arXiv subjects

Ph. Ghosez

Publications and source records attributed to Ph. Ghosez.

23 records · Page 2Linked to original sources

arXiv1996

Density Functional Theory of Polar Insulators

We examine the density-functional theory of macroscopic insulators, obtained in the large-cluster limit or under periodic boundary conditions. For polar crystals, we find that the two procedures are not equivalent. In a large-cluster case, the exact exchange-correlation potential acquires a homogeneous ``electric field'' which is absent from the usual local approximations, and the Kohn-Sham electronic system becomes metallic. With periodic boundary conditions, such a field is forbidden, and the polarization deduced from Kohn-Sham wavefunctions is incorrect even if the exact functional is used.

cond-mat.mtrl-sci↗

arXiv1996

Coulomb interaction and ferroelectric instability of BaTiO3

Using first-principles calculations, the phonon frequencies at the $Γ$ point and the dielectric tensor are determined and analysed for the cubic and rhombohedral phases of BaTiO$_{3}$. The dipole-dipole interaction is then separated à la Cochran from the remaining short-range forces, in order to investigate their respective influence on lattice dynamics. This analysis highlights the delicate balance of forces leading to an unstable phonon in the cubic phase and demonstrates the extreme sensitivity of this close compensation to minute effective charge changes. Within our decomposition, the stabilization of the unstable mode in the rhombohedral phase or under isotropic pressure has a different origin.

mtrl-th↗

arXiv1995

A scaling hypothesis for corrections to total energy and stress in plane- wave based ab initio calculations

We present a new technique aimed at preventing plane-wave based total energy and stress calculations from the effect of abrupt changes in basis set size. This s cheme relies on the interpolation of energy as a function of the number of plane waves, and on a scaling hypothesis that allows to perform the interpolation for a unique reference volume. From a theoretical point of view, the new method is compared to those already pr oposed in the literature, and its more rigorous derivation is emphasized. From a practical point of view, we illustrate the importance of the correction o n different materials (Si, BaTiO3, and He) corresponding to different types of b onding, and to different k-point samplings and cut-off energies. Then, we compare the different approaches for the calculation of the lattice par ameter, the bulk modulus, and its derivative versus pressure in bulk silicon.

mtrl-th↗

arXiv1995

Density-Polarization Functional Theory of the response of a periodic insulating solid to an electric field.

The response of an infinite, periodic, insulating, solid to an infinitesimally small electric field is investigated in the framework of Density Functional Theory. We find that the applied perturbing potential is not a unique functional of the periodic density change~: it depends also on the change in the macroscopic {\em polarization}. Moreover, the dependence of the exchange-correlation energy on polarization induces an exchange-correlation electric field. These effects are exhibited for a model semiconductor. We also show that the scissor-operator technique is an approximate way of bypassing this polarization dependence.

mtrl-th↗

arXiv1994

Born Effective Charges of Barium Titanate: band by band decomposition and sensitivity to structural features

The Born effective charge tensors of Barium Titanate have been calculated for each of its 4 phases. Large effective charges of Ti and O, also predicted by shell model calculations and made plausible by a simplified model, reflect the partial covalent character of the chemical bond. A band by band decomposition confirms that orbital hybridization is not restricted to Ti and O atoms but also involves Ba which appears more covalent than generally assumed. Our calculations reveal a strong dependence of the effective charges on the atomic positions contrasting with a relative insensitivity on isotropic volume changes.

mtrl-th↗