Sound modes broadening for Fibonacci one dimensional quasicrystals

We investigate vibrational excitation broadening in one dimensional Fibonacci model of quasicrystals (QCs). The chain is constructed from particles with two masses following the Fibonacci inflation rule. The eigenmode spectrum depends crucially on the mass ratio. We calculate the eigenstates and eigenfunctions. All calculations performed self-consistently within the regular expansion over the three wave coupling constant. The approach can be extended to three dimensional systems. We find that in the intermediate range of mode coupling constants, three-wave broadening for the both types of systems (1D Fibonacci and 3D QCs) depends universally on frequency. Our general qualitative conclusion is that for a system with a non-simple elementary cell phonon spectrum broadening is always larger than for a system with a primitive cell (provided all other characteristics are the same).Comment: 2o pages, 15 figure

Ab Initio Molecular Dynamics Simulation of Liquid Ga_xAs_{1-x} Alloys

We report the results of ab initio molecular dynamics simulations of liquid Ga_xAs_{1-x} alloys at five different concentrations, at a temperature of 1600 K, just above the melting point of GaAs. The liquid is predicted to be metallic at all concentrations between x = 0.2 and x = 0.8, with a weak resistivity maximum near x = 0.5, consistent with the Faber-Ziman expression. The electronic density of states is finite at the Fermi energy for all concentrations; there is, however, a significant pseudogap especially in the As-rich samples. The Ga-rich density of states more closely resembles that of a free-electron metal. The partial structure factors show only a weak indication of chemical short-range order. There is also some residue of the covalent bonding found in the solid, which shows up in the bond-angle distribution functions of the liquid state. Finally, the atomic diffusion coefficients at 1600K are calculated to be 2.1 \times 10^{-4} cm^2/sec for Ga ions in Ga_{0.8}As_{0.2} and 1.7 \times 10^{-4} cm^2/sec for As ions in Ga_{0.2}As_{0.8}.Comment: 29 pages, 10 eps figures, accepted for publication in Phys. Rev.

Double dynamical regime of confined water

The Van Hove self correlation function of water confined in a silica pore is calculated from Molecular Dynamics trajectories upon supercooling. At long time in the $\alpha$ relaxation region we found that the behaviour of the real space time dependent correlators can be decomposed in a very slow, almost frozen, dynamics due to the bound water close to the substrate and a faster dynamics of the free water which resides far from the confining surface. For free water we confirm the evidences of an approach to a crossover mode coupling transition, previously found in Q space. In the short time region we found that the two dynamical regimes are overimposed and cannot be distinguished. This shows that the interplay between the slower and the faster dynamics emerges in going from early times to the $\alpha$ relaxation region, where a layer analysis of the dynamical properties can be performed.Comment: 6 pages with 9 figures. RevTeX. Accepted for pulbication in J. Phys. Cond. Mat

String-like Clusters and Cooperative Motion in a Model Glass-Forming Liquid

A large-scale molecular dynamics simulation is performed on a glass-forming Lennard-Jones mixture to determine the nature of dynamical heterogeneities which arise in this model fragile liquid. We observe that the most mobile particles exhibit a cooperative motion in the form of string-like paths (``strings'') whose mean length and radius of gyration increase as the liquid is cooled. The length distribution of the strings is found to be similar to that expected for the equilibrium polymerization of linear polymer chains.Comment: 6 pages of RevTex, 6 postscript figures, uses epsf.st

A Simple Model of Liquid-liquid Phase Transitions

In recent years, a second fluid-fluid phase transition has been reported in several materials at pressures far above the usual liquid-gas phase transition. In this paper, we introduce a new model of this behavior based on the Lennard-Jones interaction with a modification to mimic the different kinds of short-range orientational order in complex materials. We have done Monte Carlo studies of this model that clearly demonstrate the existence of a second first-order fluid-fluid phase transition between high- and low-density liquid phases

Composition Dependence of the Structure and Electronic Properties of Liquid Ga-Se Alloys Studied by Ab Initio Molecular Dynamics Simulation

Ab initio molecular dynamics simulation is used to study the structure and electronic properties of the liquid Ga-Se system at the three compositions Ga$_2$Se, GaSe and Ga$_2$Se$_3$, and of the GaSe and Ga$_2$Se$_3$ crystals. The calculated equilibrium structure of GaSe crystal agrees well with available experimental data. The neutron-weighted liquid structure factors calculated from the simulations are in reasonable agreement with recent neutron diffraction measurements. Simulation results for the partial radial distribution functions show that the liquid structure is closely related to that of the crystals. A close similarity between solid and liquid is also found for the electronic density of states and charge density. The calculated electronic conductivity decreases strongly with increasing Se content, in accord with experimental measurements.Comment: REVTeX, 8 pages and 12 uuencoded PostScript figures, submitted to Phys. Rev. B. corresponding author: [email protected]

