Vassilij Belkov

Spin-galvanic effect

There is much recent interest in exploiting the spin of conduction electrons in semiconductor heterostructures together with their charge to realize new device concepts. Electrical currents are usually generated by electric or magnetic fields, or by gradients of, for example, carrier concentration or temperature. The electron spin in a spin-polarized electron gas can, in principle, also drive an electrical current, even at room temperature, if some general symmetry requirements are met. Here we demonstrate such a ‘spin-galvanic’ effect in semiconductor heterostructures, induced by a non-equilibrium, but uniform population of electron spins. The microscopic origin for this effect is that the two electronic sub-bands for spin-up and spin-down electrons are shifted in momentum space and, although the electron distribution in each sub-band is symmetric, there is an inherent asymmetry in the spin-flip scattering events between the two sub-bands. The resulting current flow has been detected by applying a magnetic field to rotate an optically oriented non-equilibrium spin polarization in the direction of the sample plane. In contrast to previous experiments, where spin-polarized currents were driven by electric fields in semiconductor, we have here the complementary situation where electron spins drive a current without the need of an external electric field.

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 -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.

Spin-Sensitive Bleaching and Monopolar Spin Orientation in Quantum Wells

Spin-sensitive bleaching of the absorption of far-infrared radiation has been observed in p-type GaAs/AlGaAs quantum well structures. The absorption of circularly polarized radiation saturates at lower intensities than that of linearly polarized light due to monopolar spin orientation in the first heavy-hole subband. Spin relaxation times of holes in p-type material in the range of tens of ps were derived from the intensity dependence of the absorption.

Quantum ratchet effects induced by terahertz radiation in GaN-based two-dimensional structures

Photogalvanic effects are observed and investigated in wurtzite (0001)-oriented GaN/AlGaN low-dimensional structures excited by terahertz radiation. The structures are shown to represent linear quantum ratchets. Experimental and theoretical analysis exhibits that the observed photocurrents are related to the lack of an inversion center in the GaN-based heterojunctions.

Magneto-gyrotropic effects in semiconductor quantum wells

Magneto-gyrotropic photogalvanic effects in quantum wells are reviewed. We discuss experimental data, results of phenomenological analysis and microscopic models of these effects. The current flow is driven by spin-dependent scattering in low-dimensional structure gyrotropic media resulting in asymmetry of photoexcitation and relaxation processes. Several applications of the effects are also considered.

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 and is in good agreement with the experimental data.

Symmetry and Spin Dephasing in (110)-Grown Quantum Wells

Symmetry and spin dephasing in (110)-grown GaAs quantum wells (QWs) are investigated applying magnetic field induced photogalvanic effect and time-resolved Kerr rotation. We show that magnetic field induced photogalvanic effect provides a tool to probe the symmetry of (110)-grown quantum wells. The photocurrent is only observed for asymmetric structures but vanishes for symmetric QWs. Applying Kerr rotation we prove that in the latter case the spin relaxation time is maximal; therefore, these structures set the upper limit of spin dephasing in GaAs QWs. We also demonstrate that structure inversion asymmetry can be controllably tuned to zero by variation of delta-doping layer positions.

Spin currents in diluted magnetic semiconductors.

We study zero-bias spin separation in (Cd,Mn)Te/(Cd,Mg)Te diluted magnetic semiconductor structures. The spin current generated by electron gas heating under terahertz radiation is converted into a net electric current by applying an external magnetic field. The experiments show that the spin polarization of the magnetic ion system enhances drastically the conversion process due to giant Zeeman splitting of the conduction band and spin-dependent electron scattering on localized Mn(2+) ions.

Cyclotron-resonance-assisted photcurrents in surface states of a three-dimensional topological insulator based on a strained high-mobility HgTe film

We report on the observation of cyclotron-resonance-induced photocurrents, excited by continuous wave terahertz radiation, in a three-dimensional topological insulator (TI) based on an 80-nm strained HgTe film. The analysis of the photocurrent formation is supported by complementary measurements of magnetotransport and radiation transmission. We demonstrate that the photocurrent is generated in the topologically protected surface states. Studying the resonance response in a gated sample, we examined the behavior of the photocurrent, which enables us to extract the mobility and the cyclotron mass as a function of the Fermi energy. For high gate voltages, we also detected cyclotron resonance (CR) of bulk carriers, with a mass about two times larger than that obtained for the surface states. The origin of the CR-assisted photocurrent is discussed in terms of asymmetric scattering of TI surface carriers in the momentum space. Furthermore, we show that studying the photocurrent in gated samples provides a sensitive method to probe the cyclotron masses and the mobility of two-dimensional Dirac surface states, when the Fermi level lies in the bulk energy gap or even in the conduction band.

Spin photocurrents in (110)-grown quantum well structures

The circular photogalvanic effect and the circular photon drag effect are observed and investigated in detail in (110)-grown quantum well structures. The experimental data are well described by phenomenological theory and microscopic models of both effects are developed being in agreement with experimental data.