Repositório RCAAP

The fusion method was used to produce PbS quantum dots (QDs) embedded in S-doped glass matrix (SiO2-Na2CO3-Al2O3-PbO2-B2O3:S). Measurements of optical absorption (OA), photoluminescence (PL) and atomic force microscopy (AFM) have been carried out in order to characterize the produced QDs. A strong red-shift observed in the optical features with an increase of the annealing time indicates an increase in QD-size. The QD sizes predicted by k.p theoretical results were confirmed by AFM observation.

We describe the Kondo resonance in quantum dots employing the atomic model. We calculate approximate Green's functions of the impurity Anderson model employing the exact solution of the system with a conduction band with zero width, and we use the completeness condition to choose the position of that band. At low temperatures, there are two solutions close to the chemical potential µ, satisfying this condition, and we choose the one with minimum Helmholtz free energy, considering that this corresponds to the Kondo solution. At high temperatures, this solution no longer exist, corresponding to the disappearance of the Kondo peak. We present curves of density of states that characterize the Kondo peak structure problem. As a simple application we calculate the conductance of a side-coupled quantum dot and we obtain good agreement with recent experimental results.

We are interested in studying the transport properties of metallic single-wall carbon nanotubes (SWCNTs) with isolated magnetic impurities. We consider a metallic zigzag SWCNT in the form of an infinitely long cylinder of diameter D, connected by two metallic electrodes under a bias voltage $E$, with a magnetic impurity located on its surface. To describe the Kondo resonance we employ an impurity version of the atomic model, previously developed to study the Kondo insulator properties in the lattice case. We calculate the approximate Green's functions of the impurity Anderson model by employing the exact solution of the atomic limit of the Anderson model, where we use the completeness condition to choose the position of the chemical potential. We consider the SWCNT Green's functions in a tight-binding approach. We calculate density of states curves that characterize well the structure of the Kondo peak and we also present the dependence of the conductance with the diameter of the SWCNT.

In this work we studied the molecular-beam epitaxy of large InAs quantum dots embedded in an InGaAs quantum well. The formation of the quantum dots was performed by pulsed deposition of InAs material, simulating therefore a low deposition rate. Room-temperature photoluminescence experiments carried out on these samples showed intense emissions with a wavelength exceeding 1.35µm. Our preliminary data indicate that even better results can be achieved by further manipulation of the growth conditions.

The conductance of a double-dot system in a ring threaded by a magnetic flux is studied. The ring is connected to two leads in such a way that each dot is embedded in one arm of the structure. The currents going through each arm of the ring are determined by the Aharonov-Bohm effect and the dot charging effects. The conductance for different values of the magnetic flux is obtained as a function of the gate potentials applied to the dots, for two situations: when the dot level energies are equal, deltaE = 0, and when they are different but with deltaE smaller than the Coulomb interaction U. The conductance shows quite distinct behavior according to having a finite or zero magnetic flux threading the ring. The system presents S = 1/2 and S = 1 Kondo phenomena for no magnetic flux and is in the S = 1/2 Kondo regime in the case of half a quantum of magnetic flux.

CdO quantum dots (QDs) incorporated in polyacrylamide were synthesized adding aqueous suspension of cadmium oxide in acrylamide:bisacrylamide copolymer. Optical properties of CdO nanocrystals were studied by optical absorption. The size ranges (2-3 nm) were calculated by the effective mass approximation.

We theoretically study many-body excitations in three different quasi-one-dimensional (Q1D) electron systems: (i) those formed on the surface of liquid Helium; (ii) in two coupled semiconductor quantum wires; and (iii) Q1D electrons embedded in polar semiconductor-based quantum wires. Our results show intersubband coupling between higher subbands and the two lowest subbands affecting even the lower energy intersubband plasmons on the liquid Helium surface. Concerning the second system, we show a pronounced extra peak appearing in the intersubband impurity spectral function for temperatures as high as 20 K. We finally show coupled intersubband plasmon-phonon modes surviving for temperatures up to 300 K.

