Oscar Hipólito

Interaction of optical phonons with electrons in GaAs quantum wires

Abstract The energy and the effective mass of an electron in a quantum-well wire of GaAs surrounded by Ga 1− x Al x As is calculated in the presence of electron-LO-phonon interaction using a variational approach. The polaron mass is found to be dramatically dependent on the sizes of the wire, and also that its magnitude is greater than that in comparable two- and three-dimensional semiconductor structures.

Electron-interface-phonon interaction in GaAs/AlAs and InAs/GaSb heterojunctions

Abstract We report for the first time a theory which treats the polaronic effects in quasi-two-dimensional semiconductor structures by taking into account the interaction of the electron with the surface, interface and bulk phonon modes. The results obtained show that the dominant electron-phonon interaction involves the interface phonon modes for the case of thin layers.

Magnetopolarons in quantum dots: comparison of polaronic effects from three to quasi-zero dimensions

The interaction of electrons, confined in a quasi-zero-dimensional quantum dot and longitudinal-optical (LO) phonons, placed in a perpendicular magnetic field, is studied within second-order perturbation theory. Analytic and numerical results are presented for the polaron correction to the Landau levels and the polaron cyclotron mass. It is shown that whether cyclotron resonance results in a resonant magnetopolaron or not depends on the ratio of the confinement energy and the LO phonon energy. The polaronic effects in a magnetic field are compared for all dimensionalities from three to quasi-zero, realizable in the experiment. It is found that these effects increase with reducing the dimensionality. Special attention is directed to the weak-magnetic-field limit and the limit of a weak confinement potential.

Charged particle interaction with surface modes

Abstract We calculate the ground state energy and the image potential of a charged particle localized near a surface of a semi-infinite material and interacting with the surface excitation modes. The method used is based on a simple unitary transformation formalism successfully employed to the bulk polaron problem. It has the advantage of being applicable to the whole range of the coupling constant. The results are compared with those obtained by using path integrals method introduced by Feynman.

Magnetopolarons in quasi-one-dimensional quantum-well wires

The interaction of quasi-one-dimensional electrons and longitudinal optical (LO) phonons placed in a perpendicular magnetic field is calculated. Results are presented for the polaron correction to the Landau levels (N) and the polaron cyclotron mass. It is shown that under the condition omega L>N Omega (where omega L is the LO phonon frequency, and Omega is the frequency of the parabolic confinement potential) level crossing occurs, resulting in a resonant magnetopolaron in the vicinity of the cyclotron frequency omega C=(( omega L/N)2- Omega 2)12/. The polaron cyclotron mass increases in the vicinity of this omega C. It is shown that in a cyclotron resonance experiment it is only possible to measure the polaron cyclotron mass if the cyclotron frequency is larger than a critical frequency, determined by the confinement potential.

Electron bound states on liquid helium

Abstract Using an exactly soluble model of image potential which has the form -Ze 2 / (z+β), we have calculated the energy levels of electrons trapped on the surface of liquid helium as a function of the parameter β. Our results show that the experimental values of Grimes et al can be fit very well by locating the effective liquid helium-vacuum interface at β = 1.01 A.

MBE growth and characterization of δ-doping in GaAs and GaAs/Si

Abstract We have investigated the δ-doped GaAs with different cap layer thickness, donor concentrations and intensities of pumping light grown by molecular beam epitaxy, using photoreflectance spectroscopy. The spectra show transitions that we assigned to confined electronic states in the potential of the δ-doping. These electronic state levels depend strongly on the cap layer thickness.

Many-polaron interaction effects in GaAs-GaAlAs quantum-well-wires

Abstract We calculate the electron-phonon contribution to the ground-state energy of a quasi- one-dimensional gas of interacting polarons by using the self-consistent-field approximation theory proposed by Singwi, Tosi, Land and Sjolander for the response function of the electron system. The present results are applied to GaAs-GaAlAs quantum-well-wires with the appropriate form factors that take into account the influence of the finite width of the electron layer. The Hartree-Fock and RPA results are also presented for comparison. We have found that screening effects on the electron-electron and electron-phonon interactions are more effective in quantum wires of small sizes. We also notice that correlation and exchange effects are more evident in wires of reduced dimensions.

Electron-phonon effects on the ground impurity level in quasi-one-dimensional semiconductor heterostructures

Abstract We calculate the effects of the electron-phonon interaction on the binding energies of an impurity placed in a quantum-well wire of rectangular cross section and finite barrier potential to the confined electrons. The calculations are performed as a function of the size of the wire for different positions of the impurity and for several values of the heights of the barrier potential. The results show that the energies are almost the same in wires with comparable cross-sectional area.

Self-trapping of an electron by the surface modes

Abstract A generalized form of the Feynmans path integral method is used in order to calculate the ground-state energy and the effective mass of an electron interacting with the surface optical modes for the whole range of the coupling constant. A phase-transition-like behaviour from nearly free electron to a self-trapping electron as the coupling constant exceeds a certain critical value αc ∼3.8 is observed.