Spin Subbands Research Articles

Negative Longitudinal Magneto-Thermoelectric Power in a Semiconductor Parabolic Quantum Well

We present a theoretical study of the electronic thermoelectric power of a semiconductor parabolic quantum well in a magnetic field. The case of a longitudinal magnetic field, with respect to the temperature gradient, has been considered. The calculations were carried out taking into account the spin-splitting of dimensionally quantized electronic energy levels. It has been shown that in the region of strong confinement the thermoelectric power decreases with increasing magnetic field, which is related to the downward shift of the lower Zeeman-split spin subband.

Strain-Induced Fermi Contour Anisotropy of GaAs 2D holes

We report measurements of magnetoresistance commensurability peaks, induced by a square array of antidots, in GaAs (311)A two-dimensional holes as a function of applied in-plane strain. The data directly probe the shapes of the Fermi contours of the two spin subbands that are split thanks to the spin-orbit interaction and strain. The experimental results are in quantitative agreement with the predictions of accurate energy band calculations, and reveal that the majority spin subband has a severely distorted Fermi contour whose anisotropy can be tuned with strain.

Novel mechanism for rapid spin flip with increasing in-plane wave vector in slightly asymmetric modulation-doped quantum wells

We calculate the electric-field-induced spin splittings in wide slightly asymmetric modulation-doped quantum wells. When spin subbands are anticrossing we demonstrate two-step spin flips as the in-plane wave vector along the [11] direction is increased by 0.002 nm-1. At the beginning of this interval the y-component flips, at the end the x-component. Simultaneously the energy separation stays roughly constant below 1 μeV and the wave functions are interchanged. A bias change of about 1 meV is sufficient to move the Fermi level from below to above the anticrossing region.

Photon-assisted spin transport in a two-dimensional electron gas

We study spin-dependent transport in a two-dimensional electron gas subjected to an external steplike potential $V(x)$ and irradiated by an electromagnetic field (EF). In the absence of EF, the electronic spectrum splits into spin subbands originating from the ``Rashba'' spin-orbit coupling. We show that the resonant interaction of propagating electrons with the component EF parallel to the barrier induces a nonequilibrium dynamic gap $(2{\ensuremath{\Delta}}_{R})$ between the spin subbands. Existence of this gap results in coherent spin-flip processes that lead to a spin-polarized current and a large magnetoresistance, i.e., the spin-valve effect. These effects may be used for controlling spin transport in semiconducting nanostructures, e.g., spin transistors, spin-blockade devices, etc., by variation of the intensity $S$ and frequency $\ensuremath{\omega}$ of the external radiation.

Rapid spin flip in a spin subband at an anticrossing region in a slightly asymmetric modulation-doped quantum well

The spin splitting due to structure and bulk inversion asymmetry is calculated for electron subbands in wide slightly asymmetric InGaSb quantum wells. At anticrossings, rapid spin flips in two steps are found as the in-plane wave vector along the [1 1] direction is increased by 0.002 nm −1. First the y-component and then the x-component is flipped. A change of bias of about 1 meV across the quantum well is sufficient move the Fermi level across the anticrossing region.

Spin-subband populations and spin polarization of quasi-two-dimensional carriers under an in-plane magnetic field

Under an in-plane magnetic field, the density of states of quasi-two-dimensional carriers deviates from the occasionally stereotypic step-like form both quantitatively and qualitatively. Here we study how this affects the spin-subband populations and the spin-polarization as functions of the temperature, $T$, and the in-plane magnetic field, $B$, for narrow to wide dilute-magnetic-semiconductor quantum wells. We examine a wide range of material and structural parameters, focusing on the quantum well width, the magnitude of the spin-spin exchange interaction, and the sheet carrier concentration. Generally, increasing $T$, the carrier spin-splitting, ${U}_{o\ensuremath{\sigma}}$, decreases, augmenting the influence of the ``minority''-spin carriers. Increasing $B$, ${U}_{o\ensuremath{\sigma}}$, increases and, accordingly, carriers populate ``majority''-spin subbands while they abandon ``minority''-spin subbands. Furthermore, in line with the density of states modification, all energetically higher subbands become gradually depopulated. We also indicate the ranges where the system is completely spin-polarized.

Current-induced inverse population of spin subbands in magnetic junctions

The spin flux matching conditions that should be satisfied at the boundaries between contacting layers of a magnetic junction to ensure the inverse population of spin subbands at experimentally achievable current densities of −107–108 A/cm2 are determined. The essence of the conditions is that an efficient spin injection takes place at the input boundary, whereas the injection at the output boundary is blocked. The fulfillment of the matching conditions for contacting layers with antiparallel magnetization orientations leads to stability in magnetic fluctuations for any forward currents.

