Biomedical subjects

E Tosatti

Publications and source records attributed to E Tosatti.

At least 19 recordsLinked to original sources

PubMed2006

Impact of molecular structure on the lubricant squeeze-out between curved surfaces with long range elasticity.

The properties of butane (C4H10) lubricants confined between two approaching solids are investigated by a model that accounts for the curvature and elastic properties of the solid surfaces. We consider the linear n-butane and the branched isobutane. For the linear molecule, well defined molecular layers develop in the lubricant film when the width is of the order of a few atomic diameters. The branched isobutane forms more disordered structures which permit it to stay liquidlike at smaller surface separations. During squeezing the solvation forces show oscillations corresponding to the width of a molecule. At low speeds (<0.1 ms) the last layers of isobutane are squeezed out before those of n-butane. Since the (interfacial) squeezing velocity in most practical applications is very low when the lubricant layer has molecular thickness, one expects n-butane to be a better boundary lubricant than isobutane. With n-butane possessing a slightly lower viscosity at high pressures, our result refutes the view that squeeze-out should be harder for higher viscosities; on the other hand our results are consistent with wear experiments in which n-butane were shown to protect steel surfaces better than isobutane.

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PubMed2005

Novel electronically driven surface phase predicted in C/Si(111).

We predict a novel electronically driven phase for the recently created C/Si(111) surface at 1/3 monolayer coverage. Whereas the isoelectronic surface Sn/Ge(111) is a 3 x 3 distorted metal and Si/SiC(0001) is an undistorted magnetic Mott insulator, the new phase combines both features. Two of three adatoms in C/Si(111) should form a distorted (3 x 3) honeycomb sublattice, the third an undistorted insulating and magnetic triangular sublattice. The generally conflicting elements, namely, band energy, favoring distortion, and strong electron correlations favoring a Mott state, actually conspire in this case. This kind of state represents the surface analog of the Fazekas-Tosatti state in the charge density wave compound 1T-TaS2.

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PubMed2005

Physics of solid and liquid alkali halide surfaces near the melting point.

This paper presents a broad theoretical and simulation study of the high-temperature behavior of crystalline alkali halide surfaces typified by NaCl(100), of the liquid NaCl surface near freezing, and of the very unusual partial wetting of the solid surface by the melt. Simulations are conducted using two-body rigid-ion Born-Mayer-Huggins-Fumi-Tosi (BMHFT) potentials, with full treatment of long-range Coulomb forces. After a preliminary check of the description of bulk NaCl provided by these potentials, which seems generally good even at the melting point, we carry out a new investigation of solid and liquid surfaces. Solid NaCl(100) is found in this model to be very anharmonic and yet exceptionally stable when hot. It is predicted by a thermodynamic integration calculation of the surface free energy that NaCl(100) should be a well-ordered, nonmelting surface, metastable even well above the melting point. By contrast, the simulated liquid NaCl surface is found to exhibit large thermal fluctuations and no layering order. In spite of that, it is shown to possess a relatively large surface free energy. The latter is traced to a surface entropy deficit, reflecting some kind of surface short-range order. We show that the surface short-range order is most likely caused by the continuous transition of the bulk ionic melt into the vapor, made of NaCl molecules and dimers rather than of single ions. Finally, the solid-liquid interface free energy is derived through Young's equation from direct simulation of partial wetting of NaCl(100) by a liquid droplet. The resulting interface free energy is large, in line with the conspicuous solid-liquid 27% density difference. A partial wetting angle near 50 degrees close to the experimental value of 48 degrees is obtained in the process. It is concluded that three elements, namely, the exceptional anharmonic stability of the solid (100) surface, the molecular short-range order at the liquid surface, and the costly solid-liquid interface, all conspire to cause the anomalously poor wetting of the (100) surface by its own melt in the BMHFT model of NaCl-and most likely also in real alkali halide surfaces.

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PubMed2005

Incommensurability of a confined system under shear.

We study a chain of harmonically interacting atoms confined between two sinusoidal substrate potentials, when the top substrate is driven through an attached spring with a constant velocity. This system is characterized by three inherent length scales and closely related to physical situations with confined lubricant films. We show that, contrary to the standard Frenkel-Kontorova model, the most favorable sliding regime is achieved by choosing chain-substrate incommensurabilities belonging to the class of cubic irrational numbers (e.g., the spiral mean). At large chain stiffness, the well known golden mean incommensurability reveals a very regular time-periodic dynamics with always higher kinetic friction values with respect to the spiral mean case.

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PubMed2005

Why are alkali halide surfaces not wetted by their own melt?

Alkali halide (100) crystal surfaces are anomalous, being very poorly wetted by their own melt at the triple point. We present extensive simulations for NaCl, followed by calculations of the solid-vapor, solid-liquid, and liquid-vapor free energies showing that solid NaCl(100) is a nonmelting surface, and that its full behavior can quantitatively be accounted for within a simple Born-Meyer-Huggins-Fumi-Tosi model potential. The incomplete wetting is traced to the conspiracy of three factors: surface anharmonicities stabilizing the solid surface; a large density jump causing bad liquid-solid adhesion; incipient NaCl molecular correlations destabilizing the liquid surface. The latter is pursued in detail, and it is shown that surface short-range charge order acts to raise the surface tension because incipient NaCl molecular formation anomalously reduces the surface entropy of liquid NaCl much below that of solid NaCl(100).

