B. Szafran

LO-phonon-induced screening of electron–electron interaction in D− centres and quantum dots

Ap roblem of screening of electron–electron interaction by LO phonons is investigated for bound two-electron systems in bulk semiconductors and semiconductor quantum dots. We consider a D − centre and a two-electron quantum dot and obtain the effective LO-phonon-induced interaction between the electrons, i.e., Veff (r12) ∼ e 2 /[eeff(r12)r12], where r12 is the interelectron distance and eeff(r12) is th ee ffective phonon dielectric function. The calculated phonon dielectric function rapidly increases for small r12 starting from the high-frequency dielectric constant, e∞ ,a nd reaches some constant value, ¯ e ,a t relatively small interelectron distances. We have found that—in most cases— ¯ is less than the static dielectric constant, es .O nly i nt he weakly ionic compounds, like GaAs, ¯ e � es .W ea rgue that—in the bound few-electron systems—¯ e better approximates the average LO-phonon-induced screening than the commonly used es .W e have also shown that the coupling with LO phonons leads to the increase of the binding energy of the two-electron system confined in the quantum dot.

Few-electron eigenstates of concentric double quantum rings

Few-electron eigenstates confined in coupled concentric double quantum rings are studied by the exact diagonalization technique. We show that the magnetic field suppresses the tunnel coupling between the rings, localizing the single-electron states in the internal ring and the few-electron states in the external ring. The magnetic fields inducing the ground-state angular momentum transitions are determined by the distribution of the electron charge between the rings. The charge redistribution is translated into modifications of the fractional Aharonov-Bohm period. We demonstrate that the electron distribution can be deduced from the cusp pattern of the chemical potentials governing the single-electron charging properties of the system. The evolution of the electron-electron correlations to the high field limit of a classical Wigner molecule is discussed.

Exciton and negative trion dissociation by an external electric field in vertically coupled quantum dots

We study the Stark effect for an exciton confined in a pair of vertically coupled quantum dots. A single-band approximation for the hole and a parabolic lateral confinement potential are adopted which allows for the separation of the lateral center-of-mass motion and consequently for an exact numerical solution of the Schrodinger equation. We show that for intermediate tunnel coupling the external electric field leads to the dissociation of the exciton via an avoided crossing of bright and dark exciton energy levels which results in an atypical form of the Stark shift. The electric-field-induced dissociation of the negative trion is studied using the approximation of frozen lateral degrees of freedom. It is shown that in a symmetric system of coupled dots the trion is more stable against dissociation than the exciton. For an asymmetric system of coupled dots the trion dissociation is accompanied by a positive curvature of the recombination energy line as a function of the electric field.

Electron spin and charge switching in a coupled quantum-dot-quantum ring system

Few-electron systems confined in a quantum dot laterally coupled to a surrounding quantum ring in the presence of an external magnetic field are studied by exact diagonalization. The distribution of electrons between the dot and the ring is influenced by the relative strength of the dot and ring confinement, and the magnetic field which induces transitions of electrons between the two parts of the system. These transitions are accompanied by changes in the periodicity of the Aharonov-Bohm oscillations of the ground-state angular momentum. The singlet-triplet splitting for a two electron system with one electron confined in the dot and the other in the ring exhibits piecewise linear dependence on the external field due to the Aharonov-Bohm effect for the ring-confined electron, in contrast to smooth oscillatory dependence of the exchange energy for laterally coupled dots in the side-by-side geometry.

Recombination energy for excitonic trions in quantum dots

A theoretical study of excitonic trions, X- and X2 + , in semiconductor quantum dots is presented. The model of a spherical quantum well of finite depth is applied to determine the influence of the three-dimensional quantum confinement on the recombination energies of the excitonic trions. A new type of variational wave function, expanded in a Gaussian basis, has been proposed. It is shown that the blue-shift of the recombination induced by the quantum confinement is much stronger for the positive trion X2 + than for the negative trion X- .

Time-dependent simulations of electron transport through a quantum ring: Effect of the Lorentz force

The time dependent Schrodinger equation for an electron passing through a semiconductor quantum ring of nonzero width is solved in the presence of a perpendicular homogenous magnetic field. We study the effects of the Lorentz force on the Aharonov-Bohm oscillations. Within the range of incident momentum for which the ring is transparent at zero magnetic field, the Lorentz force leads to a decrease of the oscillation amplitude, due to the asymmetry in the electron injection in the two arms of the ring. For structures in which the fast electrons are predominantly backscattered, the Lorentz force assists in the transport producing an initial increase of the corresponding oscillation amplitude. Furthermore, we discuss the effect of elastic scattering on a potential cavity within one of the arms of the ring. For the cavity tuned to shift maximally the phase of the maximum of the wave packet we observe a

Relative stability of negative and positive trions in model symmetric quantum wires

pi

Accuracy of the Hartree-Fock method for Wigner molecules at high magnetic fields

shift of the Aharonov-Bohm oscillations. For other cavity depths the oscillations with a period of half of the flux quantum are observed.

Dependence of the vortex structure in quantum dots on the range of the inter-electron interaction

Binding energies of negative (

Exact broken-symmetry states and Hartree-Fock solutions for quantum dots at high magnetic fields

X^-