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Performing optical spectroscopy of highly homogeneous quantum dot arrays in ultrahigh magnetic fields, an unprecedently well resolved Fock-Darwin spectrum is observed. The existence of up to four degenerate electronic shells is demonstrated where the magnetic field lifts the initial degeneracies, which reappear when levels with different angular momenta come into resonance. The resulting level shifting and crossing pattern also show evidence of many-body effects such as the mixing of configurations and exciton condensation at the resonances.

We report on a multiband microscopic theory of many-exciton complexes in self-assembled quantum dots. The single particle states are obtained by three methods: single-band effective-mass approximation, the multiband k · p method, and the tight-binding method. The electronic structure calculations are coupled with strain calculations via Bir-Pikus Hamiltonian. The many-body wave functions of N electrons and N valence holes are expanded in the basis of Slater determinants. The Coulomb matrix elements are evaluated using statically screened interaction for the three different sets of single particle states and the correlated N-exciton states are obtained by the configuration interaction method. The theory is applied to the excitonic recombination spectrum in InAs/GaAs self-assembled quantum dots. The results of the single-band effective-mass approximation are successfully compared with those obtained by using the of k · p and tight-binding methods.

Vertically stacked and coupled InAs/GaAs self-assembled quantum dots (SADs) are predicted to exhibit a strong non-parabolic dependence of the interband transition energy on the electric field, which is not encountered in single SAD structures nor in other types of quantum structures. Our study based on an eight-band strain-dependent k · p Hamiltonian indicates that this anomalous quantum confined Stark effect is caused by the three-dimensional strain field distribution which influences drastically the hole states in the stacked SAD structures. PACS: 78.67. Hc, 73.21.La, 71.70.Fk Zero-dimensional semiconductor structure, like InAs /GaAs self-assembled quantum dots (SADs) 1 have attracted considerable attention because of the new physics 2-4 of a few electron systems and potential applications in optoelectronics 5 . Recent experiment on Stark effect spectroscopy in SADs 6 has demonstrated the existence of an inverted electron-hole alignment due to the presence of gallium diffusion in InAs SADs, and established a relation between the Stark shift and the vertical electron-hole separation.The theoretical interpretation of these experimental results is based on the assumption that the applied electric field can be treated by the second-order perturbation theory, which results in a quadratical dependence of the transition energy on the applied electric field 7 ,where p is the built-in dipole moment and β measures the polarization of the electron and hole states, i.e., the quantum confined Stark effect. While this relation is well satisfied in many quantum systems including single SADs 7 , and quantum well structures 8,9 , we show in this work that it is not valid for vertically coupled SAD structures 10 where the quantum confined Stark effect deviates significantly from its quadratic dependence on the electric field. The reason for this anomalous quantum confined Stark effect is due to the three-dimensional (3D) strain field distribution in the dots and in the coupling region, which controls the localization of hole states in the respective SADs, and their sensitivity to external field. The existence of this effect is important for basic condensed matter physics because it can not be inferred from a simple superposition of the electronic properties of single SADs. It is also promising for applications in optoelectronics because interband transition energies can be significantly modulated by electric fields in quantum dot lasers and other photonic devices.The insets of Fig. 1 show schematically a single SAD structure and a system of two vertically coupled SADs that are truncated pyramids separated by a GaAs barrier of 1.8 nm, with identical base 17.4 × 17.4 nm 2 and individual height 3.6 nm. A positive electric field F is directed from the top to the bottom of the structures. Fig. 1 shows the calculated ground state transition energies for the single dot and for the stacked structure, as functions of electric fields. The electron and hole states of the system are obtained from the Schrödinger equation in the framew...

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We point out using an empirical tight-binding approach that the ground state of holes in InAs/ GaAs self-assembled quantum dots carries nonzero orbital momentum. The spin and orbital motions of the hole state are found to have opposite contributions to the hole g factor, leading to zero g factors of holes and then excitons in dots of high aspect ratio. The nonzero envelope orbital momenta of the holes are also shown to account for anisotropic circular polarization of exciton emission and nonlinear Zeeman splittings in high magnetic fields. Our theory well explains recent experiments and indicates the possibility of engineering magnetic splitting by tuning the electric confinement in nanostructures.

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Magnetic and spin-polarized transport properties in zigzag-edged graphene nanoflakes were investigated from first-principles calculations. Ferrimagnetic structure was found to be the ground state for triangular shaped graphene flakes. Magnetism is weakened by doping B or N atoms into the flakes, and it is enhanced if F atoms are doped in certain sublattices of the flakes. The magnetic properties can be rationalized by the behaviors of dopants as well as interactions between dopants and the host atoms. A perfect (100%) spin filtering effect was achieved for the pure or B doped graphene flake sandwiched between two gold electrodes. The orientation of the spin current is found to be flipped if the flake is doped with N, O, or F atoms. The orientation-tunable spin filtering effect is potentially useful in practical applications.