Spin-down Electrons Research Articles

Investigations of interface spin asymmetry and interfacial resistance in FexCo100− x/Ag interface

We investigated current-perpendicular-to-plane (CPP) magnetotransport parameters of FexCo100−x/Ag interfaces: interface specific resistance (ARF/N), extended interface resistance (AR*F/N), and spin scattering asymmetry (γ). We also investigated the dependence of interfacial parameters on the giant magnetoresistance effect with CPP geometry. For measuring these parameters, we prepared magnetic multilayers and pseudo spin valves (PSVs), combining the ferromagnetic (F) alloys Fe, Co, Fe50Co50, and Fe30Co70 with the nonmagnetic (N) metal Ag. In all cases, the largest ARF/N value was found to be ∼0.68 mΩ μm2 with an enhanced AR*F/N value of ∼1.9 mΩ μm2 for a (001)-oriented Fe/Ag interface, which suggests that spin-up and spin-down electrons on the Fermi surface have very different transmission probabilities. Such an interface can act as a spin filter through which only one type of electrons can pass.

Separated spin-up and spin-down quantum hydrodynamics of degenerated electrons: Spin-electron acoustic wave appearance.

The quantum hydrodynamic (QHD) model of charged spin-1/2 particles contains physical quantities defined for all particles of a species including particles with spin-up and with spin-down. Different populations of states with different spin directions are included in the spin density (the magnetization). In this paper I derive a QHD model, which separately describes spin-up electrons and spin-down electrons. Hence electrons with different projections of spins on the preferable direction are considered as two different species of particles. It is shown that the numbers of particles with different spin directions do not conserve. Hence the continuity equations contain sources of particles. These sources are caused by the interactions of the spins with the magnetic field. Terms of similar nature arise in the Euler equation. The z projection of the spin density is no longer an independent variable. It is proportional to the difference between the concentrations of the electrons with spin-up and the electrons with spin-down. The propagation of waves in the magnetized plasmas of degenerate electrons is considered. Two regimes for the ion dynamics, the motionless ions and the motion of the degenerate ions as the single species with no account of the spin dynamics, are considered. It is shown that this form of the QHD equations gives all solutions obtained from the traditional form of QHD equations with no distinction of spin-up and spin-down states. But it also reveals a soundlike solution called the spin-electron acoustic wave. Coincidence of most solutions is expected since this derivation was started with the same basic equation: the Pauli equation. Solutions arise due to the different Fermi pressures for the spin-up electrons and the spin-down electrons in the magnetic field. The results are applied to degenerate electron gas of paramagnetic and ferromagnetic metals in the external magnetic field. The dispersion of the spin-electron acoustic waves in the partially spin-polarized degenerate neutron matter are also considered.

新型稀磁半导体Cu掺杂LiMgN的电子结构和光学性质

Using first-principles density functional theory based on the full potential linearized augmented plane wave method and the local spin density approximation (LSDA), the geometric structures of pure LiMgN, Cu-doped LiMgN, and Cu-doped LiMgN with either excess Li or Li deficiency are optimized, and their electronic structures, half-metallic properties, formation energies and optical properties are calculated. The results indicate that Cu doping makes the Cu3d states hybridize with the Li2s and N2p states simultaneously, and this leads to sp-d orbital hybridization. The resulting system produces spin polarization impurity bands, displays half-metallic behavior and has a net magnetic moment. Li(Mg 0.9375 Cu 0.0625 )N is still a direct band gap semiconductor like LiMgN. The bottom of its conduction band and the top of its valance band are both at the G point in the Brillouin zone. The band gap of the material is 2.41 eV, which is slightly narrower than the value of 2.47 eV for LiMgN. There are a total of six impurity bands in the band gap. Three of these bands are in the spin-up band, and the other three are in the spin-down band. In the spin-down band, the Fermi level goes through the impurity bands, and thus the material shows half-metallic properties. The half-metallic energy gap is 0.09 eV. The strong sp-d orbital hybridization causes the energy level to split, and the t2g energy level of the Cu3d state electrons in the spin-down band is pushed above the Fermi level, which leads to half-filled states. The number of spin-up electrons is slightly more than that of the spin-down electrons, and the Li(Mg 0.9375 Cu 0.0625 )N system has a 1.04 μ в net magnetic moment. The properties of the Li-deficient or Li-excess systems are affected by the stoichiometric number of Li in each case. In the Li-deficient compounds, the Li vacancies strengthen the sp-d orbital hybridization, and make the impurity band width, the half-metallic properties, the net magnetic moment, and the Curie temperature increase, while the formation energy decreases from 2.23 to 1.50 eV. In contrast, the half-metallic behavior of the Li-excess Cu-doped LiMgN vanishes, and the system becomes an indirect band gap semiconductor, although its band gap narrows to 2.13 eV, and its conductive properties become stronger. These results show that the LiMgN semiconductor is a realistic candidate diluted magnetic semiconductor material with decoupled charge and spin doping, in which the charge and the spin can be injected separately via Li off-stoichiometry and Cu isovalent doping. The calculated dielectric function indicates that these Cu-doped systems have a new dielectric peak in the low-energy region. The Li deficiency enhances both the dielectric peak and the absorption of low-frequency electromagnetic waves, while there is an obvious change in the complex refractive index function.

