Dyson Orbitals, Quasi-Particle effects and Compton scattering

Dyson orbitals play an important role in understanding quasi-particle effects in the correlated ground state of a many-particle system and are relevant for describing the Compton scattering cross section beyond the frameworks of the impulse approximation (IA) and the independent particle model (IPM). Here we discuss corrections to the Kohn-Sham energies due to quasi-particle effects in terms of Dyson orbitals and obtain a relatively simple local form of the exchange-correlation energy. Illustrative examples are presented to show the usefulness of our scheme.Comment: 1 figure, 4 page

The Positronium state in quartz

The positronium state in quartz is described by a linear superposition of two states: the first describing the free positron in the crystal and the second corresponding to a positronium Bloch wavefunction in the lattice. The condition for positronium formation in the electron gas is deduced by using variational calculations of the positron binding energy to the electron system. The self annihilation parameter $\kappa$ introduced in positron lifetime experiments can be properly justified by using the mixed state. A variational method to calculate $\kappa$ is proposed.Comment: 3 pages, 1 figure, to appear in Physica Status Solidi (c), presented at the 15th International Conference of Positron Annihilation ICPA-1

The importance of local band effects for ferromagnetism in hole doped La2_2CuO4_4

Band calculations for supercells of La$_{(2-x)}$Ba$_x$CuO$_4$ show that the rigid band model for doping is less adequate than what is commonly assumed. In particular, weak ferromagnetism (FM) can appear locally around clusters of high Ba concentration. The clustering is important at large dilution and averaged models for magnetism, such as the virtual crystal approximation, are unable to stabilize magnetic moments. These results give a support to the idea that weak FM can be the cause of the destruction of superconductivity at high hole doping.Comment: 4 pages, 5 figures, accepted for publication in Physical Review Letter

How the hydrogen bond in NH4_4F is revealed with Compton scattering

In order to probe electron wave functions involved in the bonding of NH$_4$F, we have performed Compton scattering experiments in an oriented single crystal and in a powder. Ab initio calculations of the Compton profiles for NH$_4$F and NH$_4$Cl are used to enlighten the nature of the bonds in the NH$_4$F crystal. As a consequence, we are able to show significant charge transfer in the ammonium ion which is not observable using other methods. Our study provides a compelling proof for hydrogen bond formation in NH$_4$F.Comment: 4 pages, 5 figures, accepted for publication as a Regular Article in Physical Review