Thermoelectric effects in spin field-effect transistors

We investigate the thermoelectric effects in a spin field-effect transistor with ferromagnetic leads held at different temperatures. The thermopower S and thermoelectric figure of merit ZT oscillate with the increase of the Rashba spin-orbit coupling strength. The oscillation amplitude of ZT decreases with increasing the spin polarization. S and ZT are strongly influenced by the interfacial barrier strength Z, exhibiting a nonmonotonous change with Z. The thermoelectric effects are also manipulated by the magnetization configuration of the ferromagnetic leads. It is expected that the present study of the thermoelectric effects is helpful in the design of thermoelectric devices.     

Thermoelectric figure of merit in a quantum wire coupled to a graphene sheet between ferromagnetic leads

We theoretically investigate the figure of merit ZT for a quantum wireside-coupled by a graphene sheet and sandwiched between two ferromagnetic electrodes withnoncollinear magnetic moments. By using the nonequilibrium Green’s function combining withthe tight-binding Hamiltonian, we demonstrate that the ZT for the system developsan oscillating behavior and weakly depends on the wire-graphene coupling strength as wellas magnetic configuration of the leads. On the contrary, it is strongly dependent ontemperature and the polarization strength of the leads. Importantly, the maximum value ofZT for thesystem without the polarization strength (p = 0) is about 1.1 at temperature k _B T =0.015Γ ₀, which is in agreement with theexperimental measurements for silicon nanowires.     

Spin splitter regime of a mesoscopic Rashba ring

Using the non-equilibrium Greens' function formalism we calculate the spin currents in a one-dimensional ring coupled to three leads and in the presence of perpendicular magnetic flux Φ and Rashba spin-orbit coupling. A finite bias is applied between the input lead and the other two output leads. We show that the spin-orbit coupling allows one to operate this system as a spin splitter, i.e. the output leads deliver spin-polarized currents with different orientations. We find that the spin splitter operation can be tuned at integer multiples of Φ/Φ₀. Its efficiency depends not only on the value of the Rashba coupling but also on the bias applied between the input and output leads. The selected spin orientation of the output leads can be reversed by a slight change of their contact position. We discuss as well the connection between the spin splitter operation and the spectral properties of the ring.     

Power output and efficiency of quantum dot attached to ferromagnetic electrodes with non-collinear magnetic moments

Non-linear charge and heat transport through a single-level quantum dot in the Coulomb blockade regime is investigated within the framework of non-equilibrium Green function formalism and power output and efficiency of the device are studied. It is found that maximum power as well as efficiency depends on the relative orientation of magnetic moments in electrodes and can vary with polarization factor p. In general, power output is suppressed in magnetic systems and decreases with polarization. The highest efficiency can be attained in antiparallel configuration, and moreover, it does not depend on p. Spin power as well as spin efficiency of the system is introduced and discussed. It is also shown that in the Coulomb blockade regime the (spin) efficiency of the device operating under maximum power conditions varies with temperature bias in a non-monotonic way and shows a flat maximum for low ΔT.     

Epitaxially stabilized AgGaSe2 for high-efficiency spin-polarized electron source

High-quality spin-polarized electron source (SPES) is of fundamental importance in the investigation of spin-dependent phenomena. Generally speaking, an ideal material for SPES application should have both large spin–orbit and positive crystal-field splitting. Currently, almost all sources in use with accelerators are based on photoemission from GaAs and related materials such as strained GaAs grown on GaAsP or InGaAs grown on GaAs. Nevertheless, the reduced critical layer thickness of these strained films leads to poor material quality and, consequently, low quantum efficiency. Besides other ordered ternary semiconductor compounds, tetragonal chalcopyrite ternary compounds have also been considered. However, since all these compounds have zero or negative crystal-field splitting, the achieved polarization and quantum efficiency are rather low. Here we propose a new material, AgGaSe₂ in the CuAu phase, as a high-quality SPES. We show that it is possible to grow epitaxially strain-free AgGaSe₂ in the CuAu phase on ZnSe substrate. Since this material has a direct-band gap, a large spin–orbit splitting, as well as a large positive crystal-field splitting, it is predicted to be a promising material for SPES with 100% spin polarization.     

Polarization transfer in deuteron stripping reactions

Polarization phenomena involving the spins of a and b in the A(a, b)B reaction are discussed using a complete set of irreducible tensors carrying definite spin transfer. The linear model independent equations relating the cross section and the polarization observables with these tensors are shown to be particularly appropriate for the study of spin dependent interactions, preferentially associated with particular values of spin transfer. The DWBA theory of polarization transfer in deuteron stripping reactions is thoroughly discussed and among the 17 polarization observables we distinguish those likely to be more sensitive to spin dependent distortion, to have stronger deuteron D-state effects, to exhibit the sign-rule j-dependence and other forms of j-dependence. For certain deuteron polarizations, when the spin transfer is pure $\text{s =}\text{1}\text{2}$, it is shown that deuteron stripping reactions are transparent to vector polarization transfer and the outgoing nucléon polarization independent of scattering angle and deuteron energy. DWBA calculations including contributions from spin transfer $\text{3}\text{2}$ through the deuteron D-state and spin-orbit distortion show that polarization transfer in such deuteron polarizations can be explored as a method of producing fast polarized neutrons with known polarization.     

