Spin Subbands Research Articles

Spin splitting in the quantum Hall effect of disordered GaAs layers with strong overlap of the spin subbands

With minima in the diagonal conductance G_{xx} and in the absolute value of the derivative |dG_{xy}/dB| at the Hall conductance value G_{xy}=e^{2}/h, spin-splitting is observed in the quantum Hall effect of heavily Si-doped GaAs layers with low electron mobility 2000 cm^2/Vs in spite of the fact that the spin-splitting is much smaller than the level broadening. Experimental results can be explained in the frame of the scaling theory of the quantum Hall effect, applied independently to each of the two spin subbands.

Magnitude of magnetic field dependence of a possible selective spin filter inZnSe∕Zn1−xMnxSemultilayer heterostructures

Spin-polarized transport through a band-gap-matched $\mathrm{Zn}\mathrm{Se}∕{\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}∕{\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ multilayer structure is investigated. The resonant transport is shown to occur at different energies for different spins owing to the split of spin subbands in the paramagnetic layers. It is found that the polarization of current density can be reversed in a certain range of magnetic field, with the peak of polarization moving towards a stronger magnetic field for increasing the width of central ZnSe layer while shifting towards an opposite direction for increasing the width of paramagnetic layer. The reversal is limited in a small-size system. A strong suppression of the spin up component of the current density is present at high magnetic field. It is expected that such a reversal of the polarization could act as a possible mechanism for a selective spin filter device.

Spin density matrix of spin-32hole systems

For hole systems with an effective spin j=3/2, we present an invariant decomposition of the spin density matrix that can be interpreted as a multipole expansion. The charge density corresponds to the monopole moment and the spin polarization due to a magnetic field corresponds to a dipole moment while heavy hole-light hole splitting can be interpreted as a quadrupole moment. For quasi two-dimensional hole systems in the presence of an in-plane magnetic field B the spin polarization is a higher-order effect that is typically much smaller than one even if the minority spin subband is completely depopulated. On the other hand, the field B can induce a substantial octupole moment which is a unique feature of j=3/2 hole systems.

Spin splitting in GaAs (100) two-dimensional holes

We measured Shubnikov--de Haas (SdH) oscillations in GaAs (100) two-dimensional holes to determine the inversion asymmetry-induced spin splitting. The Fourier spectrum of the SdH oscillations contains two peaks, at frequencies ${f}_{\ensuremath{-}}$ and ${f}_{+},$ that correspond to the hole densities of the two spin subbands and a peak, at frequency ${f}_{\mathrm{tot}},$ corresponding to the total hole density. In addition, the spectrum exhibits an anomalous peak at ${f}_{\mathrm{tot}}/2.$ We also determined the effective masses of the two spin subbands by finding the inverse transform of the Fourier spectrum in the vicinity of ${f}_{\ensuremath{-}}$ and ${f}_{+},$ and then analyzing the temperature dependence of the SdH oscillations for each subband. We discuss our results in light of self-consistent calculations and previous experiments.

Dynamics of neutral and charged exciton line intensities

We present a study of time-dependent transmission spectra of a modulation-doped Cd1−xMnxTe/Cd1−y−zZnyMgzTe quantum well with variable hole gas concentration. The temporal evolution of the neutral and charged exciton absorption intensities was analysed. We found that the dominant effect is an ‘oscillator strength stealing’ from charged to neutral excitons. Due to the fact that charged exciton intensity is proportional to spin subband population the spin flip of heavy holes was observed.

Korringa relaxation time of magnetic ion system near a two-dimensional electron gas

Spin relaxation of Mn ions in a (Cd,Mn)Te quantum well with quasi-two-dimensional carriers (Q2DEG) is investigated. The mechanism of energy transfer is spin–flip scattering of Mn spin with electrons making transitions between spin subbands accompanied by a change in the Mn spin. A calculation of the spin–flip scattering rate shows that the Mn spin relaxation rate is proportional to the coupling constant squared, the density of states squared, and the electron temperature, the so called Korringa relaxation rate. It was found that for small Mn ion concentration, the relaxation time ≈10 −7–10 −6 s is in a good agreement with experimental results. Moreover, the relaxation rate scales with L −2, L being the well width, and it can be enhanced over its value in bulk.

Spin–plasma wave in a one-dimensional metal

It is shown that a new type weakly damped acoustic plasmons can propagate in a quasi-one-dimensional conductor in an external static magnetic field H. These are due to the presence of spin subbands in the electron spectrum and are accompanied by forced oscillations of the magnetic moment of the sample. The dependence of the velocity and amplitude of these waves on the magnetic field strength H is found.

Spin excitations of the spin-polarized electron gas in semimagnetic quantum wells.

Collective and single-particle spin-flip excitations of a two-dimensional electron gas in a semimagnetic Cd(1-x)Mn(x)Te quantum well are observed by resonant Raman scattering. Application of a magnetic field splits the spin subbands and a spin polarization is induced in the electron gas. Above 1 T the collective modes, which disperse with the in-plane wave vector, dominate the spectra. The local spin-density approximation provides a good description of our results and enables us to confirm that the energy of the low wave vector collective mode is given by the bare Zeeman energy.

