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V. V. Bel'kov

Publications and source records attributed to V. V. Bel'kov.

At least 55 records · Page 3Linked to original sources
arXiv2007

Magneto-gyrotropic photogalvanic effects due to inter-subband absorption in quantum wells

We report on the observation of the magneto-photogalvanic effect (MPGE) due to inter-subband transitions in (001)-oriented GaAs quantum wells. This effect is related to the gyrotropic properties of the structures. It is shown that inter-subband absorption of linearly polarized radiation may lead to spin-related as well as spin independent photocurrents if an external magnetic field is applied in the plane of the quantum well. The experimental results are analyzed in terms of the phenomenological theory and microscopic models of MPGE based on either asymmetric optical excitation or asymmetric relaxation of carriers in k-space. We observed resonant photocurrents not only at oblique incidence of radiation but also at normal incidence demonstrating that conventionally applied selection rules for the inter-subband optical transitions are not rigorous.

cond-mat.mtrl-sci↗
arXiv2006

Pure spin currents induced by spin-dependent scattering processes in SiGe quantum well structures

We show that spin-dependent electron-phonon interaction in the energy relaxation of a two-dimensional electron gas results in equal and oppositely directed currents in the spin-up and spin-down subbands yielding a pure spin current. In our experiments on SiGe heterostructures the pure spin current is converted into an electric current applying a magnetic field that lifts the cancellation of the two partial charge flows. A microscopic theory of this effect, taking account of the asymmetry of the relaxation process, is developed being in a good agreement with the experimental data.

cond-mat.mes-hall↗
arXiv2006

Rashba and Dresselhaus Spin-Splittings in Semiconductor Quantum Wells Measured by Spin Photocurrents

The spin-galvanic effect and the circular photogalvanic effect induced by terahertz radiation are applied to determine the relative strengths of Rashba and Dresselhaus band spin-splitting in (001)-grown GaAs and InAs based two dimensional electron systems. We observed that shifting the $δ$-doping plane from one side of the quantum well to the other results in a change of sign of the photocurrent caused by Rashba spin-splitting while the sign of the Dresselhaus term induced photocurrent remains. The measurements give the necessary feedback for technologists looking for structures with equal Rashba and Dresselhaus spin-splittings or perfectly symmetric structures with zero Rashba constant.

cond-mat.mes-hall↗
arXiv2005

Demonstration of Rashba spin splitting in GaN-based heterostructures

The circular photogalvanic effect (CPGE), induced by infrared radiation, has been observed in (0001)-oriented GaN quantum well (QW) structures. The photocurrent changes sign upon reversing the radiation helicity demonstrating the existence of spin-splitting of the conduction band in k-space in this type of materials. The observation suggests the presence of a sizeable Rashba type of spin-splitting, caused by the built-in asymmetry at the AlGaN/GaN interface.

cond-mat.other↗
arXiv2005

Magneto-Gyrotropic Photogalvanic Effects in Semiconductor Quantum Wells

We show that free-carrier (Drude) absorption of both polarized and unpolarized terahertz radiation in quantum well (QW) structures causes an electric photocurrent in the presence of an in-plane magnetic field. Experimental and theoretical analysis evidences that the observed photocurrents are spin-dependent and related to the gyrotropy of the QWs. Microscopic models for the photogalvanic effects in QWs based on asymmetry of photoexcitation and relaxation processes are proposed. In most of the investigated structures the observed magneto-induced photocurrents are caused by spin-dependent relaxation of non-equilibrium carriers.

cond-mat.mes-hall↗
arXiv2004

Can an electric current orient spins in quantum wells?

A longstanding theoretical prediction is the orientation of spins by an electrical current flowing through low-dimensional carrier systems of sufficiently low crystallographic symmetry. Here we show by means of terahertz transmission experiments through two-dimensional hole systems a growing spin orientation with an increasing current at room temperature.

cond-mat.mes-hall↗
arXiv2004

Spin relaxation times of 2D holes from spin sensitive bleaching of inter-subband absorption

We present spin relaxation times of 2D holes obtained by means of spin sensitive bleaching of the absorption of infrared radiation in p-type GaAs/AlGaAs quantum wells (QWs). It is shown that the saturation of inter-subband absorption of circularly polarized radiation is mainly controlled by the spin relaxation time of the holes. The saturation behavior has been determined for different QW widths and in a wide temperature range with the result that the saturation intensity substantially decreases with narrowing of the QWs. Spin relaxation times are derived from the measured saturation intensities by making use of calculated (linear) absorption coefficients for direct inter-subband transitions. It is shown that spin relaxation is due to the D'yakonov-Perel' mechanism governed by hole-hole scattering. The problem of selection rules is addressed.

cond-mat.other↗
arXiv2003

Magneto-Gyrotropic Photogalvanic Effect in Semiconductor Quantum Wells

We investigate both experimentally and theoretically, the magneto-gyrotropic photogalvanic effect in zinc-blende based quantum wells with $C_{2v}$ point-group symmetry using optical excitation in the terahertz frequency range. The investigated frequencies cause intra-subband but no inter-band and inter-subband transitions. While at normal incidence the photocurrent vanishes at zero magnetic field, it is shown that an in-plane magnetic field generates photocurrents both for polarized and unpolarized excitation. In general the spin-galvanic effect, caused by circularly polarized light, and the magneto-gyrotropic effect, caused by unpolarized excitation, is superimposed. It is shown that in the case of two specific geometries both effects are separable.