Coherent properties and Rabi oscillations in two-level donor systems, under terahertz excitation, are theoretically investigated. Here we are concerned with donor states in bulk GaAs and GaAs - (Ga, Al)As quantum dots. We study confinement effects, in the presence of an applied magnetic field, on the electronic and on-center donor states in GaAs - (Ga, Al)As dots, as compared to the situation in bulk GaAs, and estimate some of the associated decay rate parameters. Using the optical Bloch equations with damping, we study the time evolution of the 1s and 2p+ states in the presence of an applied magnetic field and of a terahertz laser. We also discuss the role played by the distinct dephasing rates on the photocurrent and calculate the electric dipole transition moment. Results indicate that the Rabi oscillations are more robust as the total dephasing rate diminishes, corresponding to a favorable coherence time.

We present simulations of transport through highly transmitting open cavities within a lattice Green's function formalism. The qualitative relation between the line shape of the conductance and the symmetries of the corresponding scar wavefunctions is discussed. This system presents similar scar wavefunctions at different conductance plateaus. At the high plateau index limit the scars associated with bouncing ball classic orbits suffer a transition from accessible to quasi-unaccessible from outside the square billiard.

Micron-sized spheres of mesoporous styrene-divinylbenzene (Sty-DVB) copolymer were produced by suspension polymerisation in the presence of inert diluents. Using these mesoporous microspheres as a template, optically stable CdS nanoparticles have been synthesized. To characterize these CdS nanoparticles, Raman spectroscopy and micro-photoluminescence were used. We have observed enhanced emission and lasing action at certain wavelengths that correspond to the whispering gallery (WG) modes of the microspheres. High optical stability and low threshold value make this optical system promising in microlaser applications.

We calculated optical absorption coefficients for linear and monolayer arrays of ionized quantum dots (QD) of spherical or cylindrical shape with random dimensions and occupation numbers, obtained by Monte Carlo simulation. When submitted to an oscillating electric field each ionized QD behaves as a dipole and the interaction of them changes the form of the light absorption. The effect of the many electron system is considered by Density Functional Theory. The results obtained show a shift of the absorption peak to the right for increasing densities of QD's, whatever kind of shape or array of QD. Also absorption only, occurs when the electric field has a normal component to the array.

Stationary states of an electron in thin GaAs elliptical quantum rings are calculated within the effective-mass approximation. The width of the ring varies smoothly along the centerline, which is an ellipse. The solutions of the Schrödinger equation with Dirichlet boundary conditions are approximated by a product of longitudinal and transversal wave functions. The ground-state probability density shows peaks: (i) where the curvature is larger in a constant-with ring, and (ii) in thicker parts of a circular ring. For rings of typical dimensions, it is shown that the effects of a varying width may be stronger than those of the varying curvature. Also, a width profile which compensates the main localization effects of the varying curvature is obtained.

The energy spectrum and optical absorption of confined carriers in ellipsoidal CdSe quantum dots are calculated. Our model relies on the effective mass approximation and Fermi's golden rule for the electronic structure and optical properties, respectively. We demonstrate that the electronic structure and the interband optical absorption are highly dependent on the ellipsoid aspect ratio.

We study the optical properties of carbon nanotubes within the single-band tight-binding approximation.The great number of one-dimensional-like subbands of the nanotubes lead to typical van Hove singularities on the local density of states. When a tube is under the influence of a laser beam, optical transitions are allowed between Van Hove singularities and they can be observed experimentally in absorption spectra. External magnetic and electric fields modify the energy spectrum of carbon nanotubes inducing changes on the optical phenomena at low frequencies. Results for the inter-band absorption coefficient are shown for nanotubes under an additional external electrostatic potential for parallel light polarization. This is an important problem to understand since low energy laser beams are being used to identify the chirality and diameter of the tubes and their electronic character.

