Electron Spin Polarization Effects Research Articles

The δ-doping manipulable spatial electron-spin splitter based on a single-layered semiconductor nanostructure

Introducing a δ-potential by atomic-layer doping technology, we theoretically explore its effect on Goos-Hänchen (GH) effect for electrons in a single-layered semiconductor nanostructure. Due to spin-orbit coupling, GH displacement of electrons still relies on its spins, which gives rise to an appreciable electron-spin polarization effect. Moreover, spin-dependent GH shifts can be manipulated by changing weight or position of δ-doping. Therefore, a structurally-controllable spatial electron-spin splitter can be developed for semiconductor spintronic device applications.

Bias-controllable temporal electron-spin splitter based on a magnetic nanostructure

Applying a bias along electronic transport direction, we theoretically investigate the control of a temporal electron-spin splitter (TESS), which can be realized experimentally by patterning a horizontally-magnetized ferromagnetic stripe on the surface of InAs/AlxIn1-xAs heterostructure. Due to the spin-field interaction, dwell time for electron in this TESS device is still spin related, even if a bias is applied. Thus, electron spins are separable in time dimension, giving rise to an obvious electron-spin polarization effect. Besides, both magnitude and sign of spin polarization ratio can be tuned by changing bias, leading to an electrically-controllable TESS as a tunable electron-spin polarized source for semiconductor spintronic device applications.

Dwell time and spin polarization for electron in single ferromagnetic-stripe device modulated by spin–orbit couplings

Considering both Zeeman effect and spin–orbit coupling, we calculate dwell time for electron in single ferromagnetic-stripe device (SFSD), which can be constructed by patterning a nanosized ferromagnetic stripe on the surface of GaAs/AlxGa1-xAs heterostructure. Due to an intrinsic symmetry in the SFSD, dwell time is independent of electron spins, if only Zeeman effect is involved. However, the intrinsic symmetry is broken by spin–orbit coupling, which gives rise to spin-dependent dwell time for electron in the SFSD. As a result, electron spins can be separated in time dimension, which induces an obvious electron-spin polarization effect in the SFSD. Spin polarization ratio can be efficaciously modified by interfacial confining electric-field or strain engineering, which attributes to the dependence of effective potential felt by electron in the SFSD on spin–orbit couplings. Thus, the SFSD can act as a controllable temporal electron-spin splitter, a class of electron-spin polarized sources in semiconductor spintronics.

Effect of Co doping of (Ga, In) and 2N preferred orientation on the magnetic and optical properties of ZnO

Current research on the Zn1−xMxO1−yNy (M = Ga, In) system mainly focuses on the p-type property, where Ga–N or In–N has different preferred orientations for bonding. Studies on the magnetic, bandgap, and optical properties of ZnO co doped with N are also lacking. The generalized gradient approximation (GGA+U) plane wave ultrasoft pseudopotential method based on density functional theory is used to systematically investigate the effects of the magnetic, band gap, and optical properties of the system for addressing the aforementioned problems. Results show that the formation energy of the Zn1−xMxO1−yNy (M = Ga, In) system is negative, all doping is easy, and all doped systems are stable and magnetic. The magnetism comes from the double exchange effect of electron spin polarization. Research has found that the band gap in Ga–N or In–N bonding along the c-axis direction is narrower than that along the a-axis direction. In the low-energy region, the redshift of the absorption spectrum is more significant, and the absorption or reflection coefficient and the carrier polarization and activity are stronger. In particular, In–N forms bonds along the c-axis direction and is co doped with another N in ZnO, which results in the narrowest band gap and the most significant redshift in the absorption spectrum. The absorption of sunlight can also be achieved in the ultraviolet visible near-infrared region. The absorption or reflection coefficient and the carrier polarization and activity are relatively strongest. This work has certain reference value for the design and preparation of new solar cell light absorbing materials based on ZnO.

