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Koopmans' theorem implies that the Hartree-Fock quasiparticle gap in a closed-shell system is equal to its single-particle energy gap. In this work, the theorem is generalized to optical transitions in the Hubbard model of graphene nanodots. Based on systematic configuration interaction calculations, it is proposed that the optical gap of a closed-shell graphene system within the Hubbard model is equal to its tight-binding single-particle energy gap in the absence of electron correlation. In these systems, the quasiparticle energy gap and exciton binding energy are found to be dominated by the long-range Coulomb interaction, and thus, both become small when only on-site Hubbard interactions are present. Moreover, the contributions of the quasiparticle and excitonic effects to the optical gap are revealed to nearly cancel each other, which results in an unexpected overlap of the optical and single-particle gaps of the graphene systems. [ABSTRACT FROM AUTHOR]

Spin-polarized electron transport through zigzag-edged graphene nanoislands is studied within the framework of the Pariser-Parr-Pople Hamiltonian. By including both short- and long-range electron-electron interactions, the electron conductance is calculated self-consistently for the hexagonal model on various substrates from which we are able to identify the effects of the many-body interactions in the electron transport. For the system in its lowest antiferromagnetic (AFM) state, the long-range interactions are shown to have negligible effect on the electron transport in the low-energy region in which the conductance is found quenched mainly by the short-range interactions. As the system is excited to its second AFM state, the short- and long-range interactions are found to have opposite effects on the electron transmission, i.e., the electron transmission is found to increase with either the suppression of the long-range interactions or the enhancement of the short-range interactions. When the system moves further into the ferromagnetic state, the conductance becomes spin dependent and its resonance is shown to exhibit a blue shift in an environment with stronger long-range interactions. The distinct impact of short- and long-range electron-electron interactions are attributed to their different effects on the spin polarization in the model system. [ABSTRACT FROM AUTHOR]

A microscopic theory is presented for electron cotunneling through quantum dots in the Coulomb blockade regime. Beyond the semiclassic framework of phenomenological models, a fully quantum mechanical solution for cotunneling of electrons through a one-dimensional quantum dot is obtained by using a quantum transmitting boundary method without any fitting parameters. Elastic and inelastic cotunneling conductance is calculated as a function of the energy of the incident electron. It is revealed that the cotunneling current contains a significant term proportional to V2 (V being the bias voltage) in additional to the well-known V3 term. The result also indicates that the cotunneling conductance exhibits little dependence on the spin configuration of the incident and confined electrons. [ABSTRACT FROM AUTHOR]

Electron transport in a graphene nanoisland with mixed edge profiles is studied by using the spin-unrestricted Hubbard model. The dumbbell-shaped device consists of two hexagonal nanoislands with zigzag edges connected by a armchair nanoribbon. Including the nearest-neighbor hopping and on-site Coulomb repulsion, the self-consistent calculation shows that two distinct ferromagnetic configurations allowed for the proposed device act like ON and OFF states for spin-polarized electrons. Furthermore, the spin-valve effect is demonstrated, as a proof of concept, in the device occupied by one ferromagnetic configuration without resorting to external magnetic or electric fields. [ABSTRACT FROM AUTHOR]

The possibility of electrical tuning of exciton g-factors in self-assembled InAs/GaAs quantum dots is explored theoretically by means of a tight-binding-like effective bond-orbital approach. The electron g-factor in the dots of various sizes is found to exhibit very little change over a broad range of the field strength. In contrast, the ground hole state in the dots of high aspect ratio is seen very sensitive to the applied field, its g-factor even changes the sign with the field. The distinct behavior of the electron and hole g-factors in the presence of electric field is explained in terms of nonzero envelope orbital angular momentum carried by the hole states. [ABSTRACT FROM AUTHOR]

We present a theoretical study of electron and hole g factors in stacked self-assembled InAs/GaAs quantum dots. The exciton ground and first excited states in the quantum dot molecules are found to exhibit opposite resonances in their g factors in the presence of a small vertical electric field, which is very different from the monotonic behavior of their counterparts in single quantum dots. While the g factor of the electronic ground state is seen to have little variation as the applied electric field increases, the relocalization of the hole states in coupled quantum dots is found to account for the resonant behavior of the exciton g factors. Our theoretical result agrees well with a recent experiment. [ABSTRACT FROM AUTHOR]

Electron g-factors in self-assembled InAs/GaAs quantum dots are studied theoretically by means of a tight-binding method. The anisotropy in the electron g-factor, induced by the band-mixing effect, is found to be much more sensitive to the chemical composition of the dot than the aspect ratio of the structure. The modulus of the vertical electron g-factor is usually seen smaller than the in-plane component in pure InAs dots while the situation could be reversed in highly intermixing InGaAs dots. A microscopic theory is presented to reveal the dependence of the anisotropy in the electron g-factor on the proportion of the valence-band components in the electronic states. Our study indicates the possibility of tuning electron g-factor anisotropy in quantum dots. [ABSTRACT FROM AUTHOR]

We have measured at low temperatures the conductance of electron waveguides fabricated from modulation-doped quantum wells by wet etching and regrowth. We have found that, for a waveguide with abruptly changed geometry at the entrance and exit, the conductance shows oscillations, which are superimposed on a conventional conductance plateau structure. The periods and amplitudes of conductance oscillations depend on the length to width aspect ratio of the waveguide. In addition, the amplitudes of conductance oscillations decrease with increasing temperature. We propose that the observed oscillations are caused by the formation of longitudinal resonant electron states in the waveguide, in analogy with optical Fabry-Perot effects. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]