Showing papers by "Paolo Giannozzi published in 2000"

Theory of the metal-nonmagnetic Mott-Jahn-Teller insulator transition in A 4 C 60

Abstract: We study the unconventional insulating state in ${A}_{4}{\mathrm{C}}_{60}$ with a variety of approaches, including density-functional calculations and dynamical mean-field theory. While the former predicts a metallic state, in disagreement with experiment, the latter yields a (nonmagnetic) Mott-Jahn-Teller insulator. In that state, conduction between molecules is blocked by on-site Coulomb repulsion, magnetism is suppressed by the intramolecular Jahn-Teller effect, and important excitations (such as optical and spin gap) should be essentially intramolecular. Experimental gaps of 0.5 eV and 0.1 eV, respectively, compare well with molecular ion values, in agreement with this picture.

Effects of strain and local charge on the formation of deep defects in III-V ternary alloys

Abstract: The effects of external and internal strains and of defect charges on the formation of gallium vacancies and arsenic antisites in GaAs and ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ have been investigated by ab initio density functional methods. Present results show that a proper understanding of strain and defect charge permits the development of a defect engineering of semiconductors. Specifically, they predict that arsenic antisites in InGaAs ternary alloys can form, upon $p$-type doping in the presence of an arsenic overpressure, even in the case of high-temperature epitaxial growths.

Microscopic structure of the substitutional Al defect in α quartz

Abstract: First-principles pseudopotential calculations are reported for the lattice distortion and electronic properties of the Al substitutional defect in $\ensuremath{\alpha}$ quartz. We determine microscopical properties of the center such as Al coordination, symmetry of the distorted lattice and defect-induced electronic states. The localization properties of the electronic spin density of this paramagnetic color center are investigated. Our results show that the spin density is evenly distributed on the four oxygen nearest neighbors to Al in contrast to phenomenological model results.

Structural and Electronic Properties of C60 and C60 Derivatives in the Solid Phases: Calculations Based on Density-Functional Theory

Abstract: Calculations based on density-functional theory have played (and continue to play) an important role in the characterization of fullerene-based materials. We discuss here several of these approaches applied to various metal fullenrides, and also present still unpublished results of our own calculations. In a number of cases we find that different calculations often result in different physical pictures for the same compound. Through specific comparison of the computational models and schemes, we provide the possibility to evaluate their validity. In addition, comparison with experiment clarifies to what extent major issues are still unresolved about the effects on doping in general and about the electronic behavior of fullendes and its relation with their structural properties in particular.