Vertex correction of the anisotropic magnetic impurities to the spin polarization of 2D electron gas

In terms of Kubo's formula and Green's function method, for the two-dimensional electron gas (2DEG) with Rashba spin-orbit coupling (SOC), we study the spin polarization due to the effect from magnetic impurities with anisotropic spin dependent delta type coupling to electrons when an external dc electric field in plane is applied. The vertex correction of impurities in ladder approximation is carried out in the limit of EF?1/τ, Δ. We find that the strength of spin polarization can be significantly modified by vertex correction and the spin polarization is relevant to the anisotropy coefficient γ, but the direction of net spin polarization cannot be changed.     

Spin-dependent current in resonant tunneling diode with ferromagnetic GaMnN layers

The spin-polarized tunneling current through a double barrier resonant tunneling diode (RTD) with ferromagnetic GaMnN emitter/collector is investigated theoretically. Two distinct spin splitting peaks can be observed at current-voltage (I-V) characteristics at low temperature. The spin polarization decreases with the temperature due to the thermal effect of electron density of states. When charge polarization effect is considered at the heterostructure, the spin polarization is enhanced significantly. A highly spin-polarized current can be obtained depending on the polarization charge density.     

Thermoelectric effects in a double-dot Aharonov-Bohm interferometer with Rashba spin-orbit interaction

We investigate the thermoelectric effects in a double-dot Aharonov-Bohm interferometer coupled to ferromagnetic leads held at different temperatures. The interplay of Rashba spin-orbit interaction (RSOI) and magnetic flux ϕ induces various interesting spin-dependent interference phenomena. The thermoelectric transport oscillates with ϕ. The peak of the thermopower S and figure of merit ZT splits into two new peaks and its splitting increases with the Rashba induced phase factor φ _R . With increasing φ _R S and ZT at ϕ = ± 2nπ (n = 0,1,2,...) exhibit a conversion from a peak to a valley. In the presence of the interplay of RSOI and ϕ by increasing spin polarization the splitting peaks of S (ZT) become asymmetric and ZT is greatly enhanced. The influence of the quantum dot levels on thermoelectric effects is also analyzed.     

Effects of geometry and linearly polarized cavity photons on charge and spin currents in a quantum ring with spin-orbit interactions

We calculate the persistent charge and spin polarization current inside a finite-width quantum ring of realistic geometry as a function of the strength of the Rashba or Dresselhaus spin-orbit interaction. The time evolution in the transient regime of the two-dimensional (2D) quantum ring connected to electrically biased semi-infinite leads is governed by a time-convolutionless non-Markovian generalized master equation. The electrons are correlated via Coulomb interaction. In addition, the ring is embedded in a photon cavity with a single mode of linearly polarized photon field, which is polarized either perpendicular or parallel to the charge transport direction. To analyze carefully the physical effects, we compare to the analytical results of the toy model of a one-dimensional (1D) ring of non-interacting electrons with spin-orbit coupling. We find a pronounced charge current dip associated with many-electron level crossings at the Aharonov-Casher phase ΔΦ = π, which can be disguised by linearly polarized light. Qualitative agreement is found for the spin polarization currents of the 1D and 2D ring. Quantitatively, however, the spin polarization currents are weaker in the more realistic 2D ring, especially for weak spin-orbit interaction, but can be considerably enhanced with the aid of a linearly polarized electromagnetic field. Specific spin polarization current symmetries relating the Dresselhaus spin-orbit interaction case to the Rashba one are found to hold for the 2D ring, which is embedded in the photon cavity.     

Influence of in-plane magnetic field on spin polarization in the presence of the oft-neglected k-Dresselhaus spin-orbit coupling

The influence of in-plane magnetic field on spin polarization in the presence of the oft-neglected k³-Dresselhaus spin-orbit coupling was investigated. The k³-Dresselhaus term can produce a limited spin polarization. The in-plane magnetic field plays a great role in the tunneling process. It can generate the perfect spin polarization of the electrons and the ideal transmission coefficient for spin up and down simultaneously. In energy scale, complete separation between spin up and down resonance was obtained by a relatively higher in-plane magnetic field while a comparatively lower in-plane magnetic field vanishes the spin separation. On the other hand, the spin relaxation can be suppressed by compensating the oft-neglected k³-Dresselhaus spin orbit coupling using a relatively lower in-plane magnetic field.     

Coulomb drag, magnetoresistance, and spin-current injection in magnetic multilayers

We study the effect of spin Coulomb drag on the magnetoresistance and the spin-current injection efficiency of a layered structure consisting of a nonmagnetic semiconductor sandwiched between two ferromagnetic electrodes of spin polarization p. The calculations are done within the framework of the drift-diffusion theory, which we generalize to include the spin trans-conductivity σ_↑↓. We find that for p close to 100% the spin drag enhances the magnetoresistance, while for smaller values of p it reduces it. A new approach to the measurement of σ_↑↓ is suggested.     

