A. Franceschetti

Correlation versus mean-field contributions to excitons, multiexcitons, and charging energies in semiconductor quantum dots

Single-dot spectroscopy is now able to resolve the energies of excitons, multiexcitons, and charging of semiconductor quantum dots with {approx}<1 meV resolution. We discuss the physical content of these energies and show how they can be calculated via quantum Monte Carlo (QMC) and configuration interaction (CI) methods. The spectroscopic energies have three pieces: (i) a perturbative part reflecting carrier-carrier direct and exchange Coulomb energies obtained from fixed single-particle orbitals, (ii) a self-consistency correction when the single particle orbitals are allowed to adjust to the presence of carrier-carrier interaction, and (iii) a correlation correction. We first apply the QMC and CI methods to a model single-particle Hamiltonian: a spherical dot with a finite barrier and single-band effective mass. This allows us to test the convergence of the CI and to establish the relative importance of the three terms (i)--(iii) above. Next, we apply the CI method to a realistic single-particle Hamiltonian for a CdSe dot, including via a pseudopotential description the atomistic features, multiband coupling, spin-orbit effects, and surface passivation. We include all bound states (up to 40000 Slater determinants) in the CI expansion. Our study shows that (1) typical exciton transition energies, which are {approx}1 eV, can be calculated tomorexa0» better than 95% by perturbation theory, with only a {approx}2 meV correlation correction; (2) typical electron addition energies are {approx}40 meV, of which correlation contributes very little ({approx}1 meV); (3) typical biexciton binding energies are positive and {approx}10 meV and almost entirely due to correlation energy, and exciton addition energies are {approx}30 meV with nearly all contribution due to correlation; (4) while QMC is currently limited to a single-band effective-mass Hamiltonian, CI may be used with much more realistic models, which capture the correct symmetries and electronic structure of the dots, leading to qualitatively different predictions from effective-mass models; and (5) CI gives excited state energies necessary to identify some of the peaks that appear in single-dot photoluminescence spectra.«xa0less

Excitonic transitions and exchange splitting in Si quantum dots

In a quantum dot made of an indirect gap material such as Si, the electron–hole Coulomb interaction alone can give rise to “dark” excitons even in the absence of exchange interaction. We present the predicted excitonic spectra for hydrogen-passivated Si dots and find very good agreement with the recent experiment of Wolkin, Jorne, Fauchet, Allan, and Delerue [Phys. Rev. Lett. 82, 197 (1999)]. The calculated splitting between dark and bright excitons, arising from Coulomb and exchange interactions, agrees very well with the optical data of Calcott, Nash, Canham, Kane, and Brumhead [J. Phys Condens. Matter 5, L91 (1993)].

Multi-excitons in self-assembled InAs/GaAs quantum dots: A pseudopotential, many-body approach

We use a many-body, atomistic empirical pseudopotential approach to predict the multi-exciton emission spectrum of a lens-shaped InAs/GaAs self-assembled quantum dot. We discuss the effects of i) the direct Coulomb energies, including the differences of electron and hole wave functions, ii) the exchange Coulomb energies and iii) correlation energies given by a configuration interaction calculation. Emission from the ground state of the N exciton system to the N − 1 exciton system involving e0 → h0 and e1 → h1 recombinations are discussed. A comparison with a simpler single-band, effective mass approach is presented.

An optimized configuration interaction method for calculating electronic excitations in nanostructures

The configuration interaction method has been widely used to calculate electronic excitations in nanostructures, but it suffers from a slow rate of convergence with the number of configurations in the basis set and from the inability to select a priori the most important configurations. The optimized configuration interaction method presented here removes the limitations of the conventional approach by identifying at the outset the configurations that are most relevant for describing electronic excitations. We show that the best configurations are remarkably different from the configurations that one would expect on the basis of the single-particle energy ladder, and that a small, optimized set of configurations predicts excitation energies with accuracy comparable to that for much larger, non-optimized sets of configurations. This approach opens the way to a new generation of configuration interaction methods where the configurations are pre-selected using heuristic search methods.

Design rules to achieve high-TC ferromagnetism in (Ga, Mn)As alloys

The Curie temperature T(C) of ferromagnetic semiconductor alloys depends not only on the alloy composition, but also on the spatial configuration of the magnetic impurities. Here we use a set of first-principle-calculated Curie temperatures to uncover-via a statistical, data mining approach-the rules that govern the dependence of T(C) on the configuration of Mn substitutional impurities in GaAs. We find that T(C) is lowered (raised) when the average number of first (third and fourth) nearest-neighbour Mn pairs increases, suggesting simple atom-by-atom strategies to achieve high T(C) in (Ga, Mn)As alloys.