The Structure, Dynamics and Electronic Structure of Liquid Ag-Se Alloys Investigated by Ab Initio Simulation

Ab initio molecular-dynamics simulations have been used to investigate the structure, dynamics and electronic properties of the liquid alloy Ag(1-x)Se(x) at 1350 K and at the three compositions x=0.33, 0.42 and 0.65. The calculations are based on density-functional theory in the local density approximation and on the pseudopotential plane-wave method. The reliability of the simulations is confirmed by detailed comparisons with very recent neutron diffraction results for the partial structure factors and radial distribution functions (RDF) of the stoichiometric liquid Ag2Se. The simulations show a dramatic change of the Se-Se RDF with increasing Se content. This change is due to the formation of Se clusters bound by covalent bonds, the Se-Se bond length being almost the same as in pure c-Se and l-Se. The clusters are predominantly chain-like, but for higher x a large fraction of 3-fold coordinated Se atoms is also found. It is shown that the equilibrium fractions of Se present as isolated atoms and in clusters can be understood on a simple charge-balance model based on an ionic interpretation. The Ag and Se diffusion coefficients both increase with Se content, in spite of the Se clustering. An analysis of the Se-Se bond dynamics reveals surprisingly short bond lifetimes of less than 1 ps. The changes in the density of states with composition arise directly from the formation of Se-Se covalent bonds. Results for the electronic conductivity obtained using the Kubo-Greenwood approximation are in adequate agreement with experiment for l-Ag2Se, but not for the high Se contents. Possible reasons for this are discussed.Comment: 14 pages, Revtex, 14 Postscript figures embedded in the tex

Supercooled confined water and the Mode Coupling crossover temperature

We present a Molecular Dynamics study of the single particle dynamics of supercooled water confined in a silica pore. Two dynamical regimes are found: close to the hydrophilic substrate molecules are below the Mode Coupling crossover temperature, $T_C$, already at ambient temperature. The water closer to the center of the pore (free water) approaches upon supercooling $T_C$ as predicted by Mode Coupling Theories. For free water the crossover temperature and crossover exponent $\gamma$ are extracted from power-law fits to both the diffusion coefficient and the relaxation time of the late $\alpha$ region.Comment: To be published, Phys. Rev. Lett., 4 pages, 3 figures, revTeX, minor changes in the figures, references added, changes in the tex

Surface Structure of Liquid Metals and the Effect of Capillary Waves: X-ray Studies on Liquid Indium

We report x-ray reflectivity (XR) and small angle off-specular diffuse scattering (DS) measurements from the surface of liquid Indium close to its melting point of $156^\circ$C. From the XR measurements we extract the surface structure factor convolved with fluctuations in the height of the liquid surface. We present a model to describe DS that takes into account the surface structure factor, thermally excited capillary waves and the experimental resolution. The experimentally determined DS follows this model with no adjustable parameters, allowing the surface structure factor to be deconvolved from the thermally excited height fluctuations. The resulting local electron density profile displays exponentially decaying surface induced layering similar to that previously reported for Ga and Hg. We compare the details of the local electron density profiles of liquid In, which is a nearly free electron metal, and liquid Ga, which is considerably more covalent and shows directional bonding in the melt. The oscillatory density profiles have comparable amplitudes in both metals, but surface layering decays over a length scale of $3.5\pm 0.6$ \AA for In and $5.5\pm 0.4$ \AA for Ga. Upon controlled exposure to oxygen, no oxide monolayer is formed on the liquid In surface, unlike the passivating film formed on liquid Gallium.Comment: 9 pages, 5 figures; submitted to Phys. Rev.

Metastable liquid-liquid phase transition in a single-component system with only one crystal phase and no density anomaly

We investigate the phase behavior of a single-component system in 3 dimensions with spherically-symmetric, pairwise-additive, soft-core interactions with an attractive well at a long distance, a repulsive soft-core shoulder at an intermediate distance, and a hard-core repulsion at a short distance, similar to potentials used to describe liquid systems such as colloids, protein solutions, or liquid metals. We showed [Nature {\bf 409}, 692 (2001)] that, even with no evidences of the density anomaly, the phase diagram has two first-order fluid-fluid phase transitions, one ending in a gas--low-density liquid (LDL) critical point, and the other in a gas--high-density liquid (HDL) critical point, with a LDL-HDL phase transition at low temperatures. Here we use integral equation calculations to explore the 3-parameter space of the soft-core potential and we perform molecular dynamics simulations in the interesting region of parameters. For the equilibrium phase diagram we analyze the structure of the crystal phase and find that, within the considered range of densities, the structure is independent of the density. Then, we analyze in detail the fluid metastable phases and, by explicit thermodynamic calculation in the supercooled phase, we show the absence of the density anomaly. We suggest that this absence is related to the presence of only one stable crystal structure.Comment: 15 pages, 21 figure