Anisotropic magnetoresistance in facing-target reactively sputtered epitaxial γ′-Fe4N films

The negative anisotropic magnetoresistance that comes from the spin-down conduction electrons are observed in facing-target sputtered epitaxial γ′-Fe4N films with different film thicknesses, substrates and orientations. Anisotropic magnetoresistance of γ′-Fe4N films on LaAlO3(100) is larger than other substrates. The magnitude of anisotropic magnetoresistance in (100)-oriented films is always larger than (110)-oriented films. The anisotropic magnetoresistance is intimately related to the magnetocrystalline anisotropy. Fourier coefficient C2θ and C4θ of cos2θ and cos4θ terms strongly depend on the measuring temperature. No significant influence of magnetic field on C2θ and C4θ appears. The marked change of C2θ and appearance of C4θ at low temperatures are from crystal field splitting of d orbitals induced by the lattice change due to the tensile stress from substrate and the compressive stress from decreased temperatures.A. Magnetic materials; A. Nitrides; B. Epitaxial growth; D. Electrical properties

Controlling spin–orbit interaction in a ferromagnetic Fe/Au double layer

Using spin-polarized single- and two-electron spectroscopy, we probe exchange and spin–orbit interaction in a double layer of Fe and Au on W(110) and measure the spin asymmetry of the Bloch spectral density function of the sample. In a 5 ML iron film, the spin-orbit contribution to the measured asymmetry of the (e,2e) spectra was not detectable, whereas a deposition of about 1 ML of gold introduced a substantial spin-orbit component in the measured asymmetry. At the same time, this double layer still exhibits ferromagnetic properties: (i) the spectral density function asymmetry demonstrate imbalance of spin-up and spin-down electron densities in the valence band and (ii) the Stoner excitation asymmetry has almost the same value as in a pure Fe film.

Effect of spin-orbit coupling on the wave vector and spin dependent transmission probability for the GaN/AlGaN/GaN heterostructure

The effect of the Rashba and Dresselhaus spin–orbit coupling (SOC) on the transmission of electrons through the GaN/AlGaN/GaN heterostructure is studied. It is found that the Dresselhaus SOC causes the evident dependence of the transmission probability on the spin polarization and the in-plane wave vector of electrons, and also induces evident spin splitting of the resonant peaks in the (Ez-k) plane. Because the magnitude of the Rashba SOC is relatively small, its effect on the transmission of electrons is much less. As k increases, the peaks of transmission probability for spin-up electrons (T+) shift to a higher energy region and increase in magnitude, while the peaks of transmission probability for spin-down electrons (T−) shift to a lower energy region and decrease in magnitude. The polarization efficiency (P) is found to peak at the resonant energies and increases with the in-plane wave vector. Moreover, the built-in electric field caused by the spontaneous and piezoelectric polarization can increase the amplitude of P. Results obtained here are helpful for the efficient spin injection into the III-nitride heterostructures by nonmagnetic means from the device point of view.