A structurally-controllable spin filter in a δ-doped magnetically modulated semiconductor nanostructure with zero average magnetic field

We report on a theoretical investigation of spin-polarized transport in a δ-doped magnetically modulated semiconductor nanostructure, which can be experimentally realized by depositing a ferromagnetic stripe on the top of a semiconductor heterostructure and by using the atomic layer doping technique such as molecular beam epitaxy (MBE). It is shown that although such a nanostructure has a zero average magnetic filed, a sizable spin polarization exists due to the Zeeman splitting mechanism. It is also shown that the degree of spin polarization varies sensitively with the weight and/or position of the δ-doping. Therefore, one can conveniently tailor the behaviour of the spin-polarized electron by tuning the δ -doping, and such a device can be employed as a controllable spin filter for spintronics.     

Dresselhaus spin-orbit coupling induced spin-polarization and resonance-split in n-well semiconductor superlattices

Using the transfer matrix method, we investigate the electron transmission over multiple-well semiconductor superlattices with Dresselhaus spin-orbit coupling in the potential-well regions. The superlattice structure enhances the effect of spin polarization in the transmission spectrum. The minibands of multiple-well superlattices for electrons with different spin can be completely separated at the low incident energy, leading to the 100% spin polarization in a broad energy windows, which may be an effective scheme for realizing spin filtering. Moreover, for the transmission over n-quantum-well, it is observed that the resonance peaks in the minibands split into n-folds or (n−1)-folds depending on the well-width and barrier-thickness, which is different from the case of tunneling through n-barrier structure.     

Voltage-tunable spin electron beam splitter based on antiparallel double δ-magnetic-barrier nanostructure

We report on a theoretical study of spin-dependent Goos-Hänchen (GH) shift of electrons in antiparallel double δ-magnetic-barrier (MB) nanostructure under an applied voltage, which can be experimentally realized by depositing two metallic ferromagnetic (FM) stripes on top and bottom of the semiconductor heterostructure. GH shifts for spin electron beams across this device, is exactly calculated, with the help of the stationary phase method. It is shown that a considerable spin polarization of GH shifts can be achieved in this device for two δ-MBs with unidentical magnetic strengths. It also is shown that both magnitude and sign of spin polarization of GH shifts can be controlled by adjusting the electric potential induced by the applied voltage. These interesting properties may provide an effective approach of spin injection for spintronics application, and this device can be used as a voltage-tunable spin beam splitter.     

Hyperfine field at a point interstitial impurity in ferromagnetic nickel

A simple model for the spin polarization around a charged interstitial impurity in ferromagnetic nickel is presented. It is based on screening of the charge by only s-electrons, while d-electrons, considered to be correlated and localized, are responsible, through an exchange mechanism, for the s-electron spin polarization. Agreement with recent experiments on μ⁺ precession in Ni, as well as neutron diffraction data is satisfactory.     

Particular nanowire superlattice as a spin filter

A nanowire superlattice of InAs and GaAs layers with In_0.47Ga_0.53As as the impure layers is proposed. The oft-neglected k³ Dresselhaus spin-orbit coupling causes the spin polarization of the electron but often can produce a limited spin polarization. In this nanowire superlattice, Dresselhaus term produce complete spin filtering by optimizing the distance between the In_0.47Ga_0.53As layers and the Indium (In) in the impure layers. The proposed structure is an optimized nanowire superlattice that can efficiently filter any component of electron spins according to its energy. In fact, this nanowire superlattice is an energy dependent spin filter structure.     

A method to measure neutron polarization using P-even asymmetry of γ-quantum emission in the neutron-nuclear interaction

A new method to measure polarization of cold/thermal neutrons using P-even asymmetry in nuclear reactions induced by polarized neutrons is proposed. A scheme profiting from a large correlation of the neutron spin and the circular ??-quantum polarization in the reaction (n, ??) of polarized neutrons with nuclei is analyzed. This method could be used, for instance, to measure the neutron-beam polarization in experiments with frequently varying configuration. We show that high accuracy and reliability of measurements could be expected.     

Spin-polarized transport in ferromagnet/Rashba quantum dot/ferromagnet system

The transport properties of the Datta and Das's spin transistor with the center normal region (or the quantum dot) having Rashba spin–orbit interaction and electron–electron (e–e) interaction U are investigated. We find while intra-dot level is near or above the chemical potential of the leads, the modulation efficiency of this spin transistor almost is not influenced by U. On the other hand, when the level is below the chemical potential, e–e interaction U may affect the modulator efficiency, because in this case the existence of e–e interaction can change the transport properties of the quantum dot. But the modulation efficiency still keep enough large and the spin transistor can effectively work.     

Generation and detection of spin polarization in parallel coupled double quantum dots connected to four terminals

A four-terminal parallel double quantum dots (QDs) device is proposed to generate and detect the spin polarization in QDs. It is found that the spin accumulation in QDs and the spin-polarized currents in the upper and down leads can be generated when a bias voltage is applied between the left and right leads. It is more interesting that the spin polarization in the QDs can be detected using the upper and down leads. Moreover, the direction and magnitude of the spin polarization in the QDs, and in the upper and down leads can be tuned by the energy levels of QDs and the bias.