Repositório RCAAP

We investigate the spin dynamics of a magnetic adatom on a non-magnetic surface with strong Stoner enhancement. We find a strong damping of the adatom's magnetization precession and a large shift of the resonance frequency from its bare value. Stoner enhancement in the substrate reduces the damping. We explore the damping dependence on features of the electronic structure.

We investigate the spin-resolved dynamics of spin-polarized carriers injected via a ferromagnetic scanning-tunnelling-microscope tip (STM tip) into uniformly and non-uniformly n-doped bulk semiconductor - externally driven by a current source. We propagate the injected spin packets (assumed gaussian in space at t = 0) by considering a spin hydrodynamic approach based on balance equations directly derived from a spin-dependent Boltzmann equation. We determine the spin-polarization landscapes (time and position) of the carrier population $(n_(\uparrow}-n_{\downarrow})/(n_{\uparrow}+n_{\downarrow}$ and the current density $(j_{\uparrow}-j_{\downarrow})/(j_{\uparrow}+j_{\downarrow})$. While in the uniformly-doped system the carrier spin-polarization has a slow decay, in the non-uniformly doped system it shows a drastic fall down in the interface. In contrast the current spin-polarization exhibits an enhancement for both of the systems particularly in the interface.

The Ogg-McCombe effective Hamiltonian for the electron in the conduction band together with the non-parabolic and effective-mass approximations were used in a theoretical study of the cyclotron effective mass and electron effective Landé g||-factor in semiconductor GaAs-Ga1-xAl xAs quantum wells under an applied magnetic field parallel to the growth direction of the quantum well. Calculations are performed as a function of the applied magnetic field, and for different widths of the GaAs-Ga1-xAl xAs quantum wells. Results for the electron cyclotron effective mass and g||-factor are found in quite good agreement with experimental data.

The dependence of the electron Landé g-factor on carrier confinement in quantum wells recently gained both experimental and theoretical interest. The g factor of electrons in GaAs-(Ga,Al)As quantum wells is of special interest, as it changes its sign at a certain value of the well width. In the present work, the effects of an in-plane magnetic field on the cyclotron effective mass and on the Landé g<FONT FACE=Symbol>^</FONT>-factor in single GaAs-(Ga,Al)As quantum wells are studied. Theoretical calculations are performed in the framework of the effective-mass and non-parabolic-band approximations. The Ogg-McCombe Hamiltonian is used for the conduction-band electrons in the semiconductor heterostructure, and the Landé g<FONT FACE=Symbol>^</FONT>-factor theoretically evaluated is found in good agrement with available experimental measurements.

The hydrostatic-pressure and applied electric field dependencies of the binding energy and the optical-absorption spectra, associated with transitions between the n = 1 valence subband and the donor-impurity band, in symmetrical and asymmetrical GaAs-Ga1-xAl xAs double quantum-well structures are calculated using a variational procedure within the effective-mass approximation. Results are obtained for different well and barrier widths, shallow-donor impurity positions, hydrostatic pressure, and applied electric field.

The calculation of the electronic energy levels of n-type delta-doped quantum wells in a GaAs matrix is presented. The effects of hydrostatic pressure on the band structure are taken into account specially when the host material becomes an indirect gap one. The results suggest that under the applied pressure regime the GaAs can support two-dimensional conduction channels associated to the delta-doping, with carrier densities exceeding 10(13) cm-2.

The effect of a dc electric field on the temporal evolution of an electron in a double rectangular quantum dot is explored in this work. In the framework of the effective mass approximation, first-order scattering rates for interaction between confined electron-''free'' electron and electron-longitudinal acoustic phonon at room temperature are calculated in the high tunneling regime, and used to evaluate the dynamics of the population and coherence in the first three confined levels under a short ac electric field pulse. Small values of these rates dependent upon the bias field make feasible the emission of coherent radiation at terahertz range.

In this work, the exchange energy J for a system of two laterally-coupled quantum dots, each one with an electron, is calculated analytically and in a detailed form, considering them as hydrogen-like atoms, under the Heitler-London approach. The atomic orbitals, associated to each quantum dot, are obtained from translation relations, as functions of the Fock-Darwin states. Our results agree with the reported ones by Burkard, Loss and DiVincenzo in their model of quantum gates based on quantum dots, as well as with some recent experimental reports.

The ground state energies of off-axis negatively charged donors in axially symmetrical quantum dots, with different shapes but in all cases with a small height-to-base radius aspect ratio are calculated in adiabatic approximation by using the Hylleraas-type trial function. The dependencies of the neutral and negative donor binding energies and their ratios on the base radius in the pyramid, lens and disk are calculated and compared with previously obtained results for the spherical quantum dot. We also present the contour plots of the binding energies of the neutral and negative donors with different positions along a vertical cross section in the middle of the quantum dots.

We analyze the spectrum of a neutral donor located inside or outside of a finite-barrier toroidal-shaped nanoring whose radius is much larger than the height. We derive a one-dimensional wave equation which describes the low-lying donor levels corresponding to the slow electron motion along the ring, by using the adiabatic approximation. Numerical solution of this equation has been obtained by using the trigonometric sweep method. The dependence of the energy spectrum on the donor position, radius and height ring has been studied.