Spin helical states and spin transport of the line defect in silicene lattice

We investigated the electronic structure of a silicene-like lattice with a line defect under the consideration of spin–orbit coupling. In the bulk energy gap, there are defect related bands corresponding to spin helical states localized beside the defect line: spin-up electrons flow forward on one side near the line defect and move backward on the other side, and vice versa for spin-down electrons. When the system is subjected to random distribution of spin-flipping scatterers, electrons suffer much less spin-flipped scattering when they transport along the line defect than in the bulk. An electric gate above the line defect can tune the spin-flipped transmission, which makes the line defect as a spin-controllable waveguide. • Band structure of silicene with a line defect. • Spin helical states around the line defect and their probability distribution features. • Spin transport along the line defect and that in the bulk silicene.

A spin beam splitter in graphene through the Goos–Hänchen shift

We propose a method of realizing an effective electron spin beam splitter in graphene through the Goos-Hänchen effect. The device consists of a layer of monolayer graphene on which two ferromagnetic stripes are deposited with parallel or antiparallel magnetization configuration. It is shown that the transmitted spin-up and spin-down electron beams are found at different longitudinal positions and their spatial separation can be enhanced by the number of transmission resonances formed between the two ferromagnetic stripes. The spatial separation between the spin-up and spin-down electron beams can reach values up to hundreds of wavelengths, which can be observed experimentally.

Domain Wall in Multimode Peierls State

The domain wall as a one-dimensional discommensuration of the multimode Peierls (MMP) lattice distortion is investigated numerically based on the Su–Schrieffer–Heeger model generalized to the square lattice with the half-filled electronic band. In order to create the domain wall, we consider a spin excitation where the total number of spin-up electrons is larger than that of spin-down electrons by the linear dimension of the square lattice. The extra spins are localized at the domain wall, and the localized wave functions form a mid-gap band with zero energy. The localized wave functions are analytically constructed in the same way as the edge states in the graphene nanoribbon. The properties of the localized states for the domain walls with various orientations to the MMP distortion pattern are elucidated. The number of the zero-energy states for every orientation satisfies the index theorem established in the Dirac electron system. Furthermore, the correlation effects on the domain wall are studied by introducing the on-site and nearest-neighbor Coulomb interactions to the electron–lattice model. It is revealed that the creation energy of the domain wall decreases due to the electron–electron interactions.

Strain-modulation of spin-dependent transport in graphene

We investigate strain modulation of the spin-dependent electron transport in a graphene junction using the transfer matrix method. As an analogy to optics, we define the modulation depth in the electron optics domain. Additionally, we discuss the transport properties and show that the modulation depth and the conductance depend on the spin-orbit coupling strength, the strain magnitude, the width of the strained area, and the energy of the incident electron. The conductances of the spin-down and spin-up electrons have opposite and symmetrical variations, which results in the analogous features of their modulation depths. The maximum conditions for both the modulation depth and the electron spin upset rate are also analyzed.

Evidence of Andreev bound states as a hallmark of the FFLO phase in κ-(BEDT-TTF)2Cu(NCS)2

Superconductivity is a quantum phenomena arising, in its simplest form, from pairing of fermions with opposite spin into a state with zero net momentum. Whether superconductivity can occur in fermionic systems with unequal number of two species distinguished by spin, atomic hyperfine states, flavor, presents an important open question in condensed matter, cold atoms, and quantum chromodynamics, physics. In the former case the imbalance between spin-up and spin-down electrons forming the Cooper pairs is indyced by the magnetic field. Nearly fifty years ago Fulde, Ferrell, Larkin and Ovchinnikov (FFLO) proposed that such imbalanced system can lead to exotic superconductivity in which pairs acquire finite momentum. The finite pair momentum leads to spatially inhomogeneous state consisting of of a periodic alternation of "normal" and "superconducting" regions. Here, we report nuclear magnetic resonance (NMR) measurements providing microscopic evidence for the existence of this new superconducting state through the observation of spin-polarized quasiparticles forming so-called Andreev bound states.

