arXiv subjects

Augusto Gonzalez

Publications and source records attributed to Augusto Gonzalez.

At least 37 records · Page 2Linked to original sources

arXiv2009

Atoms as perfect oscillators?

By using Supersymmetric Quantum Mechanics and Semiclassical Quantization, one may argue that the low-lying excited states of any quantum system can be modeled by a set of harmonic oscillators. In the present paper, we fit the experimental excitation spectra of atoms with atomic number 2< Z< 36 to a simple harmonic oscillator model with two parameters: the number of degrees of freedom, d, and the effective frequency, ω. The obtained \hbarωtakes values around 0.03 (in atomic units), whereas d shows clear shell filling effects, that is, takes high values for the noble gases, suggesting collective oscillations of the electrons occupying the last shell.

physics.atom-ph↗

arXiv2009

Thomas-Fermi scaling in the energy spectra of atomic ions

The energy spectra of atomic ions are re-examined from the point of view of Thomas-Fermi scaling relations. For the first ionization potential, which sets the energy scale for the true discrete spectrum, Thomas-Fermi theory predicts the following relation: E_{ioniz}=Z^2 N^{-2/3} g(N/Z), where Z is the nuclear charge, N is the number of electrons, and g is a function of N/Z. This relation does not hold for neutral atoms, but works extremely well in the cationic domain, Z>N. We provide an analytic expression for g, with two adjustable parameters, which fits the available experimental data for more than 380 ions. In addition, we show that a rough fit to the integrated density of states with a single exponential: N_{states}=exp (Delta E/Theta), where Delta E is the excitation energy, leads to a parameter, Theta, exhibiting a universal scaling a la Thomas-Fermi: Theta=Z^2 N^{-4/3} h(N/Z), where h is approximately linear near N/Z=1.

physics.atom-ph↗

arXiv2009

Polariton Lasing in a Multilevel Quantum Dot Strongly Coupled To a Single Photon Mode

We present an approximate analytic expression for the photoluminescence spectral function of a model polariton system, which describes a quantum dot, with a finite number of fermionic levels, strongly interacting with the lowest photon mode of a pillar microcavity. Energy eigenvalues and wavefunctions of the electron-hole-photon system are obtained by numerically diagonalizing the Hamiltonian. Pumping and photon losses through the cavity mirrors are described with a master equation, which is solved in order to determine the stationary density matrix. The photon first-order correlation function, from which the spectral function is found, is computed with the help of the Quantum Regression Theorem. The spectral function qualitatively describes the polariton lasing regime in the model, corresponding to pumping rates two orders of magnitude lower than those needed for ordinary (photon) lasing. The second-order coherence functions for the photon and the electron-hole subsystems are computed as functions of the pumping rate.

cond-mat.mes-hall↗

arXiv2009

Dynamics of small perturbations against a stationary polariton distribution

The dynamics of small perturbations against the stationary density matrix of a pumped polariton system with only one photon polarization is studied. Depending on the way the system is pumped and probed, decay times ranging from 30 to 5000 ps are found. The large decay times under resonant pumping are related to a bottleneck effect in the decay of the excess (probe) populations of dark polariton states. No singular behaviour at the threshold for polariton lasing is observed.

cond-mat.mes-hall↗

arXiv2009

Stationary density matrix of a pumped polariton system

The density matrix, ρ, of a model polariton system is obtained numerically from a master equation which takes account of pumping and losses. In the stationary limit, the coherences between eigenstates of the Hamiltonian are three orders of magnitude smaller than the occupations, meaning that the stationary density matrix is approximately diagonal in the energy representation. A weakly distorted grand canonical Gibbs distribution fits well the occupations.

cond-mat.mes-hall↗

arXiv2008

Exact-Diagonalization Studies of Inelastic Light Scattering in Self-Assembled Quantum Dots

