57 research outputs found
Mixtures of planetary ices at extreme conditions.
The interiors of Neptune and Uranus are believed to be primarily composed of a fluid mixture of methane and water. The mixture is subjected to pressures up to several hundred gigapascal, causing the ionization of water. Laboratory and simulation studies so far have focused on the properties of the individual components. Here we show, using first-principle molecular dynamic simulations, that the properties of the mixed fluid are qualitatively different with respect to those of its components at the same conditions. We observe a pressure-induced softening of the methane-water intermolecular repulsion that points to an enhancement of mixing under extreme conditions. Ionized water causes the progressive ionization of methane and the mixture becomes electronically conductive at milder conditions than pure water, indicating that the planetary magnetic field of Uranus and Neptune may originate at shallower depths than currently assumed
Metallic charge density waves and surface Mott insulators for adlayer structures on semiconductors: extended Hubbard modeling
Motivated by the recent experimental evidence of commensurate surface CDW in
Pb/Ge(111) and Sn/Ge(111) -adlayer structures, as well as by the
insulating states found on K/Si(111):B and SiC(0001), we have investigated the
role of electron-electron interactions, and also of electron-phonon coupling,
on the narrow surface state band originating from the dangling bond orbitals of
the adsorbate. We model the problem by an extended two-dimensional Hubbard
model at half-filling on a triangular lattice. We include an on-site Hubbard
repulsion U and a nearest-neighbor V, plus a long-ranged Coulomb tail. The
electron-phonon interaction is treated in the deformation potential
approximation. We have explored the phase diagram of the model including the
possibility of commensurate 3x3 phases, using mainly the Hartree-Fock
approximation. For U larger than the bandwidth we find magnetic insulators,
possibly corresponding to the situation in SiC and in K/Si. For smaller U, the
inter-site repulsion V can stabilize metallic CDW phases, reminiscent of the
3x3 structures of Sn/Ge, and possibly of Pb/Ge.Comment: 10 pages, 3 figures, presented at ECOSS-17 sept 199
Two state model for critical points and the negative slope of the melting-curve
We present a thermodynamic model which explains the presence of a negative
slope in the melt curve, as observed in systems as diverse as the alkali metals
and molecular hydrogen at high pressure. We assume that components of the
system can be in one of two well defined states - one associated with low
energy, the other with low volume.
The model exhibits a number of measurable features which are also observed in
these systems and are therefore expected to be associated with all negative
Clapeyron-slope systems: first order phase transitions, thermodynamic anomalies
along Widom lines.
The melt curve maximum is a feature of the model, but appears well below the
pressures where the change in state occurs in the solid: the solid-solid
transition is related to the melt line minimum. An example of the model fitted
to the electride transition in potassium is discussed
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