We describe the optical resonant manipulation of a single magnetic impurity in a self-assembled quantum dot. We show that using the resonant pumping one can address and manipulate selectively individual spin states of a magnetic impurity. The mechanisms of resonant optical polarization of a single impurity in a quantum dot involve anisotropic exchange interactions and are different to those in diluted semiconductors. A Mn impurity can act as qubit. The limiting factors for the qubit manipulation are the electron-hole exchange interaction and finite temperature. The spins of electrons in semiconductors strongly couple with electric and magnetic fields due to the spin-orbit and exchange interactions. Spintronics and quantum computation utilize these interactions to manipulate the electron spins 1,2. One important class of spintronics materials is diluted magnetic semiconductors 3 which combine high-quality semiconductor structures with magnetic properties of impurities. Since many semiconductors efficiently emit and absorb light, the spin states of electrons and magnetic impurities can be manipulated optically by using circularly-polarized light pulses 4. In diluted magnetic semiconductors such as bulk crystals, quantum wells and dots, photo-generated excitons interact with a large collection of spins of Mn impurities and therefore a large number of degrees of freedom becomes involved 5,6,7,8,9,10,11. In these systems, it is challenging to address individual spins of Mn atoms. At the same time, the quantum computational schemes are based on qubits, pairs of well-controlled quantum states. These elementary blocks, qubits, should be made interacting or decou-pled on demand. In a diluted magnetic semiconductor, even a single Mn atom has 6 spin states (I Mn = 5/3). Therefore, 15 different pairs of states (qubits) can be defined for a single Mn impurity. Here we study a system which allows us to manipulate optically a single Mn spin. This system is composed of a quantum dot (QD) and a single Mn impurity. Note that the optical properties of a QD with a single Mn impurity were recently discussed in refs. 12,13. This letter describes a single Mn impurity embedded into a self-assembled QD. Importantly, such a system permits efficient selective optical control and manipulation of individual spin states and defining a single qubit for the Mn impurity. This ability comes from the exciton spectrum of a QD with a Mn atom. An exciton in a QD has a well-defined discrete spectrum and, simultaneously , strongly interacts with the Mn spin via the exchange interaction. Since the exciton and Mn spin functions become strongly mixed, the resonant optical exci-tation strongly affect the spin state of Mn impurity. In particularly, we show that one can write spin states of Mn atom. Since spin relaxation of paramagnetic ions in the absence of carriers (i.e. after the exciton recombina-tion) is an extremely slow process (∼ 10 µs), a single Mn spin is a very promising candidate for spintronics applications. The mechanisms of Mn-spin polarizat...

The combination of semiconductor quantum well structures and strongly piezoelectric crystals leads to a system in which surface acoustic waves with very large amplitudes can interact with charge carriers in the well. The surface acoustic wave induces a dynamic lateral superlattice potential in the plane of the quantum well which is strong enough to spatially break up a two-dimensional electron system into moving wires of trapped charge. This transition is manifested in an increase of the electron transport velocity with sound amplitude, eventually reaching the sound velocity. The sound absorption by the electron system then becomes governed by nonlinearities and is strongly reduced. We study the transition from the linear towards the strongly nonlinear regime of interaction and present a theoretical description of such phenomena in a 2D system. [S0031-9007(99)08621-4] PACS numbers: 73.50. Rb, 72.50. + b, 73.50.Fq Electron transport in semiconductor quantum well structures is usually governed by drift or diffusive current flow. Also ballistic carrier motion can be observed, provided that the size of the system is smaller than the mean-free path, ranging up to some hundred microns in very pure semiconductor material. Conceptually different from those mechanisms are transport phenomena based on momentum transfer from externally propagating entities to the electron system. Such "dragging" experiments have been studied in great detail in double electron layer systems [1], where internal "Coulomb friction" between the two layers causes the dragging force. "Photon drag" induced transport was observed for intersubband transitions of a quasi-two-dimensional electron system (2DES) [2]. Nagamune et al. [3] observed the effect of a dc current on the drift of optically generated carriers in a quantum well. Acoustic charge transport (ACT) has been investigated on a variety of different systems in view of possible device applications [4]. Using surface acoustic waves (SAW), Rocke et al. [5] showed that photogenerated electron-hole pairs in a semiconductor quantum well can be efficiently trapped in the moving lateral potential of the sound wave and then be reassembled into photonic signals. Recently, SAW have been combined with Coulomb blockade to drive single electrons through a quantum point contact [6]. In addition, the interaction between SAW and mobile charges in semiconductor layered structures has become an important method to study the dynamic conductivity of low-dimensional systems in quantum wells. These studies include the integer quantum Hall effect [7], the fractional quantum Hall effect [8], Fermi surfaces of composite Fermions around a half-filled Landau level [9], and commensurability effects caused by the lateral superlattice induced by a SAW [10]. These studies, however, were restricted to the small signal limit, i.e., to a regime where the presence of the lateral potential of a piezoelectric wave does not significantly modulate the carrier density in the quantum well.Here, we would like to report on experim...

