Strong exciton binding in quantum structures through remote dielectric confinement

We propose a new type of hybrid systems formed by conventional semiconductor nanostructures with the addition of remote insulating layers, where the electron-hole interaction is enhanced by combining quantum and dielectric confinement over different length scales. Due to the polarization charges induced by the dielectric mismatch at the semiconductor/insulator interfaces, we show that the exciton binding energy can be more than doubled. For conventional III-V quantum wires such remote dielectric confinement allows exciton binding at room temperature.Comment: 4 pages, 3 PostScript figures embedded, best printed in color. Uses RevTex, multicol, and psfig styles. To appear in Phys. Rev. Let

The IBIS view of the galactic centre: INTEGRAL's imager observations simulations

The Imager on Board Integral Satellite (IBIS) is the imaging instrument of the INTEGRAL satellite, the hard-X/soft-gamma ray ESA mission to be launched in 2001. It provides diagnostic capabilities of fine imaging (12' FWHM), source identification and spectral sensitivity to both continuum and broad lines over a broad (15 keV--10 MeV) energy range. It has a continuum sensitivity of 2~10^{-7} ph cm^{-2} s^{-1} at 1 MeV for a 10^6 seconds observation and a spectral resolution better than 7 % at 100 keV and of 6 % at 1 MeV. The imaging capabilities of the IBIS are characterized by the coupling of the above quoted source discrimination capability with a very wide field of view (FOV), namely 9 x 9 degrees fully coded, 29 x 29 degrees partially coded FOV. We present simulations of IBIS observations of the Galactic Center based on the results of the SIGMA Galactic Center survey. They show the capabilities of this instrument in discriminating between different sources while at the same time monitoring a huge FOV. It will be possible to simultaneously take spectra of all of these sources over the FOV even if the sensitivity decreases out of the fully coded area. It is envisaged that a proper exploitation of both the FOV dimension and the source localization capability of the IBIS will be a key factor in maximizing its scientific output.Comment: 5 pages, LaTeX, to be published in the 4th Compton Symposium Conference Proceedings, uses aipproc.cls, aipproc.sty (included

Wigner crystallization in quantum electron bilayers

The phase diagram of quantum electron bilayers in zero magnetic field is obtained using density functional theory. For large electron densities the system is in the liquid phase, while for smaller densities the liquid may freeze (Wigner crystallization) into four different crystalline phases; the lattice symmetry and the critical density depend on the the inter-layer distance. The phase boundaries between different Wigner crystals consist of both first and second order transitions, depending on the phases involved, and join the freezing curve at three different triple points.Comment: To appear in Europhys. Lett. (11 pages in REVTEX + 2 figures in postscript

First Principles Calculations of Charge and Spin Density Waves of sqr3-Adsorbates on Semiconductors

We present ab-initio electronic structure results on the surface of sqr3 adsorbates. In particular, we address the issue of metal-insulator instabilities, charge-density-waves (CDWs) or spin-density-waves (SDWs), driven by partly filled surface states and their 2D Fermi surface, and/or by the onset of magnetic instabilities. The focus is both on the newly discovered commensurate CDW transitions in the Pb/Ge(111) and Sn/Ge(111) structures, and on the puzzling semiconducting behavior of the Pb/Ge(111), K/Si(111):B and SiC(0001) surfaces. In all cases, the main factor driving the instability appears to be an extremely narrow surface state band. We have carried out so far preliminary calculations for the Si/Si(111) surface, chosen as our model system, within the gradient corrected local density (LDA+GC) and local spin density (LSD+GC) approximations, with the aim of understanding the possible interplay between 2D Fermi surface and electron correlations in the surface + adsorbate system. Our spin- unrestricted results show that the sqr3 paramagnetic surface is unstable towards a commensurate SDW with periodicity 3x3 and magnetization 1/3.Comment: 9 pages, 4 Postscript figures, to be published in Surf. Sc

Valence band spectroscopy in V-grooved quantum wires

We present a combined theoretical and experimental study of the anisotropy in the optical absorption of V-shaped quantum wires. By means of realistic band structure calculations for these structures, we show that detailed information on the heavy- and light-hole states can be singled out from the anisotropy spectra {\em independently of the electron confinement}, thus allowing accurate valence band spectroscopy.Comment: To be published in Appl. Phys. Lett. (8 pages in REVTeX, two postscipt figures