Electron-spin polarization effect in Rashba spin-orbit coupling modulated single-layered semiconductor nanostructure

Nanothick semiconductors can grow orderly along a desired direction with the help of modern materials growth technology such as molecular beam epitaxy, which allows researchers to fabricate the so-called layered semiconductor nanostructure (LSN) experimentally. Owing to the structure inversion symmetry broken by the layered form in the LSN, the electron spins interact tightly with its momentums, in the literature referred to as the spin-orbit coupling (SOC) effect, which can be modulated well by the interfacial confining electric field or the stain engineering. These significant SOC effects can effectively eliminate the spin degeneracy of the electrons in semiconductor materials, induce the spin splitting phenomenon at the zero magnetic field and generate the electron-spin polarization in the semiconductors. In recent years, the spin-polarized transport for electrons in the LSN has attracted a lot of research interests, which is because of itself scientific importance and potential serving as spin polarized sources in the research field of semiconductor spintronics. Adopting the theoretical analysis combined with the numerical calculation, we investigate the spin-polarized transport induced by the Rashba-type SOC effect for electrons in a single-layered semiconductor nanostructure (SLSN)-InSb. The present research is to explore the new way of generating and manipulating spin current in semiconductor materials without any magnetic field, and focuses on developing new electron-spin filter for semiconductor spintronics device applications. The improved transfer matrix method (ITMM) is exploited to exactly solve Schrödinger equation for an electron in the SLSN-InSb device, which allows us to calculate the spin-dependent transmission coefficient and the spin polarization ratio. Owing to a strong Rashba-type SOC, a considerable electron-spin polarization effect appears in the SLSN-InSb device. Because of the effective potential experienced by the electrons in the SLSN-InSb device, the spin polarization ratio is associated with the electron energy and the in-plane wave vector. In particular, the spin polarization ratio can be manipulated effectively by an externally-applied electric field or the semiconductor-layer thickness, owing to the dependence of the effective potential felt by the electrons in the SLSN-InSb device on the electric field or the layer thickness. Therefore, such an SLSN-InSb device can be used as a controllable electron-spin filter acting as a manipulable spin-polarized source for the research area of semiconductor spintronics.

Analysis on the effect of electron spin polarization on a hybrid optically pumped K-Rb-21Ne co-magnetometer.

In this paper, the effect of longitudinal electron spin polarization under the combined action of alkali metal density ratio and pump laser power density on the hybrid optically pumped co-magnetometer operated in the spin-exchange relaxation-free (SERF) regime is studied. The AC response model of rotation velocity and magnetic noise of the SERF co-magnetometer system is proposed, and the factors of frequency and system bandwidth are considered. Based on the proposed response model, the error equation of the system is obtained, and the relationship between alkali metal density ratio and pump laser power density and the system noise response is theoretically analyzed and experimentally tested. The results show that when the product of pumping rate and alkali metal density ratio is greater than the electron spin relaxation rate, there is a longitudinal electron spin polarization point that minimizes the system error. In addition, the range of minimum error calculated results obtained by changing the pumping rate for the cells with different alkali metal density ratios is within 5% of the average value, that is, their minimum error potential is roughly the same within a certain range. Under the experimental conditions in this paper, due to the limitation of the electron spin relaxation rate and the operating capacity of the pump laser, the optimal alkali metal density ratio range is about 1/100-1/300.

Cooperative Effect of Electron Spin Polarization in Chiral Molecules Studied with Non-Spin-Polarized Scanning Tunneling Microscopy.