Ballistic spin transport and spin interference in ferromagnet/InAs(2DES)/ferromagnet devices

We examine the injection of electron spins from ferromagnets (F) into quasi-two-dimensional electron systems (2DES's) of semiconductors and employ the transfer-matrix formalism to obtain the carrier-density dependence of the conductances across F/InAs(2DES) single as well as F/InAs(2DES)/F double junctions in the ballistic limit. The Rashba spin-orbit interaction in the semiconductor and oblique modes in devices of finite widths are taken into account. We distinguish between spin-valve and spin-transistor geometry, in which the in-plane magnetization vectors in the ferromagnetic electrodes point perpendicular and parallel to the current direction, respectively. In case of the spin-transistor geometry, we find optimum coupling with the Rashba spin-precession state for injection straight along the channel. The distinct mismatch of majority and minority spin subbands in the ferromagnet to the band structure of the semiconductor causes spin-dependent scattering at the interface. This results in spin filtering. Within a Stoner model for ferromagnets spin filtering is stronger for iron than for Permalloy parameters and can be enhanced by an additional elastic scattering potential at the interface. In F/InAs(2DES)/F double junctions Fabry-Perot-type interferences are obtained. In case of spin-valve geometry they are due to particle interference, whereas in spin-transistor geometry spin itself is involved.

Magnetoconductance of a two-dimensional metal in the presence of spin-orbit coupling

We show that in the metallic phase of a two dimensional electron gas the spin-orbit coupling due to structure inversion asymmetry leads to a characteristic anisotropy in the magnetoconductance. Within the assumption that the metallic phase can be described by a Fermi liquid, we compute the conductivity in the presence of an in-plane magnetic field. Both the spin-orbit coupling and the Zeeman coupling with the magnetic field give rise to two spin subbands, in terms of which most of the transport properties can be discussed. The strongest conductivity anisotropy occurs for Zeeman energies of the order of the Fermi energy corresponding to the depopulation of the upper spin subband. The energy scale associated with the spin-orbit coupling controls the strength of the effect. More in particular, we find that the detailed behavior and the sign of the anisotropy depends on the underlying scattering mechanism. Assuming small angle scattering to be the dominant scattering mechanism our results agree with recent measurement on Si-MOSFET's in the vicinity of the metal-insulator transition.

Influence ofs−dinterfacial scattering on the magnetoresistance of magnetic tunnel junctions

We propose the two-band s-d model to describe theoretically a diffuse regime of the spin-dependent electron transport in magnetic tunnel junctions (MTJ's) of the form F/O/F where F's are 3d transition metal ferromagnetic layers and O is the insulating spacer. We aim to explain the strong interface sensitivity of the tunneling properties of MTJ's and investigate the influence of electron scattering at the nonideal interfaces on the degradation of the TMR magnitude. The generalized Kubo formalism and the Green's functions method were used to calculate the conductance of the system. The vertex corrections to the conductivity were found with the use of "ladder" approximation combined with the coherent-potential approximation (CPA) that allowed to consider the case of strong electron scattering. It is shown that the Ward identity is satisfied in the framework of this approximation that provides the necessary condition for a conservation of a tunneling current. Based on the known results of ab-initio calculations of the TMR for ballistic junctions, we assume that exchange split quasi-free s-like electrons with the density of states being greater for the majority spin sub-band give the main contribution to the TMR effect. We show that, due to interfacial inter-band scattering, the TMR can be substantially reduced even down to zero value. This is related to the fact that delocalized quasi-free electrons can scatter into the strongly localized d sub-band with the density of states at the Fermi energy being larger for minority spins compared to majority spins. It is also shown that spin-flip electron scattering on the surface magnons within the interface leads to a further decrease of the TMR at finite temperature.

Spin-dependent scattering in transition metals

The spin-dependent electron inelastic mean free path (IMFP) in transition metals is studied in the energy range 5–50 eV above the Fermi level. It is shown that the spin-dependent IMFP is simply related to the number of holes in both d spin subbands, whatever the detail of the d-band structure. This analysis allows us to disentangle the different scattering channels. Many experimental determinations of the spin-dependent part of the electron scattering cross section, from several teams, are analyzed in the framework of this model. The strong energy dependence of the exchange matrix element is clearly evidenced.

Spin injection and detection in a ferromagnetic metal/2DEG structure

Spin dependent transport in a high mobility two dimensional electron gas (2DEG) with an asymmetric confining potential is observed by using a ferromagnetic metal as a spin sensitive voltage probe. A high impedance junction is formed between the ferromagnet and the 2DEG on a planarized sample. The conduction band of the 2DEG is split into spin subbands by the Rashba effect, and nonequilibrium magnetization is induced by a bias current. The magnetization of the ferromagnet is controlled by an external magnetic field, and the voltage sensed in each of its two magnetic states corresponds to the up- and down-spin subband chemical potentials of the 2DEG carriers.