We present results on the influence of a magnetic field on excitons in semiconductor quantum dots, concentrating on the diamagnetic curvature. We use samples with a bimodal ensemble photoluminescence ͑PL͒ and we find that for the low-energy PL branch, the diamagnetic curvature is independent of charge, yet for the high-energy branch, the diamagnetic curvature is strongly reduced with excess charge. Guided by model calculations, we interpret the two classes as typical of the strong and intermediate confinement regimes. In the light of this, we predict that in the weak confinement regime the excitonic diamagnetic shift is strongly dependent on surplus charge, corresponding to a reversal in sign of the conventional diamagnetic shift for neutral excitons.

We report on novel valley acoustoelectric effect, which can arise in a 2D material, like a transition metal dichalcogenide monolayer, residing on a piezoelectric substrate. The essence of this effect lies in the emergence of a drag electric current (and a spin current) due to a propagating surface acoustic wave. This current consists of three contributions, one independent of the valley index and proportional to the acoustic wave vector, the other arising due to the trigonal warping of the electron dispersion, and the third one is due to the Berry phase, which Bloch electrons acquire traveling along the crystal. As a result, there appear components of the current orthogonal to the acoustic wave vector. Further, we build an angular pattern, encompassing nontrivial topological properties of the acoustoelectric current, and suggest a way to run and measure the conventional diffusive, warping, and acoustoelectric valley Hall currents independently. We develop a theory, which opens a way to manipulate valley transport by acoustic methods, expanding the applicability of valleytronic effects on acousto-electronic devices. arXiv:1906.11151v1 [cond-mat.mes-hall]

We study theoretically the magnetic field effect on a neutral, but polarizable exciton confined in quantum-ring structures. For excitons with a nonzero dipole moment, a novel magnetic interference effect occurs: The ground state of an exciton confined in a finite-width quantum ring possesses a nonzero angular momentum with increasing normal magnetic field. This effect is accompanied by a suppression of the photoluminescence in well-defined magneticfield intervals. The magnetic interference effect is calculated for type-II quantum dots and quantum rings.

A novel magnetic interference effect is proposed for a neutral, but polarizable exciton in a quantum ring with a finite width. The magnetic interference effect originates from the nonzero dipole moment in the exciton. The ground state of exciton acquires a nonzero angular momentum with increasing normal magnetic field. This leads to the suppression of the photoluminescence in defined windows of the magnetic field.

We describe theoretically the process of multi-beam reflection in a two-dimensional electron system with a lateral potential barrier. Due to spin-orbital interaction, the reflection process leads to the formation of three beams with different spin polarizations.The efficiency of spin conversion can become small for smooth lateral barriers.Nevertheless, we demonstrate that the spin-conversion effect remains strong for realistic lithographical potentials and spin-orbit interactions in etched lateral nano-structures. The system with a lateral barrier suggests useful applications as a spin-filtering device. The expected quasi-classical adiabatic behavior without spin conversion is found in the system with a very strong spin-orbit interaction. We also consider the quasi-classical motion of electrons in a system with boundaries in a magnetic field and two magnetic focusing geometries.

We study both theoretically and experimentally the nonlinear interaction between an intense surface acoustic wave and a two-dimensional electron plasma in semiconductor-piezocrystal hybrid structures. The experiments on hybrid systems exhibit strongly nonlinear acousto-electric effects. The plasma turns into moving electron stripes, the acousto-electric current reaches its maximum, and the sound absorption strongly decreases. To describe the nonlinear phenomena, we develop a coupled-amplitude method for a two-dimensional system in the strongly nonlinear regime of interaction. At low electron densities the absorption coefficient decreases with increasing sound intensity, whereas at high electron density the absorption coefficient is not a monotonous function of the sound intensity. High-harmonic generation coefficients as a function of the sound intensity have a nontrivial behavior.Theory and experiment are found to be in a good agreement. *