Molecular Spintronics: Destructive Quantum Interference Controlled by a Gate

The ability to control the spin-transport properties of a molecule bridging conducting electrodes is of paramount importance to molecular spintronics. Quantum interference can play an important role in allowing or forbidding electrons from passing through a system. In this work, the spin-transport properties of a polyacetylene chain bridging zigzag graphene nanoribbons (ZGNRs) are studied with nonequilibrium Green's function calculations performed within the density functional theory framework (NEGF-DFT). ZGNR electrodes have inherent spin polarization along their edges, which causes a splitting between the properties of spin-up and spin-down electrons in these systems. Upon adding an imidazole donor group and a pyridine acceptor group to the polyacetylene chain, this causes destructive interference features in the electron transmission spectrum. Particularly, the donor group causes a large antiresonance dip in transmission at the Fermi energy EF of the electrodes. The application of a gate is investigated and found to provide control over the energy position of this feature making it possible to turn this phenomenon on and off. The current-voltage (I-V) characteristics of this system are also calculated, showing near ohmic scaling for spin-up but negative differential resistance (NDR) for spin-down.

Self-polarized spin-nanolasers.

Besides adding a new functionality to conventional lasers, spin-polarized lasers can, potentially, offer lower threshold currents and reach higher emission intensities. However, to achieve spin-polarized lasing emission a material should possess a slow spin relaxation and a high propensity to be injected with spin-polarized currents. These are stringent requirements that, so far, have limited the choice of candidate materials for spin-lasers. Here we show that these requirements can be relaxed by using a new self-polarized spin mechanism. Fe3O4 nanoparticles are coupled to GaN nanorods to form an energy-band structure that induces the selective charge transfer of electrons with opposite spins. In turn, this selection mechanism generates the population imbalance between spin-up and spin-down electrons in the emitter's energy levels without an external bias. Using this principle, we demonstrate laser emission from GaN nanorods with spin polarization up to 28.2% at room temperature under a low magnetic field of 0.35 T. As the spin-selection mechanism relies entirely on the relative energy-band alignment between the iron oxide nanoparticles and the emitter and requires neither optical pumping with circularly polarized light nor electrical pumping with magnetic electrodes, potentially a wide range of semiconductors can be used as spin-nanolasers.

Spin-dependent dwell time through ferromagnetic graphene barrier

We investigated the dwell time of electrons tunneling through a ferromagnetic (FM) graphene barrier. The results show that the spin polarization can be efficiently controlled by the barrier width, barrier height, and the incident electron energy. Furthermore, it is found that electrons with different spin orientations will spend different times through the barrier. The difference of the dwell time between spin-up and spin-down electrons arises from the exchange splitting, which is induced by the FM strip. Study results indicate that a ferromagnetic graphene barrier can cause a nature spin filter mechanism in the time domain.

Rashba spin–orbit effect on dwell time in graphene asymmetrical barrier

We investigated the dwell time of electrons tunneling through an asymmetric barrier in monolayer graphene with the Rashba spin–orbit interaction (RSOI). The results show that the dwell time oscillates by increasing the barrier width, RSOI strength, and the bias voltage. In addition, when an external electric field is present, the spin polarization can be observed, whereas for the RSOI alone it is zero. Furthermore, it is found that electrons with different spin orientations will spend different times through the barrier. The difference of the dwell time between spin-up and spin-down electrons arises from the Rashba spin–orbit interaction. It can be concluded that Rashba spin–orbit effect can cause a natural spin filter mechanism in the time domain.

High-efficiency spin filtering in salophen-based molecular junctions modulated with different transition metal atoms

Abstract Based on the density functional theory and nonequilibrium Green's function methods, we investigate the spin transport properties of the molecular junctions constructed by a homologous series of 3d transition metal(II) salophens (TM-salophens, TM = Co, Fe, Ni and Mn) sandwiched between two gold electrodes. It is found that among the four molecular junctions only Co-salophen junction can act as an efficient spin filter distinctively. The conductance through Co-salophen molecular junction is dominated by spin-down electrons. The mechanism is proposed for these phenomena.