We report exact diagonalization studies of inelastic light scattering in few-electron quantum dots under the strong confinement regime characteristic of self-assembled dots. We apply the orthodox (second-order) theory for scattering due to electronic excitations, leaving for the future the consideration of higher-order effects in the formalism (phonons, for example), which seem relevant in the theoretical description of available experiments. Our numerical results stress the dominance of monopole peaks in Raman spectra and the breakdown of selection rules in open-shell dots. The dependence of these spectra on the number of electrons in the dot and the incident photon energy is explicitly shown. Qualitative comparisons are made with recent experimental results.

cond-mat.mes-hall↗

arXiv2008

Excitonic States in Medium-Size Quantum Dots

In a quantum dot with dozens of electrons, an approximation beyond Tamm-Dankoff is used to construct the quantum states with an additional electron-hole pair, i.e. the "excitonic" states. The lowest states mimic the non-interacting spectrum, but with excitation gaps renormalized by Coulomb interactions. At higher excitation energies, the computed density of energy levels shows an exponential increase with energy. In the interband absorption, we found a background level in the quasicontinuum of states rising linearly with the excitation energy. Above this background, there are distinct peaks related to single resonances or to groups of many states with small interband dipole moments.

cond-mat.mes-hall↗

arXiv2007

Micropillar resonator in a magnetic field: Zero and Finite temperature cases

In this work, we present a theoretical study of a quantum dot-microcavity system which includes a constant magnetic field in the growth direction of the micropillar. First, we study the zero temperature case by means of a selfconsistent procedure with a trial function composed by a coherent photon field and a BCS function for the electron-hole pairs. The dependence of the ground-state energy on the magnetic field and the number of polaritons is found. We show that the magnetic field can be used as a control parameter of the photon number, and we make explicit the scaling of the total energy with the number of polaritons. Next, we study this problem at finite temperatures and obtain the scaling of the critical temperature with the number of polaritons.

cond-mat.mes-hall↗

arXiv2006

Photon emission as a source of coherent behaviour of polaritons

We show that the combined effect of photon emission and Coulomb interactions may drive an exciton-polariton system towards a dynamical coherent state, even without phonon thermalization or any other relaxation mechanism. Exact diagonalization results for a finite system (a multilevel quantum dot interacting with the lowest energy photon mode of a microcavity) are presented in support to this statement.

cond-mat.mes-hall↗

arXiv2006

A multiexcitonic quantum dot in an optical microcavity

We theoretically study the coupled modes of a medium-size quantum dot, which may confine a maximum of ten electron-hole pairs, and a single photonic mode of an optical microcavity. Ground-state and excitation energies, exciton-photon mixing in the wave functions and the emission of light from the microcavity are computed as functions of the pair-photon coupling strength, photon detuning, and polariton number.

cond-mat.mes-hall↗

arXiv2006

Inelastic light scattering by electronic excitations in artificial atoms

In the paper, we present theoretical calculations of the cross section for inelastic light scattering by electronic excitations in a quantum dot charged with 42 electrons. The many-electron states involved in the computations are obtained in the framework of the Random Phase Approximation. The evaluation of energy-weighted sum rules allowed us to distinguish among single-particle (SPEs), collective charge (CDEs) and collective spin (SDEs) electronic excitations. Raman spectra in polarized and depolarized geometries are computed for a range of incident laser energies. The below-band-gap excitation regime reveals advantages for the identification of individual excited states contributing to the Raman spectra. The computed polarization ratios for Raman intensities show the breakdown of selection rules in the presence of a magnetic field. A new jump rule of Raman intensities at the band gap may help identifying the nature (charge or spin) of electronic excitations. Under extreme resonance (laser energy very close to the band gap), Raman spectra are dominated by SPEs. For incident laser energies 40 -- 50 meV above band gap, a correct desciption of Raman spectra is shown to require the proper account of energy widths of intermediate states.

cond-mat.mes-hall↗

arXiv2005

Nonperturbative methods in Quantum Mechanics of three or more particles

In the present report, a set of theoretical results obtained in the period from 1991 to 2005 are reviewed. The physical systems under study include quark models of hadrons, inert atom clusters, atomic traps, and electrons and excitons confined in quantum dots. They contain three or more particles, and are described by nonrelativistic Quantum Mechanics. In the smallest systems (6 particles or less), exact diagonalization complemented with the Lanczos algorithm, and some analytical approaches (1/D-expansion, improved semiclassical quantization, etc.) are used. In the larger systems (from 7 to hundreds of particles), we employ approximate methods, such as two-point Pade approximants, variational Monte Carlo estimations, Hartree - Fock and RPA schemes, BCS functions and the Bethe - Goldstone algorithm, etc. With the help of these methods, a variety of physical properties have been computed, i.e., energy spectra, particle densities, spin textures, light absorption and emission, inelastic light scattering, and some dynamical features.