cond-mat↗
arXiv2003

Experimental Separation of Rashba and Dresselhaus Spin-Splittings in Semiconductor Quantum Wells

The relative strengths of Rashba and Dresselhaus terms describing the spin-orbit coupling in semiconductor quantum well (QW) structures are extracted from photocurrent measurements on n-type InAs QWs containing a two-dimensional electron gas (2DEG). This novel technique makes use of the angular distribution of the spin-galvanic effect at certain directions of spin orientation in the plane of a QW. The ratio of the relevant Rashba and Dresselhaus coefficients can be deduced directly from experiment and does not relay on theoretically obtained quantities. Thus our experiments open a new way to determine the different contributions to spin-orbit coupling.

cond-mat↗
arXiv2003

Spin-galvanic effect due to optical spin orientation

Under oblique incidence of circularly polarized infrared radiation the spin-galvanic effect has been unambiguously observed in (001)-grown $n$-type GaAs quantum well (QW) structures in the absence of any external magnetic field. Resonant inter-subband transitions have been obtained making use of the tunability of the free-electron laser FELIX. It is shown that a helicity dependent photocurrent along one of the $<110>$ axes is predominantly contributed by the spin-galvanic effect while that along the perpendicular in-plane axis is mainly due to the circular photogalvanic effect. This strong non-equivalence of the [110] and [1$\bar{1}$0] directions is determined by the interplay between bulk and structural inversion asymmetries. A microscopic theory of the spin-galvanic effect for direct inter-subband optical transitions has been developed being in good agreement with experimental findings.

cond-mat↗
arXiv2003

Spin-sensitive Bleaching and Spin-Relaxation in QW's

Spin-sensitive saturation of absorption of infrared radiation has been investigated in p-type GaAs QWs. It is shown that the absorption saturation of circularly polarized radiation is mostly controlled by the spin relaxation time of the holes. The saturation behavior has been investigated for different QW widths and in dependence on the temperature with the result that the saturation intensity substantially decreases with narrowing of QWs. Spin relaxation times were experimentally obtained by making use of calculated (linear) absorption coefficients for inter-subband transitions.

cond-mat↗
arXiv2003

Resonant inversion of the circular photogalvanic effect in n-doped quantum wells

We show that the sign of the circular photogalvanic effect can be changed by tuning the radiation frequency of circularly polarized light. Here resonant inversion of the photogalvanic effect has been observed for direct inter-subband transition in n-type GaAs quantum well structures. This inversion of the photon helicity driven current is a direct consequence of the lifting of the spin degeneracy due to k-linear terms in the Hamiltonian in combination with energy and momentum conservation and optical selection rules.

cond-mat↗
arXiv2003

Optical Spin Orientation under Inter- and Intra-Subband Transitions in QWs

It is shown that absorption of circularly polarized infrared radiation achieved by inter-subband and intra-subband (Drude-like) transitions results in a monopolar spin orientation of free carriers. The monopolar spin polarization in zinc-blende-based quantum wells (QWs) is demonstrated by the observation of the spin-galvanic and circular photogalvanic effects. It is shown that monopolar spin orientation in n-type QWs becomes possible if an admixture of valence band states to the conduction band wave function and the spin-orbit splitting of the valence band are taken into account.

cond-mat↗
arXiv2003

Spin-photocurrent in p-SiGe quantum wells under terahertz laser irradiation

A detailed study of the circular photogalvanic effect (CPGE) in SiGe structures is presented. It is shown that the CPGE becomes possible due to the built-in asymmetry of quantum wells (QWs) in compositionally stepped samples and in asymmetrically doped structures. The photocurrent arises due to optical spin orientation of free carriers in QWs with spin splitting in k-space. It is shown that the effect can be applied to probe the macroscopic in-plane symmetry of low dimensional structures and allowing to conclude on Rashba or Dresselhaus terms in the Hamiltonian.

cond-mat↗
arXiv2003

Spin-Galvanic Effect in Quantum Wells

It is shown that a homogeneous non-equlibrium spin-polarization in semiconductor heterostructures results in an electric current. The microscopic origin of the effect is an inherent asymmetry of spin-flip scattering in systems with lifted spin degeneracy caused by k-linear terms in the Hamiltonian.

cond-mat↗
arXiv2003

Monopolar Optical Orientation of Electronic Spins in Semiconductors

It is shown that absorption of circularly polarized infrared radiation due to intraband (Drude-like) transitions in n-type bulk semiconductors and due to intra-subband or inter-subband transitions in quantum well (QW) structures results in a monopolar spin orientation of free electrons. Spin polarization in zinc-blende-structure based QWs is demonstrated by the observation of the spin-galvanic and the circular photogalvanic effects. The monopolar spin orientation in n-type materials is shown to be possible if an admixture of valence band states to the conduction band wave function and the spin-orbit splitting of the valence band are taken into account.

cond-mat↗
arXiv2003

Spin-dependent tunnelling through a symmetric barrier

The problem of electron tunnelling through a symmetric semiconductor barrier based on zinc-blende-structure material is studied. The $k^3$ Dresselhaus terms in the effective Hamiltonian of bulk semiconductor of the barrier are shown to result in a dependence of the tunnelling transmission on the spin orientation. The difference of the transmission probabilities for opposite spin orientations can achieve several percents for the reasonable width of the barriers.

cond-mat.mes-hall↗