Atomistic models of hydrogenated amorphous silicon nitride from first principles

We present a theoretical study of hydrogenated amorphous silicon nitride (a-SiNx:H), with equal concentrations of Si and N atoms (x=1), for two considerably different densities (2.0 and 3.0 g/cm3). Densities and hydrogen concentration were chosen according to experimental data. Using first-principles molecular-dynamics within density-functional theory the models were generated by cooling from the liquid. Where both models have a short-range order resembling that of crystalline Si3N4 because of their different densities and hydrogen concentrations they show marked differences at longer length scales. The low-density nitride forms a percolating network of voids with the internal surfaces passivated by hydrogen. Although some voids are still present for the high-density nitride, this material has a much denser and uniform space filling. The structure factors reveal some tendency for the nonstoichiometric high-density nitride to phase separate into nitrogen rich and poor areas. For our slowest cooling rate (0.023 K/fs) we obtain models with a modest number of defect states, where the low (high) density nitride favors undercoordinated (overcoordinated) defects. Analysis of the structural defects and electronic density of states shows that there is no direct one-to-one correspondence between the structural defects and states in the gap. There are several structural defects that do not contribute to in-gap states and there are in-gap states that do only have little to no contributions from (atoms in) structural defects. Finally an estimation of the size and cooling rate effects on the amorphous network is reported.

Conduction bands and invariant energy gaps in alkali bromides

Electronic structure calculations of the alkali bromides LiBr, NaBr, KBr, RbBr and CsBr are reported. It is shown that the conduction band has primarily bromine character. The size of the band gaps of bromides and alkali halides in general is reinterpreted

Bulk and surface electronic structure of 1T-TiS2 and 1T-TiSe2

Ab initio band-structure calculations were performed for bulk, single slab, and thin films of TiX2 (X=S, Se) using the localized spherical wave method. According to these calculations, bulk TiS2 and TiSe2 are semimetallic. The calculations show that TiS2 thin films are semiconductors, but thin films of TiSe2 are semimetallic. The indirect gap for single slab TiS2 is about 1.0 eV, and the gap becomes smaller with increasing number of layers. When the number of layers increases to 11, the TiS2 thin films are semimetallic. All but the surface layers are found to be electrically neutral. The density of states as a function of the energy for the surface layer is different from that of the bulk. The Madelung energy of the Ti atoms on the surface is about 0.35 eV lower than that for the Ti atoms in the bulk. The calculations are compared with photoemission spectra, reported in the literature

Ab initio electronic-structure calculations on the Nb/Cu multilayer system

Ab initio electronic-structure calculations are reported for coherent and incoherent Nb/Cu multilayem. An incoherent unit cell describing three Nb BCC (110) layers and three Cu FCC (111) layers is constructed for the layers in the Nishiyama- Wasserman orientation and with relaxed atomic positions at the interface. It is found that the total density of states is a combination of the broadened DOS curves of the parent metals and that at the interface Nb has a decreased, and Cu has an increased, density of states at the Fermi energy. These results are in agreement with experimetital results and for Nb can be explained by a broadening of the density of states. Possible coherency for small modulation wavelengths is investigated by calculating the total energies for overall BCC [110] and overall FCC [111] Nb/Cu multilayers consisting of one layer of each metal and comparing these with the total energy results of the incoherent structure. The positive interface energy found for the Nb/Cu system favours the incoherent multilayer over the coherent multilayers, where a positive structural energy is also involved

Reversed spin polarization at the Co(001)-HfO2(001) interface

Ab initio electronic-structure calculations on the Co(001)-HfO2(001) interface are reported. The spin polarization of conduction electrons is positive at the interface, i.e., it is reversed with respect to the spin polarization in bulk Co. The electronic structure is very sensitive to the interface structure; without atomic relaxations the reversed spin polarization is not found. The possible relation with spin-polarized tunneling and magnetoresistance is discussed

UAl2:Fine structure of the f bands

The electronic structure of the C15, or cubic-Laves-phase material, UAl2 has been calculated using the linearized relativistic augmented-plane-wave method. The anomalous behavior of the electrical resistivity, specific heat, and magnetic susceptibility can be explained by the fine structure of the density of states near the Fermi energy alone, without the necessity of the introduction of drastic spin fluctuations or many-body effects

Meta-analysis: A Tool for Upgrading Inputs of Macroeconomic Policy Models

Meta-analysis is a research method to synthesise previously obtained research results. It is best seen as a statistical approach towards reviewing and summarising the literature. This paper aims to introduce and critically review the research method of meta-analysis and to illustrate its potential use in applied economic policy analysis. Special attention will be paid to the possibilities for value transfer and the possibilities to improve the calibration of existing macroeconomic policy models

Analyseer de analyses

Meta-analyse wordt in toenemende mate toegepast binnen de economische wetenschap. Er is op veel onderzoeksterreinen echter nog een wereld te winnen