Polyalanine molecules (PA) with an α-helix conformation have recently attracted a great deal of interest, as the propagation of electrons through the chiral backbone structure comes along with spin polarization of the transmitted electrons. By means of scanning tunneling microscopy and spectroscopy under ambient conditions, PA molecules adsorbed on surfaces of epitaxial magnetic Al2O3/Pt/Au/Co/Au nanostructures with perpendicular anisotropy were studied. Thereby, a correlation between the PA molecules ordering at the surface with the electron tunneling across this hybrid system as a function of the substrate magnetization orientation as well as the coverage density and helicity of the PA molecules was observed. The highest spin polarization values, P, were found for well-ordered self-assembled monolayers and with a defined chemical coupling of the molecules to the magnetic substrate surface, showing that the current-induced spin selectivity is a cooperative effect. Thereby, P deduced from the electron transmission along unoccupied molecular orbitals of the chiral molecules is larger as compared to values derived from the occupied molecular orbitals. Apparently, the larger orbital overlap results in a higher electron mobility, yielding a higher P value. By switching the magnetization direction of the Co layer, it was demonstrated that the non-spin-polarized STM can be used to study chiral molecules with a submolecular resolution, to detect properties of buried magnetic layers and to detect the spin polarization of the molecules from the change in the magnetoresistance of such hybrid structures.

Effect of electron spin polarization in laser-assisted electron–proton scattering

This work aims to study theoretically the electron–proton scattering for initially spin-polarized electrons in the presence of a circularly polarized electromagnetic field. Using the first Born approximation and the Dirac–Volkov states for dressed electrons, we derive an analytic expression for the differential cross-section with the help of the spin-helicity formalism. Meanwhile, a well-known concept of spin-flip and spin non-flip differential cross sections is applied. The influence of the electromagnetic field on the degree of polarization of the scattered electron is investigated, and the effect of electron spin polarization is examined. We found that the initial spin of the electron can be affected depending on its kinetic energy during the scattering process. This work is a continuation of a recent paper in which we studied the unpolarized electron–proton scattering in the presence of a laser field (Dahiri, et al., 2021).

Manipulating Electron-Spin Polarization via a δ-Potential in an Embedded Magnetic-Electric-Barrier Microstructure

We theoretically explore how to manipulate electron-spin polarization by a δ-potential in an embedded magnetic-electric-barrier microstructure (EMEBM), which can be fabricated experimentally by patterning a ferromagnetic (FM) stripe and a Schottky-metal (SM) stripe on top and bottom of GaAs/AlxGa1_xAs heterostructure, respectively. Due to spin-orbit coupling (SOC) an appreciable electron-spin polarization effect still remains, even through a δ-potential is contained. Besides, electron-spin polarization ratio can be controlled by δ-potential, which may result in a structurally-manipulable electron-spin filter for spintronics device applications.

Manipulating Dresselhaus-spin-orbit-coupling induced electron-spin polarization via δ-doping in 3-layered semiconductor heterostructure

Abstract We report a theoretical exploration on the control of the Dresselhaus －spin-orbit －coupling (SOC) induced spin polarization via the δ-doping for electrons in the 3-layered semiconductor heterostructure (3-LSH). It is shown that an obvious electron-spin polarization effect still exists when a δ-doping is introduced inside. Moreover, both magnitude and sign of the spin polarization can be controlled by adjusting the weight or position of the δ-doping. Therefore, spin-polarized electrons can be manipulated by the δ-doping technique, and the considered 3-LSH can be used as a structurally-controllable spin filter, which may be helpful for spintronics device applications.

Control of electron-spin polarization via δ-potential in 3-layered semiconductor heterostructure modulated by Rashba spin-orbit coupling

We theoretically explore to manipulate the electron-spin polarization by a δ-potential in 3-layered semiconductor heterostructure modulated by the Rashba spin-orbit-coupling. An obvious electron-spin polarization effect still exists even if the δ-potential is embedded inside. Both size and sign of the electron-spin polarization can be tuned by changing the weight or position of the δ-potential. Therefore, a controllable spin filter can be obtained for spintronics device applications.

Enantiospecific Response in Cross-Polarization Solid-State Nuclear Magnetic Resonance of Optically Active Metal Organic Frameworks.

