Optical exciton Aharonov-Bohm effect, persistent current, and magnetization in semiconductor nanorings of type I and II

The optical exciton Aharonov-Bohm effect, i. e. an oscillatory component in the energy of optically active (bright) states, is investigated in nanorings. It is shown that a small effective electron mass, strong confinement of the electron, and high barrier for the hole, achieved e. g. by an InAs nanoring embedded in an AlGaSb quantum well, are favorable for observing the optical exciton Aharonov-Bohm effect. The second derivative of the exciton energy with respect to the magnetic field is utilized to extract Aharonov-Bohm oscillations even for the lowest bright state unambiguously. A connection between the theories for infinitesimal narrow and finite width rings is established. Furthermore, the magnetization is compared to the persistent current, which oscillates periodically with the magnetic field and confirms thus the non-trivial (connected) topology of the wave function in the nanoring.Comment: 12 pages, 11 figure

Acoustomagnetoelectric effect in two-dimensional materials: Geometric resonances and Weiss oscillations

We study electron transport in two-dimensional materials with parabolic and linear (graphene) dispersions of the carriers in the presence of surface acoustic waves and an external magnetic field using semiclassical Boltzmann equations approach. We observe an oscillatory behavior of both the longitudinal and Hall electric currents as functions of the surface acoustic wave frequency at a fixed magnetic field and as functions of the inverse magnetic field at a fixed frequency of the acoustic wave. We explain the former by the phenomenon of geometric resonances, while we relate the latter to the Weiss-like oscillations in the presence of the dynamic superlattice created by the acoustic wave. Thus we demonstrate the dual nature of the acoustomagnetoelectric effect in two-dimensional electron gas.Comment: Manuscript: 9 pages, 2 figure

Non-circular semiconductor nanorings of type I and II: Emission kinetics in the exciton Aharonov-Bohm effect

Transition energies and oscillator strengths of excitons in dependence on magnetic field are investigated in type I and II semiconductor nanorings. A slight deviation from circular (concentric) shape of the type II nanoring gives a better observability of the Aharonov-Bohm oscillations since the ground state is always optically active. Kinetic equations for the exciton occupation are solved with acoustic phonon scattering as the major relaxation process, and absorption and luminescence spectra are calculated showing deviations from equilibrium. The presence of a non-radiative exciton decay leads to a quenching of the integrated photoluminescence with magnetic field.Comment: The first version submitted to Phys. Rev. B on April 16, 2007. Revised (this) version on July 31, 200

Single and vertically coupled type II quantum dots in a perpendicular magnetic field: exciton groundstate properties

The properties of an exciton in a type II quantum dot are studied under the influence of a perpendicular applied magnetic field. The dot is modelled by a quantum disk with radius $R$, thickness $d$ and the electron is confined in the disk, whereas the hole is located in the barrier. The exciton energy and wavefunctions are calculated using a Hartree-Fock mesh method. We distinguish two different regimes, namely $d<<2R$ (the hole is located at the radial boundary of the disk) and $d>>2R$ (the hole is located above and below the disk), for which angular momentum $(l)$ transitions are predicted with increasing magnetic field. We also considered a system of two vertically coupled dots where now an extra parameter is introduced, namely the interdot distance $d_{z}$. For each $l_{h}$ and for a sufficient large magnetic field, the ground state becomes spontaneous symmetry broken in which the electron and the hole move towards one of the dots. This transition is induced by the Coulomb interaction and leads to a magnetic field induced dipole moment. No such symmetry broken ground states are found for a single dot (and for three vertically coupled symmetric quantum disks). For a system of two vertically coupled truncated cones, which is asymmetric from the start, we still find angular momentum transitions. For a symmetric system of three vertically coupled quantum disks, the system resembles for small $d_{z}$ the pillar-like regime of a single dot, where the hole tends to stay at the radial boundary, which induces angular momentum transitions with increasing magnetic field. For larger $d_{z}$ the hole can sit between the disks and the $l_{h}=0$ state remains the groundstate for the whole $B$-region.Comment: 11 pages, 16 figure

Kinetics of exciton photoluminescence in type-II semiconductor superlattices

The exciton decay rate at a rough interface in type-II semiconductor superlattices is investigated. It is shown that the possibility of recombination of indirect excitons at a plane interface essentially affects kinetics of the exciton photoluminescence at a rough interface. This happens because of strong correlation between the exciton recombination at the plane interface and at the roughness. Expressions that relate the parameters of the luminescence kinetics with statistical characteristics of the rough interface are obtained. The mean height and length of roughnesses in GaAs/AlAs superlattices are estimated from the experimental data.Comment: 3 PostScript figure

Nonlinear acousto-electric transport in a two-dimensional electron system

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.Comment: 27 pages, 6 figure

Valley Acoustoelectric Effect

We report on the 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 acoustoelectronic devices