Manifestation of coherent and spin-dependent effects in the conductance of ferromagnets adjoining a superconductor

Electrotransport was investigated in the macroscopic ferromagnets (F) Fe and Ni in the presence of F∕In interfaces with different area. The measurements were performed in two configurations—with a constant current flowing parallel and perpendicular to the interfaces. It was found that the transition of In into the superconducting (S) state is accompanied by an increase of the resistance of the ferromagnets. In the first configuration the increase concerned the change of the resistance of the region between the boundaries of the potential probes (superconducting “mirrors”) and was of the order of the typical, weakly localized, contribution to the conductivity of ferromagnets from subgap singlet excitations, arising with Andreev reflection, for macroscopic distances between the “mirrors” at the coherence length characteristic for metals with a strong difference of the energy dispersion of the spin subbands. In the second configuration, where the conductivity of the F/S interface is also included in the measurements, the nonequilibrium resistive contribution of the latter, associated with the spin polarization of the region of the ferromagnet under the interface, was studied. The observed increase of the resistance corresponded to the theoretically predicted magnitude of the change occurring in the resistance of a single-domain region with spin-polarized electrons as a result of spin accumulation on the F/S interface under the conditions of limitations of Andreev reflections. The coefficients of current polarization and the coherence length in the exchange field were found for Fe and Ni from the experiemental data and the lower limit of the spin relaxation length was found for the samples investigated.

Spin-Triplet Pairing States in Ferromagnet/Ferromagnet/d-Wave Superconductor Heterojunctions with Noncollinear Magnetizations

Tunneling conductance in clean ferromagnet/ ferromagnet/d-wave superconductor (F/F/d-wave S) double tunnel junctions is studied by use of four-component Bogoliubov-de Gennes equations. The novel Andreev reflection appears due to noncollinear magnetizations, in which the incident electron and the Andreev-reflected hole come from the same spin subband, resulting in spin-triplet pairing states near the F/S interface. In the highly polarized Fs case, the conductance within the energy gap exhibits a conversion from a zero-bias dip in the parallel magnetizations to a spilt zero-bias peak in the perpendicular magnetizations.

Efficient switching of Rashba spin splitting in wide modulation-doped quantum wells

The authors demonstrate that the size of the electric-field-induced Rashba spin splitting in an 80nm wide modulation-doped InGaSb quantum well can depend strongly on the spatial variation of the electric field. In a slightly asymmetric quantum well it can be an order of magnitude stronger than for the average uniform electric field. For even smaller asymmetry spin subbands can have wave functions and/or expectation values of the spin direction that are completely changed as the in-plane wave vector varies. The Dresselhaus effect [Phys. Rev. 100, 580 (1955)] can give an anticrossing at which the spin rapidly flips.

Disturbance of spin equilibrium by current through the interface of noncollinear ferromagnets

Boundary conditions are derived that determine the penetration of spin current through an interface of two non-collinear ferromagnets with an arbitrary angle between their magnetization vectors. We start from the well-known transformation properties of an electron spin wave functions under the rotation of a quantization axis. It allows directly find the connection between partial electric current densities for different spin subbands of the ferromagnets. No spin scattering is assumed in the near interface region, so that spin conservation takes place when electron intersects the boundary. The continuity conditions are found for partial chemical potential differences in the situation. Spatial distribution of nonequilibrium electron magnetizations is calculated under the spin current flowing through a contact of two semi-infinite ferromagnets. The distribution describes the spin accumulation effect by current and corresponding shift of the potential drop at the interface. These effects appear strongly dependent on the relation between spin contact resistances at the interface.

Dynamical properties of spin and subbands populations in 1D quantum wire

AbstractIn this paper we study the spin and subbands populations, as functions of time, for electrons in a quasi‐1D quantum wire, with spin–orbit coupling (SOC), to which a perpendicular magnetic field is applied. The system is governed by the Hamiltonian which, in the strong magnetic field limit, resembles the Jaynes–Cummings model (JCM) in quantum optics (QO). Using a procedure similar to that in QO, we explicitly present the time‐evolution operator, thereby calculating the spin states and subbands populations as functions of time. We show that the populations exhibit oscillations, depending on the interaction parameters, scale lengths and, particularly, the initial states of the system. Specifically, if the electrons are initially prepared in a maximal coherent superposition of spin states, the expectation values periodically collapse and revive. The collapse‐revivals are most profound for the spin along the magnetic field and subbands populations. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The role of orbital ordering in the formation of electron structure in undoped LaMnO3 manganites in the regime of strong electron correlations

The electron structure of undoped LaMnO{sub 3} and slightly doped La{sub 1-x}Sr{sub x}MnO{sub 3} manganites has been calculated within the framework of a generalized tight binding method with explicit allowance for strong intraatomic electron correlations. According to the results of these calculations, the ground state in orbitally disordered undoped LaMnO{sub 3} ferromagnets would be metallic despite the Mott-Hubbard correlation gap in the spectrum of quasiparticles. Owing to the orbital ordering, the insulating state is stabilized in both antiferromagnetic and paramagnetic phases. In-gap states of a polaron nature with a spectral weight proportional to the dopant concentration have been found near the top of the valence band in La{sub 1-x}Sr{sub x}MnO{sub 3}. As the doping level increases, a metal state appears in the ferromagnetic phase, which has a metallic character for one spin subband and an insulating character for the other subband (representing the so-called half-metallic state)

Investigation of the super-efficient Rashba effect by simulation of Shubnikov–de Haas oscillations in a two-dimensional hole gas

The Shubnikov–de Haas effect is frequently used for two-dimensional systems to determine individual subband populations, e.g. when the subbands are split by the Rashba effect. We have previously shown that the Rashba effect can give a wave vector splitting for holes that is up to three orders of magnitude larger than for electrons at the same electric field. To reach the optimum we have made a careful design of a modulation-doped quantum well with a top gate in which the negative differential Rashba effect is utilized. From the calculated hole Landau levels we determine the density of states at the Fermi energy and demonstrate a clear difference between the symmetric case without bias and the asymmetric case with a gate voltage of 100 meV, where the spin subband populations differ by a factor 3.

