Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Bose-Einstein condensation in a gas of sodium atoms

We review work on In2O3:Sn films prepared by reactive e‐beam evaporation of In2O3 with up to 9 mol % SnO2 onto heated glass. These films have excellent spectrally selective properties when the deposition rate is ∼0.2 nm/s, the substrate temperature is ≳150 °C, and the oxygen pressure is ∼5×10−4 Torr. Optimized coatings have crystallite dimensions ≳50 nm and a C‐type rare‐earth oxide structure. We cover electromagnetic properties as recorded by spectrophotometry in the 0.2–50‐μm range, by X‐band microwave reflectance, and by dc electrical measurements. Hall‐effect data are included. An increase of the Sn content is shown to have several important effects: the semiconductor band gap is shifted towards the ultraviolet, the luminous transmittance remains high, the infrared reflectance increases to a high value beyond a certain wavelength which shifts towards the visible, phonon‐induced infrared absorption bands vanish, the microwave reflectance goes up, and the dc resisitivity drops to ∼2×10−4 Ω cm. The corre...

Evaporated Sn‐doped In2O3 films: Basic optical properties and applications to energy‐efficient windows

The possibilities of the Hohenberg-Kohn-Sham local density theory are explored in view of recent advances in the theory of the interacting electron gas. The authors discuss and provide numerical data for the effective exchange-correlation potentials mu xc for ground state problems and Vxc for excitation spectra. The potential mu xc(r) is written as a factor beta ( rho (r)), which depends on the local density rho (r), times the Kohn-Sham-Gaspar potential mu x=-e2kF/ pi . The factor beta , which is represented by a simple analytical formula, varies between 1 for very high densities and about 1.5 for very low densities, where mu xc thus approaches the Slater approximation. The potential Vxc is given in the form of a table. The explicit predictions of the approximate theories are mapped in their linearized versions. The theory is also developed for the case when the ion cores are treated separately, and the local density approximation is applied only to the valence electrons.

http://iopscience.iop.org/article/10.1088/0022-3719/4/14/022/pdf

Explicit local exchange-correlation potentials

The General Utility Lattice Program (GULP) has been extended to include the ability to simulate polymers and surfaces, as well as adding many other new features, and the current status of the program is fully documented. Both the background theory is described, as well as providing a concise review of some of the previous applications in order to demonstrate the range of its use. Examples are presented of work performed using the new compatibilities of the software, including the calculation of Born effective charges, mechanical properties as a function of applied pressure, calculation of frequency-dependent dielectric data, surface reconstructions of calcite and the performance of a linear-scaling algorithm for bond-order potentials.

The general utility lattice program (GULP)

Structure and nanostructure in ionic liquids.

Publisher Summary In an idealized picture, an ionic crystal is regarded as composed of spherical, nonoverlapping ions, bearing net charges of integral amount. The electrostatic interactions of these pointlike charges give rise to a net binding (Madelung energy), as each ion is preferentially surrounded by ions of opposite charge. In typical ionic crystals, the Madelung energy is in fact quite close to the observed cohesive energy. The Born model was originally proposed for the purpose of evaluating the lattice energy of crystals, which approach this idealized picture, and because of its success and simplicity it has subsequently been applied to the description of a variety of physical properties of ionic crystals, with varying degrees of success. In the framework of the Born model, it is convenient to describe the lattice energy of an ionic crystal as composed of energy terms arising from interactions between the ions. In addition to the net binding provided by the electrostatic interactions of point-like ionic charges, the binding arising from a synchronization of the electronic motions in the ions is considered explicitly in some versions of the model. The contribution of this so-called “van der Waals interaction” to the lattice energy is small, of the order of a few per cent. The ratio between the polarization energy that is connected with the mutually induced electronic dipoles on a pair of ions and the Coulomb interaction energy of the net charges of the ions is definitely less than the ionic polarizability per unit volume. The attractive forces in the crystal are balanced by the so-called overlap repulsive forces, which oppose the interpenetration of the ions.