Phonon-induced spin relaxation rates and electron g-factor tuning of quantum dots are studied as function of in-plane and perpendicular magnetic fields for different dot sizes. We consider Rashba and Dresselhaus spin-orbit mixing in wide and narrow-gap semiconductors, and show how Zeeman sublevels can relax via piezoelectric (GaAs) and deformation (InSb) potential coupling to acoustic phonons. We find that strong confinement may induce minima in the rates at particular values of the magnetic field (due to a magnetic field-induced cancellation of the spin-orbit effects), where spin relaxation times can reach seconds. We also report on g-factor anisotropy. We obtain good agreement with available experimental values.

Electronic band-edge structure and optical properties of Si1-xGe x are investigated theoretically emloying a full-potential linearized augmented plane wave (FPLAPW) method. The exchange-correlation potential in the local density approximation (LDA) is corrected by an on-site Coulomb potential (i.e., within the LDA+U SIC approach) acting asymmetrically on the atomic-like orbitals in the muffin-tin spheres. The electronic structure of the Si1-xGe x is calculated self-consistently, assuming a Td symmetrized Hamiltonian and a linear behavior of the valence-band eigenfunctions for Si, SiGe, and Ge with respect to Ge composition x, assuming randomly alloyed crystal structure. i.e., a "virtual-crystal like" approximation (VCA). We show that this approach yields accurate band-gap energies, effective masses, dielectric function, and optical properties of Si1-xGe x. We perform absorption measurements showing the band-gap energy for x < 0.25.

In this work optical and semiconductor characteristics of gallium oxide based thin films are studied. This material has important electro-optic applications and offers possibilities for semiconductor applications in heavy metal oxide semiconductor technology. In this study thin films of PbO-Bi2O3-Ga2O3 were deposited by reactive sputtering. Two targets with different concentrations of Nd2O3 (0.2 and 1.0 wt%) were prepared. Thin films were deposited by pure Ar plasma, at 5 mTorr pressure and RF power of only 20 W. Films were deposited on three inch diameter substrates of (100) silicon wafers. Optical UV-Vis-Nir transmittance and FTIR analyses showed that the films are transparent in the visible and middle infrared region of the electromagnetic spectrum. The refractive index of the films was of about 2.3, as measured by ellipsometry; Rutherford Backscattering Spectroscopy (RBS) analyses were employed for the compositional analysis of the films. For the electrical characterization it was used metallic electrodes, and the final structure obtained is a capacitor structure where the gallium oxide film plays the role of a dielectric. In the current versus voltage (IxV) analyses, the samples with 1.0 wt% of Nd2O3 show semiconductor characteristics similar to the ones obtained for the degenerated transistors. The electric characteristics of these films allow applications with electro-optic devices (phototransistors and photo detectors).

We study organic microcavities in the strong coupling regime. The cavity exciton-polariton dispersion relations and quantum states are derived in using a microscopic theory. We consider the two cases of anisotropic organic crystals with one and two molecules per unit cell. In general, the cavity exciton-polaritons are a coherent superposition of both cavity mode polarizations and of both Davydov exciton branches. The obtained polarization mixing is in contrast to the case of typical inorganic semiconductor cavities in which no mixing between the TM and TE polarizations occurs. By applying the quasi-mode approximation, we derive the transmission, reflection, and absorption coefficients for high quality organic cavities.

Recent studies suggest that base pairing is an efficient electronic delocalization mechanism. However, defects may break down such effect. In the present work we show how a simple model of defects suppresses the delocalization, which survives only for low defect concentrations.

A study of ultraviolet and visible Raman spectroscopy was made in amorphous carbon nitride (a-C:Nx) and amorphous hydrogenated carbon nitride (a-C:Nx:H) thin films. Two growth techniques were used: 1) glow discharge was used for depositing hydrogenated films with diamond-like (at -200V of bias) and graphite-like (-800V of bias) structure and 2) ion beam assisted deposition (IBAD) to deposit non-hydrogenated films and to obtain high nitrogen concentrations (~30%). The difference in energy between the visible and UV Raman, the so called G dispersion parameter, was investigated for both series of films. This parameter brings additional information concerning the structure of the films regarding size and concentration of graphitic clusters, and qualitative information on the sp²/sp³ concentration. The characterization of the structural properties in these films indicates an increase of graphite clusters with the incorporation of nitrogen.