Modeling the polymorphism of pentacene

Thin films of pentacene are known to crystallize in at least four different polymorphs. All polymorphs are layered structures that are characterized by their interlayer spacing d(001). We develop a model that rationalizes the size of the interlayer spacing in terms of intralayer shifts of the pentacene molecules along their long molecular axes. It explains the wide variety of interlayer spacings, without distorting the herringbone pattern that is characteristic of many acenes. Using two simple theoretical models, we attempt to relate the intralayer shifts with the dominant, although weak, interatomic interactions (van der Waals, weak electrostatic, and covalent). For two polymorphs, a consistent picture is found. A full understanding of the other two, substrate-induced, polymorphs probably requires consideration of interlayer interactions

Defects in half-metals and finite temperature

The influence of intrinsic defects in half-metals is calculated in the case of NiMnSb. Of the 14 cases of intrinsic defects, five affect the half-metallic properties. They are energetically very unlikely to occur. Circumstances are discussed under which defects may even have a beneficial effect on the spin polarization of the conduction electrons. Non-intrinsic defects, like deliberate doping by rare-earth atoms, as well as the effect of nano-structured contacts may influence the magnon spectrum, improving the behaviour at finite temperature.</p

Anionogenic Ferromagnets

Magnetism in molecules and solids is understood to originate from atoms in that part of the periodic table where a particular value of the angular momentum appears first (i.e., the 2p, 3d, and 4f series). In contrast to the many magnetic compounds containing transition metal or lanthanide atoms, ferromagnetism based on atoms from the 2p series is very rare. We report density functional calculations that show the existing compound rubidium sesquioxide is a ferromagnet with an estimated Curie temperature of 300 K, unprecedented in p-electron magnetism. The magnetic moment is carried by the anion. Rubidium sesquioxide is a conductor, but only for the minority spin electrons (a so-called “half-metal”). Half-metals play an important role in spintronics, that is, electronics that exploits the electron spin. Since the magnetic moment resides on a light element (oxygen), spin-orbit interactions are considerably reduced compared to other half-metals. Consequently spin relaxation is expected to be suppressed by up to 2 orders of magnitude in comparison with materials presently used in spintronics.

Transport coefficients of liquids from first principles

The use of first-principles molecular dynamics to calculate the viscosity of liquid metals using the Green-Kubo relations is described. The first-principles techniques are based on density functional theory the pseudopotential approximation, and plane-wave basis sets. The statistical-mechanical basis of the Green-Kubo relations is summarised, and extensive first-principles molecular dynamics simulations of liquid aluminium are presented to demonstrate that the method works in practice. Calculated viscosity results are reported for two important systems: liquid iron at Earth's core conditions, and liquid selenium at states on the liquid-vapour curve. The significance of the viscosity results for an understanding of these systems is discussed. (C) 1999 Elsevier Science B.V. All rights reserved

First-principles Molecular-Dynamics Simulation of Liquid Li12Si7

We have studied the structural, dynamical, and electronic properties of liquid Li12Si7 by means of first-principles molecular-dynamics simulations. We find that the Si atoms give rise to a covalently bonded network that can be described as originating from interconnected short chains and stars. The elements of the crystal structure that survive in the liquid state are the preferential twofold Si coordination and the angle between Si bonds. The structure factor is in overall agreement with neutron-scattering data, except for a slight nonuniform shift of peak positions. Electronic-structure calculations show that the Fermi energy is in a minimum of the electronic density of states. The resulting dc conductivity is in good agreement with the resistivity measurements by Meyer et al. Our results appear consistent with the extended Zintl principle recently proposed by Hafner and Jank

Interfacial charge transfer and Schottky barriers at c-Si/a-In heterojunctions

Metal-Semiconductor (M/S) heterojunctions, better known as Schottky junctions play a crucial role in modern electronics. At present, the mechanisms behind the M/S junctions are still a subject of discussion. In this work, we investigate the interfaces between semiconducting crystalline Si and amorphous metallic indium, Si{0 0 1}/a-In and Si{1 1 1}/a-In using both ab initio molecular dynamics simulations and a Schottky-Mott approach. The simulations reveal the formation of a distinct border between the Si substrates and amorphous In at the interfaces. The In atoms adjacent to the interfaces exhibit atomic ordering. Charge transfer occurs from In to Si, forming c-Si−q/a-In+q charge barriers at the interfaces. This indicates that a crystalline p-Si/a-In heterojunction will have rectifying properties, which agrees with an analysis using the Schottky-Mott model which predicts a Schottky barrier height of 1.3 eV for crystalline p-Si/a-In using the calculated work function for a-In (3.82 eV). We further discuss the interfacial charge transfer, related hole-depletion regions in Si adjacent to the interfaces and the Schottky-Mott approximations

First-principles calculation of the phonon spectrum of MgAl2O4 spinel

The phonon spectrum of a MgAl2O4 spinel has been calculated from first principles using density-functional theory. The spinel was perfectly ordered with space group Fd3m. The five Raman active and four infrared active modes allowed by symmetry are umambiguously identified. Agreement with available Raman, infrared, and inelastic neuron scattering dat is good. The fifth Raman mode (missing in experiment) is located at 570 cm-1

Electronic structure of Li12Si7

Ab initio localized-spherical-wave calculations on crystalline Li12Si7 are reported. The crystal consists of two one-dimensional units, (Li6Si5)(infinity) and (Li12Si4)(infinity), that contain five-membered Si rings and four-membered Si stars, respectively. In the density of states the region below -5 eV is largely determined by the local ringlike and starlike arrangements of the Si atoms; above -5 eV there is a signature of delocalization of states across the subunits. Additional calculations on the separate units show that Li12Si7 can be described as [Li12Si4](4+)[Li6Si5](2-)(2) where the charge transfer places the Fermi energy in the gaps of the aromatic subunits. A simple molecular orbital model is presented in order to rationalize the occurrence of these gaps. A short comparison to the theoretical density of states of liquid Li12Si7 is also presented