Local Density Model Research Articles

Energy loss and straggling accompanied by charge exchange

Energy loss and straggling of heavy ions have been studied. The conventional target excitation contributions are estimated on the basis of the dielectric function method together with the local electron density model. In addition, electron capture contributions are estimated in a classical atomic picture, which are expressed by simple analytical forms. The average charge of traversing ions is determined by an empirical formula. Electron capture contributions becomes large and comparable at relatively low velocities, where they are proportional to the average charge of ions. At high velocities, on the other hand, these contributions are negligible. Charge exchange processes are found to contribute to the slowing-down as corrections.

Molecular dynamics of flow in micropores

The method of nonequilibrium molecular dynamics is used to study the viscosity and flow properties of strongly inhomogeneous liquids, a particular case of which is a liquid confined in a micropore only a few molecular diameters wide. Fluid inhomogeneity is introduced by imposing an external potential that in one case simulates flat solid walls and in the other case causes density peaks in the middle of a thin liquid film. For comparison a homogeneous fluid is also simulated. In both types of inhomogeneous fluid, the shear stress and effective viscosity are smaller than in the homogeneous fluid. The density profiles and the diffusivities in the micropore were found to be independent of flow, even at the extremely high rates, 1010–1011 s−1 of the simulation. The Green–Kubo relation is found to be valid for the diffusivity under the flow studied. We propose a local average density model (LADM) of viscosity and diffusivity, in which the local transport coefficients are those of homogeneous fluid at a mean density obtained by averaging the local density over a molecular volume. LADM predicts qualitatively correct velocity profiles, effective viscosities, and shear stresses using only equilibrium density profiles and molecular diameters. An analogous local equilibrium version of Enskog’s theory of diffusivity agrees well with the simulated pore diffusivities. Recently Vanderlick and Davis generalized Enskog’s theory of diffusivity to strongly inhomogeneous fluids. Their theoretical pore diffusion coefficient is also in good agreement with simulation results.

Many-electron model of equilibrium metal-semiconductor contacts and semiconductor heterojunctions.

A many-electron model is proposed to describe the electronic structure of metal-semiconductor interfaces and semiconductor heterojunctions. The model is utilized to calculate the self-consistent one-electron potential in the vicinity of the interface. This potential arises from two sources: a short-range dipole contribution due to valence-electron rearrangements at the interface relative to the corresponding vacuum surfaces, and a long-range space-charge contribution caused by band-bending effects extending thousands of angstroms from the interface.The short-range, valence-electron dipole contribution is evaluated by minimizing the interface energy calculated within a local-density description of the valence-electron charge density. The long-range space-charge contribution is determined both by the valence-electron rearrangements, being included in the interface energy through an electrostatic energy term, and by the thermodynamic equilibrium boundary conditions. The boundary conditions ensuring thermal, mechanical, and electron-transfer equilibrium are imposed explicitly. The chemical potentials required to satisfy the electron-transfer boundary condition are obtained from the local-density model at the constant temperature and pressure characteristic of the equilibrium interface.For numerical calculations the ion-core charges are approximated by a uniform positive charge density. The interfaces between the metal and semiconductor as well as the metal-vacuum and semiconductor-vacuum surfaces are defined by abrupt discontinuities in this uniform positive charge density. A version of local-density theory suitable for both metals and semiconductors is utilized to describe the valence-electron charge densities and is extended to incor- porate the dopants in the semiconductor as well. Consequently, a unified model of both the microscopic interface dipole and macroscopic space-charge region is achieved. Numerical calculations reveal that the microscopic interface dipole is independent of the doping in the space-charge region for dopant densities N\ensuremath{\lesssim}${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$.These calculations further predict the validity of Schottky's phenomenological boundary condition for metal-semiconductor contacts, the applicability of the electron-affinity rule for semiconductor heterojunctions, and systematic correlations between Schottky-barrier heights and heterojunction band offsets. The latter three results emerge as consequences of the rigorous and complete imposition of thermodynamic equilibrium boundary conditions and of the use of a model of the ion-core charge density in which atomic reconstructions at the interface relative to the vacuum surfaces are not considered explicitly.

On first-row diatomic molecules and local density models

The total Xα energy accurate to 0.3 eV is computed for H2, B2, C2, N2, O2, CO, and F2. Relative to experiment, the Xα model (α=0.7) is accurate to within ΔRe=0.1 bohr, ΔDe=2 eV, and Δωe=300 cm−1 for these molecules. Except for the lightest first-row diatomic molecules, the Xα and experimental dissociation energies are bracketed by those of the Hartree–Fock model (from below) and the Local Spin Density model (from above).

Non-exponential range of localized electron states in liquid chains

The range of electron states near a band edge in liquid chains has been found to depend, not on an exponential decay factor as previously believed, but on the `beat period' of a related Bloch function. This finding affects extension of the work to real liquids, and the use of such approximative methods as the local density model.