We report herein on a NMR-based enantiospecific response for a family of optically active metal-organic frameworks. Cross-polarization of the 1H-13C couple was performed, and the intensities of the 13C nuclei NMR signals were measured to be different for the two enantiomers. In a direct-pulse experiment, which prevents cross-polarization, the intensity difference of the 13C NMR signals of the two nanostructured enantiomers vanished. This result is due to changes of the nuclear spin relaxation times due to the electron spin spatial asymmetry induced by chemical bond polarization involving a chiral center. These experiments put forward on firm ground that the chiral-induced spin selectivity effect, which induces chemical bond polarization in the J-coupling, is the mechanism responsible for the enantiospecific response. The implications of this finding for the theory of this molecular electron spin polarization effect and the development of quantum biosensing and quantum storage devices are discussed.

Rashba spin-orbit coupling induced electron-spin polarization in a realistic 3-layered semiconductor heterostructure

In layered semiconductor heterostructure, there exists spin-orbit coupling (SOC), which can induce electron-spin polarization. Considering a realistic 3-layered semiconductor heterostructure, InSb/InxGa1-xAs/GaSb, we present a theoretical study of spin polarized transport by the Rashba-SOC type. An obvious electron-spin polarization effect is found in such a 3-layered system, and its degree is related to in-plane wave vector, Rashba-SOC strength and intermediate-layer parameters. Both magnitude and sign of spin polarization can be manipulated by tuning interfacial confining electric field or fabricating intermediate layer properly. These interesting features may be useful for exploring new way of spin injection and designing controllable spin filter for spintronics applications.

Manipulating spin polarization of titanium dioxide for efficient photocatalysis

Photocatalysis has been regarded as a promising strategy for hydrogen production and high-value-added chemicals synthesis, in which the activity of photocatalyst depends significantly on their electronic structures, however the effect of electron spin polarization has been rarely considered. Here we report a controllable method to manipulate its electron spin polarization by tuning the concentration of Ti vacancies. The characterizations confirm the emergence of spatial spin polarization among Ti-defected TiO2, which promotes the efficiency of charge separation and surface reaction via the parallel alignment of electron spin orientation. Specifically, Ti0.936O2, possessing intensive spin polarization, performs 20-fold increased photocatalytic hydrogen evolution and 8-fold increased phenol photodegradation rates, compared with stoichiometric TiO2. Notably, we further observed the positive effect of external magnetic fields on photocatalytic activity of spin-polarized TiO2, attributed to the enhanced electron-spin parallel alignment. This work may create the opportunity for tailoring the spin-dependent electronic structures in metal oxides.

Langmuir instability in partially spin polarized bounded degenerate plasma

Some new features of waves inside the cylindrical waveguide on employing the separated spin evolution quantum hydrodynamic model are evoked. Primarily, the instability of Langmuir wave due to the electron beam in a partially spin polarized degenerate plasma considering a nano-cylindrical geometry is discussed. Besides, the evolution of a new spin-dependent wave (spin electron acoustic wave) due to electron spin polarization effects in the real wave spectrum is elaborated. Analyzing the growth rate, it is found that in the absence of Bohm potential, the electron spin effects or exchange interaction reduce the growth rate as well as k-domain but the inclusion of Bohm potential increases both the growth rate and k-domain. Further, we investigate the geometry effects expressed by R and pon and find that they have opposite effects on the growth rate and k-domain of the instability. Additionally, how the other parameters like electron beam density or streaming speed of beam electrons influence the growth rate is also investigated. This study may find its applications for the signal analysis in solid state devices at nanoscales.

Landauer-Büttiker approach for hyperfine mediated electronic transport in the integer quantum Hall regime

The interplay of spin-polarized electronic edge states with the dynamics of the host nuclei in quantum Hall systems presents rich and non-trivial transport physics. Here, we develop a Landauer-B\"uttiker approach to understand various experimental features observed in the integer quantum Hall set ups featuring quantum point contacts. The approach developed here entails a phenomenological description of spin resolved inter-edge scattering induced via hyperfine assisted electron-nuclear spin flip-flop processes. A self-consistent simulation framework between the nuclear spin dynamics and edge state electronic transport is presented in order to gain crucial insights into the dynamic nuclear polarization effects on electronic transport and in turn the electron-spin polarization effects on the nuclear spin dynamics. In particular, we show that the hysteresis noted experimentally in the conductance-voltage trace as well as in the resistively detected NMR lineshape results from a lack of quasi-equilibrium between electronic transport and nuclear polarization evolution. In addition, we present circuit models to emulate such hyperfine mediated transport effects to further facilitate a clear understanding of the electronic transport processes occurring around the quantum point contact. Finally, we extend our model to account for the effects of quadrupolar splitting of nuclear levels and also depict the electronic transport signatures that arise from single and multi-photon processes.

