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.
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.
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