Observation of an underlying relativistic effect in the valence bands of Pt

We have measured the photoelectron spin polarization emitted by unpolarized UV radiation from the valence-bands of the well ordered Pt(0 0 1)-(5 × 1) surface and the disordered surface destroyed by Ar ions bombardment. Almost identical spin polarizations have been observed in both cases. This observation suggests that the electron spin polarization in photoemission caused by unpolarized light is determined by a short-range order of atoms. This finding has an obvious implication that the electron spin polarization in photoemission caused by unpolarized light can be used to study the bulk electronic structure of the nonmagnetic materials.     

Spin polarization in directional photoemission from non-magnetic crystals by unpolarized light

A spin-dependent three-step photoemission formalism for non-magnetic crystals is outlined, in which unpolarized Bloch electrons are diffracted at the surface. Assuming no spin dependence of the excitation process in the bulk, appreciable spin polarization features are obtained for W(001) at off-normal emission, in fairly good agreement with the first angle- and energy-resolved measurements of photo-electron spin polarization (using unpolarized light).     

Spin-to-orbital angular momentum conversion and spin-polarization filtering in electron beams

We propose the design of a space-variant Wien filter for electron beams that induces a spin half-turn and converts the corresponding spin angular momentum variation into orbital angular momentum of the beam itself by exploiting a geometrical phase arising in the spin manipulation. When applied to a spatially coherent input spin-polarized electron beam, such a device can generate an electron vortex beam, carrying orbital angular momentum. When applied to an unpolarized input beam, the proposed device, in combination with a suitable diffraction element, can act as a very effective spin-polarization filter. The same approach can also be applied to neutron or atom beams.     

Anomalous spin dephasing in (110) GaAs quantum wells: anisotropy and intersubband effects

A strong anisotropy of electron spin decoherence is observed in GaAs/(AlGa)As quantum wells grown on a (110) oriented substrate. The spin lifetime of spins perpendicular to the growth direction is about one order of magnitude shorter compared to spins along [110]. The spin lifetimes of both spin orientations decrease monotonically above temperatures of 80 and 120 K, respectively. The decrease is very surprising for spins along the [110] direction and cannot be explained by the usual Dyakonov-Perel dephasing mechanism. A novel spin dephasing mechanism is put forward that is based on scattering of electrons between different quantum well subbands.     

Measurement of the correlation between electron spin and photon linear polarization in atomic-field bremsstrahlung

Atomic-field bremsstrahlung has been studied with a longitudinally polarized electron beam. The correlation between the initial orientation of the electron spin and the angle of photon polarization has been measured at the photon high energy tip region. In the time reversal this corresponds to a so-far unobserved phenomenon of production of longitudinally polarized electrons by photoionization of unpolarized atoms with linearly polarized photons. The results confirm the fully relativistic calculations for radiative recombination and suggest a new method for electron beam polarimetry.     

Angular distribution of low energy photoelectrons with defined spin orientation

On the basis of symmetry arguments general expressions are derived for the angular distribution of photoelectrons in defined spin states from atoms or molecules for the two cases of circularly and linearly polarized light. It is prooved that linearly polarized light or unpolarized light ionizing an isotropic ensemble of atoms or molecules will allways produce unpolarized electrons in all directions in the electric dipole approximation.     

Cooper-pair injection into quantum spin Hall insulators

We theoretically study tunneling of Cooper pairs from a superconductor spanning a two-dimensional topological insulator strip into its helical edge states. The coherent low-energy electron-pair tunneling sets off positive current cross correlations along the edges, which reflect an interplay of two quantum-entanglement processes. Most importantly, superconducting spin pairing dictates a Cooper pair partitioning into the helical edge liquids, which transport electrons in opposite directions for opposite spin orientations. At the same time, Luttinger-liquid correlations fractionalize electrons injected at a given edge into counterpropagating charge pulses carrying definite fractions of the elementary electron charge.     

Electron paramagnetic resonance study of the nuclear spin dynamics in an AlAs quantum well

The nuclear spin dynamics in an asymmetrically doped 16-nm AlAs quantum well grown along the [001] direction has been studied experimentally using the time decay of the Overhauser shift of paramagnetic resonance of conduction electrons. The nonzero spin polarization of nuclei causing the initial observed Overhauser shift is due the relaxation of the nonequilibrium spin polarization of electrons into the nuclear subsystem near electron paramagnetic resonance owing to the hyperfine interaction. The measured relaxation time of nuclear spins near the unity filling factor is (530 ± 30) min at the temperature T = 0.5 K. This value exceeds the characteristic spin relaxation times of nuclei in GaAs/AlGaAs heterostructures by more than an order of magnitude. This fact indicates the decrease in the strength of the hyperfine interaction in the AlAs quantum well in comparison with GaAs/AlGaAs heterostructures.     