Spin Current and Its Electrical Detection in Semiconductors

The idea of utilizing the electron spin in semiconductor devices leads to the growth of the field semiconductor spintronics. This may result in the development of novel electronic devices with advantages over conventional electronics. However, the basic requirements necessary in developing semiconductor spintronic devices are the efficient generation/injection of spins (or spin current) in/into a semiconductor such that they can be transported reliably, i.e., without spin relaxation, over reasonable distances and, finally, detection of them. Since in spintronic devices, the information is carried by the electron spin, an electrical means of detecting spin current in semiconductors is desirable for fully exploring the possibility of utilizing spin degree of freedom and for possible device applications. Here, I describe an experiment which detects the spin current electrically in semiconductors by investigating the effect of a longitudinal electric field on the spin-polarized electrons generated by a circularly polarized light. The experiment observes the effect as a pure anomalous Hall voltage resulting from non-equilibrium magnetization induced by the spin-carrier electrons accumulating at the transverse edges of the sample as a result of asymmetries in scattering for spin-up and spin-down electrons in the presence of spin-orbit interaction. The results are presented by discussing the dominant spin relaxation mechanisms in semiconductors, and in relation with the recent understanding in this area.

Spin-dependent tunneling rates for electrostatically defined GaAs quantum dots

The tunneling rates for spin-up and -down electrons are investigated for a GaAs quantum dot in an in-plane magnetic field by using a real-time single-electron counting scheme with a nearby charge detector. An extremely small spin-polarized current on the order of attoamperes is analyzed with the spin and energy dependences of the tunneling rates. Fully spin-polarized current is obtained when only a spin-up Zeeman sublevel is located in the transport window. When both Zeeman sublevels are allowed to contribute to the transport, we find that the tunneling rate for spin-up electrons is considerably higher than that for spin-down electrons. This partially spin-polarized current can be explained by the exchange-enhanced spin splitting in low-density regions near the tunneling barriers.

Spin-dependent delay time in ferromagnet/insulator/ferromagnet heterostructures

We study theoretically spin-dependent group delay and dwell time in ferromagnet/insulator/ferromagnet (FM/I/FM) heterostructure. The results indicate that, when the electrons with different spin orientations tunnel through the FM/I/FM junction, the spin-up process and the spin-down process are separated on the time scales. As the self-interference delay has the spin-dependent features, the variations of spin-dependent dwell-time and spin-dependent group-delay time with the structure parameters appear different features, especially, in low incident energy range. These different features show up as that the group delay times for the spin-up electrons are always longer than those for spin-down electrons when the barrier height or incident energy increase. In contrast, the dwell times for the spin-up electrons are longer (shorter) than those for spin-down electrons when the barrier heights (the incident energy) are under a certain value. When the barrier heights (the incident energy) exceed a certain value, the dwell times for the spin-up electrons turn out to be shorter (longer) than those for spin-down electrons. In addition, the group delay time and the dwell time for spin-up and down electrons also relies on the comparative direction of magnetization in two FM layers and tends to saturation with the thickness of the barrier.

Tuning the band structure, magnetic and transport properties of the zigzag graphene nanoribbons/hexagonal boron nitride heterostructures by transverse electric field.

The paper presents the results of ab initio study of the opportunities for tuning the band structure, magnetic and transport properties of zigzag graphene nanoribbon (8-ZGNR) on hexagonal boron nitride (h-BN(0001)) semiconductor heterostructure by transverse electric field (E(ext)). This study was performed within the framework of the density functional theory (DFT) using Grimme's (DFT-D2) scheme. We established the critical values of E(ext) for the 8-ZGNR/h-BN(0001) heterostructure, thereby providing for semiconductor-halfmetal transition in one of electron spin configurations. This study also showed that the degeneration in energy of the localized edge states is removed when E(ext) is applied. In ZGNR/h-BN (0001) heterostructure, value of the splitting energy was higher than one in ZGNRs without substrate. We determined the effect of low E(ext) applied to the 8-ZGNR/h-BN (0001) semiconductor heterostructure on the preserved local magnetic moment (LMM) (0.3μ(B)) of edge carbon atoms. The transport properties of the 8-ZGNR/h-BN(0001) semiconductor heterostructure can be controlled using E(ext). In particular, at a critical value of the positive potential, the electron mobility can increase to 7× 10(5) cm(2)/V s or remain at zero in the spin-up and spin-down electron subsystems, respectively. We established that magnetic moments (MMs), band gaps, and carrier mobility can be altered using E(ext). These abilities enable the use of 8-ZGNR/h-BN(0001) semiconductor heterostructure in spintronics.