M. Pacheco

Spectra of acceptors in quantum dots: the effect of a magnetic field

We report calculations of the energy spectra of shallow-acceptor impurities in quantum dots in the presence of external magnetic fields. We calculate the binding energies of the ground and excited acceptor states in the effective-mass approximation using a formalism based on a four-band model that includes the coupling of the spin to the magnetic field as well as the valence-band mixing. The potential of the acceptor is taken to be the screened Coulomb potential of a point charge, and we take into account the mismatch of the dielectric constant through the method of image charges. We present a complete analysis of the acceptor binding energies as a function of the lateral confinement of the quantum dot and as a function of the magnetic field strength. We also discuss the role of the mixing of heavy holes and light holes in determining the acceptor spectrum in the different regimes of confinement.

Magnetoexciton transitions in GaAs-Ga1-xAlxAs quantum wells

A theoretical study of the internal transitions of confined magnetoexcitons in GaAs-Ga1-xAlxAs quantum wells is presented, with the magnetic field applied along the growth direction of the semiconductor heterostructure. The various exciton-envelope wavefunctions are described as products of electron and hole solutions of the associated quantum-well potentials and symmetry-adapted Gaussian functions. The magnetoexciton states are simultaneously obtained by diagonalizing the appropriate Hamiltonian in the effective-mass approximation. Exciton internal transitions are theoretically investigated by studying the allowed magnetoexcitonic transitions using far-infrared (terahertz) radiation circularly polarized in the plane of the quantum well. Theoretical results are obtained for both the intramagnetoexciton transition energies and oscillator strengths associated with excitations from 1s-like to 2s-, 2p±-, and 3p±-like magnetoexciton states, and from 2p-- to 2s-like exciton states. The present results are compared with previous theoretical work and available optically detected resonance measurements.

Electron transport in quantum antidots made of four-terminal graphene ribbons

Electronic and transport properties of two- and four-terminal graphene nanoribbons are studied taking into account different configurations of quantum antidot potentials, designed at a central conductor. Local density of states maps the electronic distribution changes induced by the antidot potentials and highlights localization effects at the neighboring vacancy sites. Depending on the position, extension, and symmetry of such antidots, we found delocalization of electronic states leading to particular conduction paths along the central region. The origin of dips and maxima in the conductance, and full transport suppression was studied within a microscopic scenario using real-space Green function formalism. The combination of antidot potentials and extra terminals in the device model has shown to generate a variety of transport responses that may be experimentally tuned.

Magnetoabsorption spectra of intraexcitonic transitions in GaAs-(Ga,Al)As semiconductor quantum wells

We present a theoretical study, within the effective-mass approximation, of the magnetoabsorption spectra of intraexcitonic terahertz transitions of light-hole and heavy-hole confined magnetoexcitons in GaAs-(Ga,Al)As quantum wells. The semiconductor quantum wells are studied under magnetic fields applied in the growth direction of the semiconductor heterostructure. The various magnetoexciton states are obtained in the effective-mass approximation by an expansion of the exciton-envelope wave functions in terms of products of hole and electron quantum-well states with appropriate Gaussian functions for the various excitonic states. Intramagnetoexciton transitions are theoretically studied by exciting the allowed excitonic transitions with σ+ (or σ−) far-infrared radiation circularly polarized in the plane of the GaAs-(Ga,Al)As quantum well. Theoretical results are obtained for the intramagnetoexciton transition energies and magneto-absorption spectra associated with excitations from 1s-like to 2p±, and 3p±...

Excitonic effects in a superlattice in parallel electric and magnetic fields

Working in the effective-mass approximation the authors study the problem of excitons in a superlattice in parallel electric and magnetic fields. Using the translation symmetry of the excitonic effective Hamiltonian and the electric field induced localization, they find an expression for the envelope excitonic function in terms of packet-like functions constructed of non-interacting electron-hole pairs in a one-dimensional superlattice under an electric field.

Graphene nanoribbon thermopower as a tool for molecular spectroscopy

In this work, we study thermoelectric properties of graphene nanoribbons with side-attached organic molecules. By adopting a single-band tight binding Hamiltonian and the Greens function formalism, we calculated the transmission and Seebeck coefficients for different hybrid systems. The corresponding thermopower profiles exhibit a series of sharp peaks at the eigenenergies of the isolated molecule indicating that the system can be proposed as a molecular thermo-device. We have studied the effects of the temperature on the thermoelectric response, and considered random configurations of molecule distributions, in different disorder regimes. We have found that the main features of the thermopower are robust under temperature and disorder.