2,4-二[a-(羟甲硫基)乙基]次卟啉-区及基2,4-二{a-(3-(4-羟基四氢噻唑基)]乙基}次卟啉-区的电子自旋共振及其光化动力学的研究

本文用电子自旋共振研究了2,4-二[α-(羧甲硫基)乙基]次卟啉-Ⅸ及2,4-二{α-[3-(4-羧基四氢噻唑基)]乙基}次卟啉-Ⅸ在不同温度时经紫外-可见光辐照的自由电子变化情况。本文还测定了上述样品在有氧存在下及真空条件下辐照的自旋浓度(N)为10¹⁶~10¹⁷ Spin/g。本文讨论了上述样品在光辐照时的动力学行为。     

Observation of intrinsic inverse spin Hall effect

We report observation of intrinsic inverse spin Hall effect in undoped GaAs multiple quantum wells with a sample temperature of 10 K. A transient ballistic pure spin current is injected by a pair of laser pulses through quantum interference. By time resolving the dynamics of the pure spin current, the momentum relaxation time is deduced, which sets the lower limit of the scattering time between electrons and holes. The transverse charge current generated by the pure spin current via the inverse spin Hall effect is simultaneously resolved. We find that the charge current is generated well before the first electron-hole scattering event. Generation of the transverse current in the scattering-free ballistic transport regime provides unambiguous evidence for the intrinsic inverse spin Hall effect.     

Excitation of the spin density and current by coherent light pulses in quantum wells

We study the orbital and spin dynamics of charge carriers induced by non-overlapping linearly polarized light pulses in semiconductor quantum wells. It is shown that such an optical excitation with coherent pulses leads to a spin orientation of photocarriers and an electric current. The effects are caused by the interference of optical transitions driven by individual pulses. The distribution of carriers in the spin and momentum spaces depends on the crystallographic orientation of quantum wells and can be efficiently controlled by the pulse polarizations, time delay and phase shift between the pulses, as well as an external magnetic field.     

Spin gunn effect

We predict that the flow of unpolarized current in electron-doped GaAs and InP at room temperature is unstable at high electric fields to the dynamic formation of spin-polarized current pulses. Spin-polarized current is spontaneously generated because the conductivity of a spin-polarized electron gas differs from that of an unpolarized electron gas, even in the absence of spin-orbit interaction. Magnetic fields are not required for the generation of these spin-polarized current pulses, although they can help align the polarization of sequential pulses along the same axis.     

Accuracy of circular polarization as a measure of spin polarization in quantum dot qubits

A quantum dot spin light emitting diode provides a test of carrier spin injection into a qubit and a means for analyzing carrier spin injection and local spin polarization. Even with 100% spin-polarized carriers the emitted light may be only partially circularly polarized due to the geometry of the dot. We have calculated carrier polarization-dependent optical matrix elements for InAs/GaAs self-assembled quantum dots (SAQDs) for electron and hole spin injection into a range of quantum dot sizes and shapes, and for arbitrary emission directions. Calculations for typical SAQD geometries with emission along [110] show light that is only 5% circularly polarized for spin states that are 100% polarized along [110]. Measuring along the growth direction gives near unity conversion of spin to photon polarization and is the least sensitive to uncertainties in SAQD geometry.     

Spin polarization of two-dimensional electron system in different Laughlin states and around filling factor

The spin configuration of the ground state of a two-dimensional electron system is investigated for different FQHE states from an analysis of circular polarization of time-resolved luminescence. The method clearly distinguishes between fully spin polarized, partially spin polarized and spin unpolarized FQHE ground states. We demonstrate that FQHE states which are spin unpolarized or partially polarized at low magnetic fields become fully spin polarized at high fields. Temperature dependence of the spin polarization reveals a nonmonotonic behavior at . At and the electron system is found to be fully spin polarized. This result does not indicate the existence of any skyrmionic excitations in high magnetic field limit. However, at the observed spin depolarization of electron system at and becomes broader for lower magnetic fields, so that full spin polarization remains only in a small vicinity of . Such a behavior could be considered as a precursor of skirmionic depolarization, which would dominate for smaller ratios between Zeeman and Coulomb energies.We demonstrate that the spin polarization of 2D-electron system at and can be strongly affected by hyperfine interaction between electrons and optically spin-oriented nuclears. This result is due to the fact that hyperfine interaction can both enhance and suppress effective Zeeman splitting in fixed external magnetic field.     