Effect of spontaneous spin depopulation on the ground-state energy of a two-dimensional spintronic system

We calculate the ${r}_{s}$ expansion of the ground-state energy per particle of a two-dimensional electron gas with spin-orbit interaction induced by the Rashba coupling. At high areal electron density, we obtain the energy for noninteracting electrons, their exchange energy, and the lowest-order approximation for the correlation energy. A closed-form expression is obtained for the energy of noninteracting electrons. However, we must calculate the exchange and correlation energy numerically. As the density is increased and ${r}_{s}$ decreases, the ground-state energy changes rapidly at some value ${r}_{s}={r}_{s}^{*}$ and the Fermi energy ${E}_{F}$ changes from negative to positive. When ${E}_{F}l0$, only the lower spin subband is occupied. An interesting effect occurs in the presence of electron-electron interaction as ${E}_{F}$ increases through $E=0$ and the upper spin subband suddenly gets a finite population rather than increasing gradually.

Spin waves of the spin‐polarized electron gas in semimagnetic quantum wells

AbstractSpin flip waves and spin density fluctuations of a two‐dimensional spin‐polarized electron‐gas in a semimagnetic Cd1–0.008Mn0.008Te quantum well are observed by Resonant Raman Scattering. Application of a magnetic field splits the spin subbands and a static spin polarization is induced. Wavevector and field experimental dispersions are very well reproduced by Local Spin Density Approximation calculations including recent Quantum Monte Carlo evaluation of coulomb correlations corrections. We show how to extract the spin polarization degree. A maximum value of 35% is deduced from measurements. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Beyond the Rashba model

We analyze some common approximations made in connection with the Rashba effect, where a macroscopic electric field gives rise to a spin splitting. We demonstrate that the size of the Rashba splitting is not given by the expectation value of the electric field or some other average electric field, as is commonly assumed. Instead we find that the local electric field near an interface of a wide asymmetric modulation-doped quantum well can give rise to a Rashba splitting that is an order of magnitude larger than expected from the average electric field. The localization of the wave functions of the spin subbands can be quite sensitive to the parallel wave vector. Clear deviations from an energy-independent wave vector splitting occur when nonparabolicity is taken into account.

Multiple Phases with the Same Quantized Hall Conductance in a Two-Subband System

In a two-dimensional electron system with two occupied subbands, the experimentally determined phase diagram in the density-magnetic field plane exhibits rich topological features. Ringlike structures are observed at even integer filling factors in the phase diagram. Even with the identical quantized Hall resistance values as those given by the ordinary integer quantum Hall effect due to the Landau level quantization, the activation energies of these states within the rings are much smaller. These ring structures cannot be accounted for by the simple single particle picture. We argue that ferromagnetic quantum Hall states, due to the interaction of two energy levels with opposite spin and different subband indices, are responsible for these unusual structures.

Anomalous spin polarization of GaAs two-dimensional hole systems

We report measurements and calculations of the spin-subband depopulation, induced by a parallel magnetic field, of dilute GaAs two-dimensional (2D) hole systems. The results reveal that the shape of the confining potential dramatically affects the values of in-plane magnetic field at which the upper spin subband is depopulated. Most surprisingly, unlike 2D electron systems, the carrier-carrier interaction in 2D hole systems does not significantly enhance the spin susceptibility. We interpret our findings using a multipole expansion of the spin density matrix, and suggest that the suppression of the enhancement is related to the holes' band structure and effective spin j=3/2.

Domain-wall magnetoresistance of Co nanowires

Using density functional theory implemented within a tight-binding linear muffin-tin orbital method we perform calculations of electronic, magnetic and transport properties of ferromagnetic free-standing fcc Co wires with diameters up to 1.5 nm. We show that finite-size effects play an important role in these nanowires resulting in oscillatory behavior of electronic charge and the magnetization as a function of the wire thickness, and a non-monotonic behavior of spin-dependent quantized conductance. We calculate the magnetoresistance (MR) of a domain wall (DW) modeled by a spin-spiral region of finite width sandwiched between two semi-infinite Co wire leads. We find that the DW MR decreases very rapidly, on the scale of a few interatomic layers, with the increasing DW width. The largest MR value of about 250% is predicted for an abrupt DW in the monatomic wire. We show that, for some energy values, the density of states and the conductance may be non-zero only in one spin channel, making the MR for the abrupt DW infinitely large. We also demonstrate that for the abrupt DW a large MR may occur due to the hybridization between two spin subbands across the DW interface. We do not find, however, such a behavior at the Fermi energy for the Co wires considered.