with fast-relaxing electronic spin and anisotropic susceptibility tensor provide a rich source of structural information that can be derived from pseudo-contact shifts, residual dipolar couplings, dipole-dipole Curie spin cross-correlation, and paramagnetic relaxation enhancements. The present study draws attention to a cross-correlation effect between nuclear relaxation due to anisotropic chemical shielding (CSA) and due to the anisotropic dipolar shielding (DSA) caused by the electronic Curie spin. This CSA x DSA cross-correlation contribution seems to have been overlooked in previous interpretations of paramagnetic relaxation enhancements. It is shown to be sufficiently large to compromise the 1/r6 distance dependence usually assumed. The effect cannot experimentally be separated from auto-correlated DSA relaxation. It can increase or decrease the observed paramagnetic relaxation enhancement. Under certain conditions, the effect can dominate the entire paramagnetic relaxation, resulting in nuclear resonances narrower than in the absence of the paramagnetic center. CSAxDSA cross-correlation becomes important when paramagnetic relaxation is predominantly due to the Curie rather than the Solomon mechanism. Therefore the effect is most pronounced for relaxation by metal ions with large magnetic susceptibility and fast-relaxing electron spin. It most strongly affects paramagnetic enhancements of transverse relaxation in macromolecules and of longitudinal relaxation in small molecules.     

自旋电子学简介及其研究进展

自旋电子学主要研究电子自旋在固体物理中的作用,是一门结合磁学与微电子学的交叉学科,其研究对象包括电子的自旋极化、自旋相关散射、自旋弛豫以及与此相关的性质及其应用等.基于电子自旋的自旋电子器件能够大大提高信息处理速度和存储密度,而且具有非易失性、低能耗等优点.简单介绍了自旋电子学的概念及其研究内容,综述了自旋电子学目前的研究及应用进展.     

Modulation of the distance dependence of paramagnetic relaxation enhancements by CSA x DSA cross-correlation

Paramagnetic metal ions with fast-relaxing electronic spin and anisotropic susceptibility tensor provide a rich source of structural information that can be derived from pseudo-contact shifts, residual dipolar couplings, dipole-dipole Curie spin cross-correlation, and paramagnetic relaxation enhancements. The present study draws attention to a cross-correlation effect between nuclear relaxation due to anisotropic chemical shielding (CSA) and due to the anisotropic dipolar shielding (DSA) caused by the electronic Curie spin. This CSA x DSA cross-correlation contribution seems to have been overlooked in previous interpretations of paramagnetic relaxation enhancements. It is shown to be sufficiently large to compromise the 1/r6 distance dependence usually assumed. The effect cannot experimentally be separated from auto-correlated DSA relaxation. It can increase or decrease the observed paramagnetic relaxation enhancement. Under certain conditions, the effect can dominate the entire paramagnetic relaxation, resulting in nuclear resonances narrower than in the absence of the paramagnetic center. CSAxDSA cross-correlation becomes important when paramagnetic relaxation is predominantly due to the Curie rather than the Solomon mechanism. Therefore the effect is most pronounced for relaxation by metal ions with large magnetic susceptibility and fast-relaxing electron spin. It most strongly affects paramagnetic enhancements of transverse relaxation in macromolecules and of longitudinal relaxation in small molecules.     

Electron spin relaxation of radicals in γ-irradiated malonic acid and methyl malonic acid

Radicals generated by γ-irradiation of malonic acid and methyl malonic acid were studied as a function of temperature by inversion recovery, echo-detected saturation recovery and electron-electron double resonance (ELDOR) at X-band, and by continuous-wave saturation recovery at X-band and S-band. ELDOR reductions for malonic acid radical in polycrystalline and single-crystal samples indicate that nuclear spin relaxation is faster than both electron spin relaxation and cross relaxation between 93 and 233 K. Deuteration of the carboxylate protons caused the maximum ELDOR reduction to shift from about 110 to 150 K, consistent with the assignment of the rapid nuclear spin relaxation to hydrogen-bonded proton dynamics. ELDOR enhancements for both radicals formed in methyl malonic acid indicate that cross relaxation is faster than both electron spin relaxation and nuclear spin relaxation between 77 and 220 K. Enhanced cross relaxation and electron spin relaxation are attributed to the rotation of methyl groups at a rate comparable to the electron Larmor frequency. The temperature dependence of the enhancement of 1/T _1e was analyzed to determine the activation energies for methyl rotation. The same radical is formed in irradiated methyl malonic acid and L-alanine, but the barrier to rotation of the α-methyl is 500 K in the methyl malonic acid host and 1500 K in the L-alanine host, which indicates a large impact of the lattice on the barrier to rotation.     

Electron–electron interaction induced spin thermalization in quasi-low-dimensional spin valves

We study the spin thermalization, i.e., the inter-spin energy relaxation mediated by electron–electron scattering in small spin valves. When one or two of the dimensions of the spin valve spacer are smaller than the thermal coherence length, the direct spin energy exchange rate diverges and needs to be regularized by the sample dimensions. Here we consider two model systems: a long quasi-1D wire and a thin quasi-2D sheet.     

Double electron-electron resonance in electron spin echo: Conformations of spin-labeled poly-4-vinilpyridine in glassy solutions

Double electron-electron resonance in electron spin echo has been used to study the glassy solutions of poly-4-vinylpyridine doped by nitroxyl radicals frozen in liquid nitrogen. The phase relaxation of spin labels due to spin-spin interaction of unpaired electrons has been studied. The intramolecular and intermolecular contributions of the dipole-dipole interaction of spin labels into relaxation process have been separated. It has been established that both the intramolecular and intermolecular spin-spin interaction of spin labels lead to the dependence of echo signal on timeT of the exp (?aT ^q ) type. It is shown that for the intramolecular interaction the experimentalq value is 0.3, for the intermolecular one it is 2. The assumption has been made of the linear structure of polymeric molecules due to the presence of a sufficiently high density of an electric charge on polymeric molecules.