Using density functional (DF) calculations and Monte Carlo (MC) simulations we have investigated the main electronic and structural properties of silicon interacting with single walled carbon nanotubes. We have investigated the silicon adsorption externally and internally in the nanotubes surface. The total energies calculations and charge density plot present that the adsorption is most stable externally with silicon forming four bonds with the C atoms and the Si-C bond distances are similar to the ones in the SiC crystal. When silicon is adsorbed in a semiconductor nanotube, a new occupied electronic level inside the band gap is observed. For the metallic tube, the electronic silicon levels are close to the Fermi energy, increasing the metallic tube character.

We analyze the effect of the potential shape on the ground state energy of the off-axis neutral donor in GaAs/Ga1-xAl xAs cylindrical nanotube in the presence of the uniform magnetic field applied along the symmetry axis. To take into account the mixing of the low lying subbands we express the wave function as a product of combination of 1s and 2p x,y wave functions with an unknown envelope function that depends only on electron-ion separation. By using variational principle and the functional derivative procedure we derive a one-dimensional differential equation for the envelope function, which we solve numerically by using of the trigonometric sweep method. Results of calculation of the ground state binding energy dependencies on the distance from the donor position to the axis and on the strength of the external magnetic field for square-well, soft-edge-barrier and parabolic bottom potentials are presented. It is shown that the additional peaks in the curves of the density of impurity states appear due to the presence of the repulsive core is nanotube.

Multiwall Carbon Nanotubes (MWCNTs) were synthesized by Chemical Vapor Deposition (CVD). Two different procedures were used to grow MWCNT films roughly, aligned in the direction normal to the SiO2/Si(111) substrate. Inverse Photoemission Spectroscopy measurements, on these samples, show the existence of resonances which could be traced back to a flat graphene sheet. The unoccupied valence band is fairly similar to that shown by graphite except by an additional intensity in the vicinity of the Fermi level. This resonance could be interpreted both as tubes tips end effects or van Hove singularities in the density of states.

A theoretical discussion of electronic and transport properties of a particular family of double-wall carbon nanotubes, named commensurate structures of the armchair type (n,n)@(2n,2n) is addressed. A single pi-band tight binding hamiltonian is considered and the magnetic field is theoretically described by following the Peierls approximation into the hopping energies. Our emphasis is put on investigating the main effects of the geometrical aspects and relative positions of the tubes on the local density of states and on the conductance of the system. By considering intershell interactions between a set of neighboring atoms on the walls of the inner and outer tubes, we study the possibility of founding Aharonov-Bohm effects in the DWCNs when a magnetic field is applied along the axial direction.

The localization properties of the single-particle and collective electron excitations were investigated in the intentionally disordered GaAs/AlGaAs superlattices by weak field magnetoresistance and Raman scattering. The localization length of the individual electron was found to be considerably larger than that one of the collective excitations. This suggests that the disorder has weaker effect on the electrons than on their collective motion and that the interaction which gives rise to the collective effects increases localization.

The formation of the miniband electron energy structure was explored in doped InGaAs/InP superlattices with different periods. The analysis of the Raman data allowed us to conclude that in spite of the defect structure of the layers constituting the superlattices, their super-periodicity was well defined. The quantitative proof of the conditions for break-down of the Raman selection rules is presented: the emergence of the selection rules of the coupled plasmon-LO phonon vibrations was demonstrated to occur due to the increase of their coherence lengths. In addition, the expected anisotropy of the effective electron masses was found by high-field magnetoresistance.

In the present work, by means of a phenomenological model, we simulate the hysteresis loop of an hexagonal array of Ni nanowires. Our model is based on the assumption that the hysteresis loop of a single wire is a rectangular box with a particular value of the coercive field, and the effect of the array is to generate a distribution of the coercive fields. Our results are in good agreement with experimental data.

We calculate the conductance of three-dimensional semiconductor quantum wires considering different effective masses in the contacts and in the channel. We show that, with respect to the case with equal masses in the channel and in the contacts, the amplitude of the conductance oscillations increases if the electron effective mass in the channel is larger and decreases if it is smaller than in the contacts. Effects on the density of probability are also shown. These effects of the effective mass discontinuity are explained in terms of kinetic confinement and transmission coefficient modulation.

In this article we study the electronic transport through a triple quantum-dot molecule parallel-coupled to leads under a magnetic field. Analytical expressions are obtained for both the conductance and total density of states for the molecule in equilibrium at zero temperature. As a result of quantum interference of resonances belonging to different channels, this configuration exhibits bound states in the continuum (BICs). We examine the broadenings of the molecular states around the conditions under which BICs occur, finding long-lived states extremely robust under variations of the magnetic flux.

The slave boson (SB) technique is employed to study the Zeeman spin splitting in a quantum dot. Unlike traditional SB method, each spin is renormalized differently. Two geometries are compared: side connected and embedded. In both cases, it's shown a non linear behavior of the splitting as a function of the magnetic field applied. The results are in line with the latest experiments.