Cohesion of Ionic Solids in the Born Model

The dielectric function of a degenerate electron gas in the random-phase approximation, and the one proposed by Hubbard, which takes exchange effects into account, have been extensively used in the study of metallic properties. However, both dielectric functions lead to an overestimate of the short-range correlations between particles. This is manifest from the fact that the pair-correlation function is negative for small interparticle separations over the whole range of metallic densities, and implies an overestimate of the correlation energy. An improved expression of the dielectric function is given, which includes explicitly, in an approximate way, the short-range correlations arising from both Coulomb and exchange effects by being a functional of the structure factor. The structure factor and the dielectric function can then be determined in a self-consistent manner. The numerical solution of the self-consistent scheme yields a pair-correlation function which is positive for all values of the density up to ${r}_{s}=4$. For ${r}_{s}g4$, it is very slightly negative at small separations, but it is so small that it can be considered to be zero for all practical purposes. New estimates of the correlation energy are given for the entire metallic density range, and are smaller than the earlier estimates. These results are used to recalculate the cohesive energy of the alkali metals. A discussion of the plasmon dispersion relation, the compressibility, and the liquid-solid transition, both for the electron system and for an astrophysically interesting system of protons over a background of electrons, is also given.

Electron Correlations at Metallic Densities

Modern techniques of liquid state physics have been successfully used to probe the microscopic structure and dynamics of a variety of multicomponent liquids in which relative ordering of the species is present near freezing. The alkali halides are prototypes for this specific type of short-range order in relation to the nature of bonding, but the systems in question also include other monovalent and polyvalent metal-ion halides, alkali-based intermetallic compounds and chalcogen-based alloys. A viewpoint is taken in this review which gives attention to the relations between liquid and solid phase properties across melting for compound systems at stoichiometric composition. In addition, large deviations for stoichiometry can be realised in the liquid phase, to display trends of evolution of structure, bonding and electronic states with composition.

https://iopscience.iop.org/article/10.1088/0034-4885/49/9/002/pdf

Structure and dynamics of molten salts

The crystal structures formed by compounds of strongly electropositive and strongly electronegative elements are described crystallographically in terms of interpenetrating Bravais lattices occupied by the various chemical species. The coexistence of crystalline long-range order and chemical order up to melting can be explained by electronic charge transfer and local compensation of positive and negative ionic charges. An examination of the local structures found in crystalline metal halides shows that charge compensation occurs in two qualitatively distinct ways. The first involves halogen sharing and high coordination numbers for the metal ions; the structure of alkali, alkaline-earth, and lanthanide metal halides are ex­ amples of this type of order. In the second, charge compensation takes place within well-defined molecular units, either monomeric ones, as for example in HgC12 and SbCI3, or dimerie ones, as in AlBr3' Studies of molten salts have generally been based on the assumption that the melting transition, which involves the loss of crystalline long­ range order" largely preserves the type of local order found in the crystal. Determinations of the structure of metal halide meits carried out over the past two decades with neutron diffraction have largely substantiated this

Ordering in Metal Halide Melts

The coupling between plasmons and polar phonons in a degenerate semiconductor is studied starting from an electron-phonon Hamiltonian which is valid in the long-wavelength limit. A truncated form of the Hamiltonian proves to be a good approximation near the crossover point of the uncoupled modes; and expressions for the phonon strengths and the sum rules are very simply derived from it. Plasmon damping is introduced phenomenologically and its effect on the behavior of the coupled modes is investigated. Model calculations of the effect of damping on the dispersion curves and reflectivity are made for the case of the degenerate semiconductor GaSb.

M. P. Tosi

Papers

Cohesion of Ionic Solids in the Born Model

Electron Correlations at Metallic Densities

Structure and dynamics of molten salts

Ordering in Metal Halide Melts

Interaction of Plasmons and Optical Phonons in Degenerate Semiconductors