Influence of molecular structure on double ionization of N2 and O2 by high intensity ultrashort laser pulses

The electron momentum correlation after nonsequential double ionization of N2 and O2 in ultrashort light pulses at light intensities near 1.5 x 10(14) W/cm(2) has been investigated. The experimental results reveal distinctive differences between the molecular species and between molecules and atoms of similar ionization threshold. We provide evidence that recollision double ionization is the essential mechanism and trace the origin of the differences back to the symmetry of the orbitals occupied by the valence electrons.     

Conductance of a large point contact with Rashba effect

We study the scattering of an electron of a 2DEG through a large point contact separating a region where the electrons are free and a region where the Rashba spin-orbit coupling is present. The scattering depends dramatically on the electron incidence angle showing double refraction within the Rashba region. For incidence not normal to the interface the electron spin state is not conserved. The calculated conductance exhibits an oscillating behavior as a function of spin state of the incident electrons with different spin down and spin up currents. Our model describes both a ferromagnetic semimetallic source and a simple metallic injection electrode. In the first case the electrons are injected in a pure spin state and in the second one they are unpolarized, that is in a statistical mixture of spin up and down states. In both the cases the passage through the large point contact produces spin polarized currents.Received: 30 July 2003, Published online: 23 December 2003PACS: 85.75.Hh Spin polarized field effect transistors - 72.25.-b Spin polarized transport - 73.23.Ad Ballistic transport     

Spin manipulation of free two-dimensional electrons in Si/SiGe quantum wells

Understanding the mechanisms controlling the spin coherence of electrons in semiconductors is essential for designing structures for quantum computing applications. Using a pulsed electron paramagnetic resonance spectrometer, we measure spin echoes and deduce a spin coherence time (T2) of up to 3 mus for an ensemble of free two-dimensional electrons confined in a Si/SiGe quantum well. The decoherence can be understood in terms of momentum scattering causing fluctuating effective Rashba fields. Further confining the electrons into a nondegenerate (other than spin) ground state of a quantum dot can be expected to eliminate this decoherence mechanism.     

Metastable and spin-polarized states in electron systems with localized electron–electron interaction

We study the formation of spontaneous spin polarization in inhomogeneous electron systems with pair interaction localized in a small region that is not separated by a barrier from surrounding gas of non-interacting electrons. Such a system is interesting as a minimal model of a quantum point contact in which the electron–electron interaction is strong in a small constriction coupled to electron reservoirs without barriers. Based on the analysis of the grand potential within the self-consistent field approximation, we find that the formation of the polarized state strongly differs from the Bloch or Stoner transition in homogeneous interacting systems. The main difference is that a metastable state appears in the critical point in addition to the globally stable state, so that when the interaction parameter exceeds a critical value, two states coexist. One state has spin polarization and the other is unpolarized. Another feature is that the spin polarization increases continuously with the interaction parameter and has a square-root singularity in the critical point. We study the critical conditions and the grand potentials of the polarized and unpolarized states for one-dimensional and two-dimensional models in the case of extremely small size of the interaction region.     

Anisotropic hysteretic Hall effect and magnetic control of chiral domains in the chiral spin states of Pr2Ir2O7

We uncover a strong anisotropy in both the anomalous Hall effect (AHE) and the magnetoresistance of the chiral spin states of Pr(2)Ir(2)O(7). The AHE appearing below 1.5 K at a zero magnetic field shows hysteresis which is most pronounced for fields cycled along the [111] direction. This hysteresis is compatible with the field-induced growth of domains composed by the 3-in 1-out spin states which remain coexisting with the 2-in 2-out spin ice manifold once the field is removed. Only for fields applied along the [111] direction, we observe a large positive magnetoresistance and Shubnikov-de Haas oscillations above a metamagnetic critical field. These observations suggest the reconstruction of the electronic structure of the conduction electrons by the field-induced spin texture.