A. Latgé

Binding energies and density of impurity states in spherical GaAs-(Ga,Al)As quantum dots

The binding energies of hydrogenic donor in both finite and infinite GaAs‐(Ga,Al)As spherical quantum dots are calculated as a function of the donor position for different radii within the effective‐mass approximation. It is observed an enhancement of the binding energy of donors in quantum dots when compared to results in quantum wells and quantum‐well wires, which is an expected consequence of the higher geometrical electronic confinement in these systems. The density of impurity states as a function of the donor binding energy was also calculated. As a general feature it presents structures associated with special impurity positions that may be important in the understanding of the absorption and photoluminescence experiments of doped quantum dots.

Quantum dots under electric and magnetic fields: Impurity-related electronic properties

A systematic study of the ground-state binding energies of a hydrogenic impurity in quantum dots subjected to external electric and magnetic fields is presented. The quantum dot is modeled by superposing a lateral parabolic potential and a square-well potential and the energies are calculated via a variational approach within the effective-mass approximation. The interplay between the confinement effects due to the applied fields and the quantum-size confinements on the binding energies is analyzed. The role played by the impurity position along the well direction on the impurity energies is also discussed. We have shown that by changing the strength of the external electric and magnetic fields, a large spread in the range of the donor binding energy may be obtained, for a particular choice for the lateral confinement. The presented results could be used to tailor energy states in optoelectronic devices.

Conductance and persistent current of a quantum ring coupled to a quantum wire under external fields

The electronic transport of a noninteracting quantum ring side coupled to a quantum wire is studied via a single-band tunneling tight-binding Hamiltonian. We found that the system develops an oscillating band with antiresonances and resonances arising from the hybridization of the quasibound levels of the ring and the coupling to the quantum wire. The positions of the antiresonances correspond exactly to the electronic spectrum of the isolated ring. Moreover, for a uniform quantum ring the conductance and the persistent current density were found to exhibit a particular odd-even parity related with the ring order. The effects of an in-plane electric field were also studied. This field shifts the electronic spectrum and damps the amplitude of the persistent current density. These features may be used to control externally the energy spectra and the amplitude of the persistent current.

Transport properties of graphene nanoribbons with side-attached organic molecules

In this work we address the effects on the conductance of graphene nanoribbons (GNRs) of organic molecules adsorbed at the ribbon edge. We studied the case of armchair and zigzag GNRs with quasi-one-dimensional side-attached molecules, such as linear poly-aromatic hydrocarbons and poly(para-phenylene). These nanostructures are described using a single-band tight-binding Hamiltonian and their electronic conductance and density of states are calculated within the Greens function formalism based on real-space renormalization techniques. We found that the conductance exhibits an even-odd parity effect as a function of the length of the attached molecules. Furthermore, the corresponding energy spectrum of the molecules can be obtained as a series of Fano antiresonances in the conductance of the system. The latter result suggests that GNRs can be used as a spectrograph sensor device.

Laser-dressed-band approach to shallow-impurity levels of semiconductor heterostructures

Abstract We present a simple theoretical approach to treat the interaction of a laser field with a semiconductor system, in which the effect of the laser field is incorporated within a renormalization of the semiconductor effective mass. As an application, we discuss the effects of laser dressing on the transition energies between the 1 s - and 2 p ± -like states of hydrogenic donors in GaAsGa 1− x Al x As QWs, in the presence of an external homogeneous magnetic field. It is shown that the modifications on the intradonor transitions due to weak intensity-laser dressing may be as important as the effects of a strongly applied magnetic field.

Upper bound for the conductivity of nanotube networks

Films composed of nanotube networks have their conductivities regulated by the junction resistances formed between tubes. Conductivity values are enhanced by lower junction resistances but should reach a maximum that is limited by the network morphology. By considering ideal ballistic-like contacts between nanotubes, we use the Kubo formalism to calculate the upper bound for the conductivity of such films and show how it depends on the nanotube concentration as well as on their aspect ratio. Highest measured conductivities reported so far are approaching this limiting value, suggesting that further progress lies with metallic nanowires rather than carbon nanotubes.

Hydrostatic pressure effects on electron states in GaAs–(Ga,Al)As double quantum rings

Here we address a theoretical analysis of the effects of applied hydrostatic pressure on electron states in concentric GaAs–(Ga,Al)As double quantum rings, under axial magnetic fields. Emphasis is put on the dependence of such effects on the system geometry confinement described within a hard potential model and following an effective-mass approximation. The energy of the ground and excited electronic states were found to decrease with the applied hydrostatic pressure, due mainly to an effective reduction in the barrier potential confinement. Also, while the increase in the magnetic field opens the electron states degeneracy with different angular momenta, the increase in the applied hydrostatic pressure does not alter significantly the energy of these states. For both symmetric and asymmetric double quantum rings, one found that the electron-heavy hole transition energies augment with the applied hydrostatic pressure, mainly due to the increase in the GaAs gap.

Magnetic-field and laser effects on the electronic and donor states in semiconducting quantum dots

Light shifts induced in the electronic and shallow on-center donor states in spherical semiconductor quantum dots, including magnetic field effects, are theoretically investigated. The interaction of light with the spherical GaAs–(Ga, Al)As quantum dot is treated within a dressed-band approach in which the Kane band structure scheme is used to model the GaAs bulk semiconductor whereas the dressing by the laser field is treated through the renormalization of the GaAs energy gap and conduction/valence effective masses. This nonperturbative approach is valid far from resonances and has been successfully adopted for other confined semiconductor heterostructures. The discrete nature of the electronic and impurity states, characteristic of quantum dot systems, and the possibility of adding extra confining effects by laser and applied magnetic fields opens up a promising route of applicability and/or manipulation of quantum-dot states in recent quantum-computer proposals.

Energy spectrum in a concentric double quantum ring of GaAs-(Ga,Al)As under applied magnetic fields

The energy spectrum of an electron in a two-dimensional concentric double quantum ring is exactly calculated in the presence of an axial magnetic field using linear combinations of confluent hypergeometric functions. The influence of the geometric confinement on the energy spectrum is analyzed as a function of the inner and outer ring radii. The applied magnetic field modifies the electron energy spectrum and we discuss the appearance of the well-known Aharonov-Bohm oscillations. Interesting behavior of the probability amplitude of the electron state was found reflecting the competition between geometric and magnetic confinements. Electronic tunneling between the coupled rings may be reinforced or suppressed by conveniently modulating the barrier size separating the two rings, leading to drastic change in the electronic charge distribution through the nanostructure.

Magnetic-field effects on transport in carbon nanotube junctions

Here we address a theoretical study on the behavior of electronic states of heterojunctions and quantum dots based on carbon nanotubes under magnetic fields. Emphasis is put on the analysis of the local density of states, the conductance, and on the characteristic curves of current voltage. The heterostructures are modeled by joining zigzag tubes through single pentagon-heptagon pair defects, and described within a simple tight-binding calculation. The conductance is calculated using the Landauer formula in the Green-functions formalism. The theoretical approach used incorporates the atomic details of the topological defects by performing an energy relaxation via Monte Carlo calculation. The effect of a magnetic field on the conductance gap of the system is investigated and compared to those of isolated constituent tubes. It is found that the conductance gap of the studied carbon nanotube heterostructure exhibits oscillations as a function of the magnetic flux. However, unlike the pristine tubes case, they are not Aharonov-Bohm periodic oscillations.