cond-mat.mes-hall↗

arXiv2005

Selection and jump rules in electronic Raman scattering from GaAs/Al_{x}Ga_{1-x}As artificial atoms

A theoretical description of electronic Raman scattering from GaAs/Al_{x}Ga_{1-x}As artificial atoms under the influence of an external magnetic field is presented. Raman spectra with laser excitation energy in the interval E_{gap}-30 meV to E_{gap} are computed in the polarized and depolarized geometry. The polarization ratios for the collective and single-particle excitations indicate a breakdown of the Raman polarization selection rules once the magnetic field is switched on. A Raman intensity jump rule at the band gap is predicted in our calculations. This rule can be a useful tool for identifying the physical nature (charge or spin) of the electronic excitations in quantum dots in low magnetic fields.

cond-mat.mes-hall↗

arXiv2005

Electronic Raman scattering in quantum dots revisited

We present theoretical results concerning inelastic light (Raman) scattering from semiconductor quantum dots. The characteristics of each dot state (whether it is a collective or single-particle excitation, its multipolarity, and its spin) are determined independently of the Raman spectrum, in such a way that common beliefs used for level assignments in experimental spectra can be tested. We explore the usefulness of below band gap excitation and an external magnetic field to identify charge and spin excited states of a collective or single-particle nature.

cond-mat.mes-hall↗

arXiv2005

Higher Landau levels contribution to the energy of interacting electrons in a quantum dot

Properly regularized second-order degenerate perturbation theory is applied to compute the contribution of higher Landau levels to the low-energy spectrum of interacting electrons in a disk-shaped quantum dot. At ``filling factor'' near 1/2, this contribution proves to be larger than energy differences between states with different spin polarizations. After checking convergence of the method in small systems, we show results for a 12-electron quantum dot, a system which is hardly tractable by means of exact diagonalization techniques.

cond-mat.mes-hall↗

arXiv2004

Effects of Coulomb interactions on the splitting of luminescence lines

We study the splitting between the right-hand and left-hand circularly polarized luminescence lines in a quantum dot under relatively weak confinement regime and resonant high-power excitation. When the dot is populated with an even number of electron-hole pairs (biexciton and higher excitations), the splitting measures basically the Zeeman energy. However, in the odd number of pairs case, we have, in addition to the Zeeman and Overhauser shifts, a contribution to the splitting coming from Coulomb interactions. This contribution is of the order of a few meV, and shows distinct signatures of shell-filling in the quantum dot.

cond-mat.mes-hall↗

arXiv2004

Mean-field dynamics of a quantum dot - microcavity system

Mean-field evolution equations for the exciton and photon populations and polarizations (Bloch-Lamb equations) are written and numerically solved in order to describe the dynamics of electronic states in a quantum dot coupled to the photon field of a microcavity. The equations account for phase space filling effects and Coulomb interactions among carriers, and include also (in a phenomenological way) incoherent pumping of the quantum dot, photon losses through the microcavity mirrors, and electron-hole population decay due to spontaneous emission of the dot. When the dot may support more than one electron-hole pair, asymptotic oscillatory states, with periods between 0.5 and 1.5 ps, are found almost for any values of the system parameters.

cond-mat.mes-hall↗

arXiv2004

Inelastic light scattering and the off-resonance approximation

Inelastic (Raman) light scattering intensities for a 42-electron quantum dot under off-resonance conditions and in different spin and angular momentum channels are computed in order to test whether final collective states become the dominant peaks in the process. The results of the calculations set a limit to the spread use of the off-resonant approximation for the theoretical description of Raman processes.

cond-mat.mes-hall↗