Optically Induced Nuclear Spin Polarization in the Quantum Hall Regime: The Effect of Electron Spin Polarization through Exciton and Trion Excitations.

We study nuclear spin polarization in the quantum Hall regime through the optically pumped electron spin polarization in the lowest Landau level. The nuclear spin polarization is measured as a nuclear magnetic field B(N) by means of the sensitive resistive detection. We find the dependence of B(N) on the filling factor nonmonotonic. The comprehensive measurements of B(N) with the help of the circularly polarized photoluminescence measurements indicate the participation of the photoexcited complexes, i.e., the exciton and trion (charged exciton), in nuclear spin polarization. On the basis of a novel estimation method of the equilibrium electron spin polarization, we analyze the experimental data and conclude that the filling factor dependence of B(N) is understood by the effect of electron spin polarization through excitons and trions.

Spin polarization of electrons tunneling through magnetic-barrier nanostructures

We present a theoretical investigation of the spin-dependent transport properties of electrons in nanostructures consisting of realistic magnetic barriers created by a lithographic patterning of ferromagnetic or superconducting films. It is shown that a significant electron-spin polarization effect can be induced by such magnetic-barrier nanostructures with a symmetric magnetic field. It is also shown that an applied bias voltage or an external magnetic field can greatly change the degree of the electron-spin polarization in magnetic-barrier nanostructures. When the applied bias voltage increases, the electron-spin polarization shifts toward the low-energy end and gradually decreases, while, with an increase of the external magnetic field, the electron-spin polarization shifts toward the high-energy direction and successively enlarges. It is also found that the degree of the electron-spin polarization can be tuned with the electric barrier induced by a constant voltage applied to the metallic stripe of system.

Spin-polarization effect and wave-vector filtering effect in composite magnetic-barrier structures

We propose a composite magnetic-barrier structure, which is alternately arranged with two different kinds of magnetic barriers. We theoretically investigate the transport properties for electrons tunnelling through this type of magnetic-barrier structure with and without the interaction of the electronic spin and the magnetic field. It is shown that the composite magnetic barriers greatly enhance the wave-vector filtering properties in contrast to the corresponding non-composite magnetic barriers consisting of two identical magnetic barriers. It is also shown that the composite magnetic barriers exhibit a rather evident electron spin-polarization effect, especially in low-energy region, while the corresponding non-composite magnetic barriers do not possess the electron spin-polarization effect.

Electron spin–orbit split minibands in semiconductor asymmetric superlattices

Semiconductor superlattices can be either symmetric or asymmetric with respect to specular reflection along the growth direction. The electronic miniband structure of asymmetric superlattices is in general spin dependent, due to spin‐orbit interaction. Using Kane’s kip model for the bulk and standard envelope function formalism, we have calculated the spin dependent transmission probability for electrons crossing different III‐V politype multibarrier nanostructures. We have obtained spin dependent intervals of energy with nonzero transmission, corresponding to the minibands of allowed electronic states in the superlattice. Spin‐orbit split minibands for InGaAs superlattices, with asymmetric double barrier unit cells and different pairs of latticematched barrier materials, are obtained from the transmission and reflection coefficients for the unit cell. The miniband structure is well reproduced by the transfer matrix calculation with already three unit cells. The symmetric‐asymmetric crossover as well as the miniband formation from the double barrier spin split resonances were also investigated. The effect of electron spin polarization by resonant tunneling is shown to be enhanced with the use of multibarrier or superlattice structures.