A strong adsorption of the water molecules to the pyrite surface is shown by a molecular dynamic simulation of the water-iron pyrite FeS2 interface. Water molecules closest to the pyrite surface are bound by an electrostatic interaction to the iron atoms in grooves running parallel to one of the crystal axes. The grooves are about two atoms wide and are directed along 010 for the (001) surface. The position of the water-surface potential minimum and the energy of adsorption were determined by optimization for a single water molecule at the interface. At room temperature and normal density there are altogether three distinguishable layers of water above the surface. One is associated with the groove: one with H bonding to the sulphur atoms comprising the ridges separating the grooves, and the third with the soft wall boundary between the absorbed water layers and bulk region of water. Simulations were also used to explore the effect of a temperature range significant for geophysical studies.

No abstract

No abstract

The effects of hydrostatic pressure on the electronic band structure of the semiconductor mineral iron pyrite FeS2 have been investigated theoretically by an ab initio full-potential linearized-augmented plane wave (FPLAPW) method within a local approximation (LDA/GGA) to the density functional theory. The calculations predict that at a pressure of 94.1 GPa the indirect band gap of pyrite FeS2 vanishes and the material becomes a metal. This is due to the presence of the S–S and Fe–S bonds, which provide novel energy band distortions in the process of attaining the metallic state. Analysis indicates that, under increasing high pressure, the conduction bands (3pz of sulfur and 3dx2−y2+3dxy of iron) intrude downwards into the valence bands, which are predominantly 3d in nature. At normal pressure, the lattice constant, the bulk modulus, sulfur position parameter u, S–S bond length, and the indirect band gap of pyrite FeS2 are calculated using a fully relaxed unit cell and found to be equal to 541.8 pm, 159.7 GPa, u = 0.383, 219.5 pm and 0.45 eV, respectively. Apart from the gap, which is too small (the usual ‘LDA error’), these results agree well with recent experiments. The effective masses of an electron at selected points in the conduction band are reported.

The condensation of indirect excitons in double quantum wells is studied in an electric field created by electrodes of different shapes. The finite value of the exciton lifetime, the pumping and nonuniformity of the electric field under the electrode are taken into account. It is shown that islands of exciton condensed phase emerge under electrodes when the pumping exceeds a certain threshold value. They appear first under the rim where the potential energy of excitons has a dip. Calculations predict a complicated evolution of the exciton density distribution: from the gaseous phase at low laser intensities to the condensed phase in the whole area under the electrode at larger intensities. Therefore, the configurations of the exciton condensed phase may be manipulated by choosing the setups with conductive electrodes of different shapes via forming specific potentials of the electrical field and controlled by the level of the laser irradiation.

No abstract

Energy spectra of the size quantization of excitons in Xe, Kr, Ar and Ne precipitates in Al and their manifestation in reflection spectra are calculated taking into account polariton effects (dipole-dipole exciton interactions), spatial dispersion and mixing of the electronic excitations of the inclusion with collective excitations of the surrounding metal (plasmons). It was shown that: 1) the proximity of energies of the exciton levels and the plasmons localized on the inclusion (surface plasmons) results in the gigantic shift (up to 1 eV) of the levels of coupled excitations, 2) the transfer of the oscillator strength from the plasmon level to the exciton levels leads to the amplification of optical transitions in bubbles by several orders of magnitude. Depending on the position of the surface polariton with respect to the surface plasmon the spectra either show additional broadenning due to the coupling with quantized exciton levels (Ar, Ne) or the polariton level is pushed out of the exciton band and manifests itself as a narrow dip in the reflection spectra.

For the models of a number of polar crystals the theoretical studies of the effect of LO-phonon dispersion on frequency and temperature dependence of the exciton attenuation is carried out. It is shown that, with increasing dispersion, the edge of the phonon emission processes shifts to the long-wave side of the spectrum. For those frequencies higher than corresponding to the exciton band bottom this results in the nonzero attenuation even for zero temperature. Phonon dispersion mainly increases attenuation but it is noticed that for low temperature there exists a temperature range where attenuation is decreased with the growth of dispersion. Frequency dependence of the attenuation in a wide interval of exciton radii from 10 up to 200 Å is illustrated.