The anomalous 0.5 and 0.7 conductance plateaus in quantum point contacts

The anomalous 0.5 and 0.7 conductance plateaus in quantum point contacts in zero magnetic field are analyzed within a phenomenological model. The model utilizes the Landauer–Büttiker formalism and involves enhanced spin correlations and thermal depopulation of spin subbands. In particular we can account for the plateau values 0.5 and 0.7, as well as the unusual temperature and magnetic field dependences of the 0.7 plateau. Finally, the model predicts the possibility of coexisting 0.5 and 0.7 plateaus.

Highly anisotropic Zeeman splitting of two-dimensional hole systems

In two-dimensional hole systems the anisotropic dispersion E( k) gives rise to a highly anisotropic Zeeman splitting with respect to different orientations of an in-plane magnetic field B relative to the crystal axes. We obtain good, qualitative agreement between theory and experimental data, taken in GaAs 2D hole systems grown on (1 1 3) substrates, showing the anisotropic depopulation of the upper spin subband as a function of in-plane B .

Spin-splitting in GaAs two-dimensional holes

We present quantitative measurements and calculations of the spin-orbit-induced zero-magnetic-field spin splitting in two-dimensional (2D) hole systems in modulation-doped GaAs (3 1 1) A quantum wells. The results show that the splitting is large and tunable. In particular, via a combination of back- and front-gate biases, we can tune the splitting while keeping the 2D hole density constant. The data also reveal a surprising result regarding the magnetoresistance (Shubnikov–de Haas) oscillations in a 2D system with spin-split energy bands: the frequencies of the oscillations are not simply related to the population of the spin-subbands. Next we concentrate on the metallic-like behavior observed in these 2D holes and its relation to spin-splitting. The data indicate that the metallic behavior is more pronounced when two spin-subbands with unequal populations are occupied. Our measurements of the magnetoresistance of these 2D hole systems with an in-plane magnetic field corroborate this conclusion: while the system is metallic at zero magnetic field, it turns insulating when one of the spin subbands is depopulated at high magnetic field.

Highly anisotropic g-factor of two-dimensional hole systems

Coupling the spin degree of freedom to the anisotropic orbital motion of two-dimensional (2D) hole systems gives rise to a highly anisotropic Zeeman splitting with respect to different orientations of an in-plane magnetic field B relative to the crystal axes. This mechanism has no analog in the bulk band structure. We obtain good, qualitative agreement between theory and experimental data, taken in GaAs 2D hole systems grown on (113) substrates, showing the anisotropic depopulation of the upper spin subband as a function of in-plane B.

Effect of external uniaxial stress on the electronic properties and symmetry of p-GaAs/AlxGa1−xAs quantum wells

The results of experimental and theoretical investigations of the energy spectrum and electronic properties of symmetric p-[001] Al0.5Ga0.5As/GaAs/Al0.5Ga0.5As heterostruc-tures under uniaxial [110] compression are presented. The stress-induced piezoelectric field breaks the confining potential symmetry in the quantum well and lifts the degeneracy of the hole subbands. A redistribution of holes in the spin subbands of the ground state takes place, which is revealed in the different shifts of the Shubnikov-de Haas oscillation maxima corresponding to the different spin subbands. The [110] uniaxial compression significantly modifies the band structure, which leads to a strong aniso-tropy of the Fermi surface. The electrical resistance becomes strongly anisotropic under applied compression, decreasing in the direction parallel to the compression and increasing in the perpendicular direction.

Oscillator strengths of charged excitons: combining magnetoabsorption and photoluminescence dynamics in semimagnetic quantum wells

We present a systematic study of the oscillator strength of the positively charged excitons (X +) in Cd 1− x Mn x Te quantum wells. CW-absorption and time-resolved photoluminescence measurements were combined as two approaches for the determination of the oscillator strength. By varying (in a small magnetic field) the spin subband hole distribution at constant total concentration, we observe an increase of the oscillator strength in absorption, proportional to the hole concentration in one spin subband (the one which allows the X + formation). The measurements done for different total hole concentrations show an important decrease of the X + oscillator strength per carrier when increasing the total concentration. On the contrary the radiative lifetime, measured in time-resolved PL experiment, is found to be constant over the whole range of hole gas concentrations, and equal to the value which we deduce from transmission in the limit of vanishing hole concentration.

Potentiometric measurements of the spin-split subbands in a two-dimensional electron gas

The density of states of carriers in a high-mobility InAs single quantum well is spin split by the Rashba effect. The asymmetry favors down-spin states for carriers with positive momentum and up-spin states for carriers with negative momentum. Imposing a bias current causes inequivalent shifts of the spin subband chemical potential, which are detected using a ferromagnetic film electrode and an open circuit voltage measurement. Measurements made on three samples over a range $77<T<296\mathrm{K}$ demonstrate spin detection at a ferromagnet-semiconductor interface and corroborate earlier experimental results of spin-dependent interfacial resistance.