B. Barbiellini

Variational wave function for a two-electron quantum dot

We have applied the variational quantum Monte Carlo technique to a two-electron quantum dot. A simple variational wave function is presented and we compare the results obtained using it with exact numerical diagonalizations. The comparison shows that almost exact results are obtained for lowest energy states of di⁄erent relative angular momenta. ( 1998 Elsevier Science B.V. All rights reserved.

Positron annihilation and correlation in high-Tc oxides

Spectra of positron annihilation in YEa.

Correlation effects for positron annihilation with core and semicore electrons

u&+ and Nd

Fermi surfaces of high-Tc superconductors by positron 2D-ACAR

uO, are calculated using self-consistent LMIC calculations and compared with various experimental data. Methods for including effects of correlation and enhancement arc presented and applied in addition to the independent panicle model. Such effects are not crucial for the question of Fermi surface breaks in these materials. However from comparison of lifetimes between theory and experiment in the oxides and other materials we conclude that lattice imperfections can mask the Fermi surface which has been observed in recent experiments.

Correlation effects in positron-electron systems: A quantum Monte-Carlo study

We present a theory for calculating the momentum distribution of annihilating electron-positron pairs in solids. The theory leads to a scheme in which the momentum density within the independent particle model for a given electron state is weighted by the annihilation rate of that state. The annihilation rate is calculated within the generalized gradient approximation. The scheme describes the suppression of the electron-positron correlations due to the charge inhomogeneity and explains the trends observed for positron annihilation with core and semicore electrons.

Positron annihilation and positron-electron correlation effects in high-Tc oxides

Abstract We review the work aimed at identifying the Fermi surface of high- T c superconducting oxides using positron 2D-ACAR technique. We also present our recent observation of the pillbox in YBa 2 Cu 3 O 7 − δ and the first identification of Fermi surface signals in Nd 2 − x Ce x CuO 4 − δ obtained by 2D-ACAR.

Theoretical and experimental study of positron annihilation with core electrons in solids

We present Quantum Monte-Carlo results for various positron-electron systems. We discuss the energetics of the bound state of a positronium (Ps) with finite atomic systems: a massive point charge Z, hydrogen (H) and lithium (Li). We also analyze the screening cloud around the positron in the positronium hydride (HPs) and in the homogeneous electron gas in order to determine the positron annihilation rates.

Gradient correction for positron states in solids

Abstract We have calculated electron and positron states in the Nd2 − xCexCuO4 compound, for the perfect bulk lattice as well as when vacancy defects are included. We study the sensitivity of the results to different approximations in the positron potential. We also compute the annihilation rate and the electron-positron momentum density using the local density approximation and the newly developed gradient correction for the electron-positron contact term. Our main goal is to give predictions for the Fermi surface signals.