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PubMed2004

Sealing is at the origin of rubber slipping on wet roads.

Loss of braking power and rubber skidding on a wet road is still an open physics problem, as neither the hydrodynamic effects nor the loss of surface adhesion that are sometimes blamed really manage to explain the 20-30% observed loss of low-speed tyre-road friction. Here we report a novel mechanism based on sealing of water-filled substrate pools by the rubber. The sealed-in water effectively smoothens the substrate, thus reducing the viscoelastic dissipation in bulk rubber induced by surface asperities-well established as a major friction contribution. Starting with the measured spectrum of asperities one can calculate the water-smoothened spectrum and from that the predicted friction reduction, which is of the correct magnitude. The theory is directly supported by fresh tyre-asphalt friction data.

Accidents, Traffic↗

PubMed2004

Trapping of excitons at chemical defects in polyethylene.

In a previous paper we studied an injected electron-hole pair in crystalline polyethylene (PE) and found that the exciton becomes weakly self-trapped in a narrow interchain pocket comprised between two gauche defects. Despite the large energy stored in the trapped excitation, there did not appear to be a direct nonradiative channel for electron-hole recombination. Actual polyethylene systems of practical use are, however, neither crystalline nor pure. To understand the fate of an electron-hole pair in the impure case, we studied by ab initio simulations the evolution of an exciton trapped on three common chemical defects found in polyethylene: a grafted carbonyl (C=O); an intrachain vinyl group (C=C); a grafted carboxyl (COOH). Ab initio simulations lead to predict three different outcomes: trapping, nonradiative recombination, and homolitic bond-breaking, respectively. This suggests that extrinsic self-trapping of electron-hole pairs over chemical defects inside the quasicrystalline fraction of PE could be relevant for electrical damage in high-voltage cables.

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PubMed2004

Magnetism in atomic-size palladium contacts and nanowires.

We have investigated Pd nanowires theoretically, and found that, unlike either metallic or free atomic Pd, they exhibit Hund's rule magnetism. In long, monostrand nanowires, we find a spin moment of 0.7 mu(B) per atom, whereas for short, monostrand nanowires between bulk leads, the predicted moment is about 0.3 mu(B) per nanowire atom. In contrast, a coaxial (6,1) nanowire was found to be nonmagnetic. The origin of the nanowire magnetism is analyzed.

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PubMed2003

Electron-phonon interaction at the Be(0001) surface.

We present a first principle study of the electron-phonon (e-p) interaction at the Be(0001) surface. The real and imaginary parts of the e-p self-energy (Sigma) are calculated for the Gamma; surface state in the binding energy range from the Gamma; point to the Fermi level. Our calculation shows an overall good agreement with several photoemission data measured at high and low temperatures. Additionally, we show that the energy derivative of Re Sigma presents a strong temperature and energy variation close to E(F), making it difficult to measure its value just at E(F).

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PubMed2003

Role of the external pressure on the dewetting of soft interfaces.

We consider dewetting at soft interfaces, e.g., a rubber ball squeezed in a fluid against a flat hard substrate. We show that the applied squeezing pressure, which was not taken into account in earlier studies of the dewetting transition, may have a big effect on the squeeze-out dynamics.

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PubMed2002

Ferroelectricity and isotope effects in hydrogen-bonded KDP crystals.

Based on an accurate first principles description of the energetics in H-bonded potassium-dihydrogen-phosphate crystals, we conduct a first study of nuclear quantum effects and of the changes brought about by deuteration. Tunneling is allowed only for clusters involving correlated protons and heavy ion displacements, the main effect of deuteration being a depletion of the proton probability density at the O-H-O bridge center, which in turn weakens its proton-mediated covalent bonding. The ensuing lattice expansion couples self-consistently with the proton off-centering, thus explaining both the giant isotope effect and its close connection with geometrical effects.

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PubMed2002

Disproportionation phenomena on free and strained Sn/Ge(111) and Sn/Si(111) surfaces.

Distortions of the sqrt[3]x sqrt[3] Sn/Ge(111) and Sn/Si(111) surfaces are shown to reflect a disproportionation of an integer pseudocharge, Q, related to the surface band occupancy. A novel understanding of the (3 x 3)-1U ("1 up, 2 down") and 2U ("2 up, 1 down") distortions of Sn/Ge(111) is obtained by a theoretical study of the phase diagram under strain. Positive strain keeps the unstrained value Q=3 but removes distortions. Negative strain attracts pseudocharge from the valence band causing first a (3 x 3)-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a (sqrt[3] x sqrt[3])-3U ("all up") state with Q=6. The possibility of a fluctuating phase in unstrained Sn/Si(111) is discussed.

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PubMed2002

Strongly correlated superconductivity.

High-temperature superconductivity in doped Mott insulators such as the cuprates contradicts the conventional wisdom that electron repulsion is detrimental to superconductivity. Because doped fullerene conductors are also strongly correlated, the recent discovery of high-critical-temperature, presumably s-wave, superconductivity in C60 field effect devices is even more puzzling. We examine a dynamical mean-field solution of a model for electron-doped fullerenes that shows how strong correlations can indeed enhance superconductivity close to the Mott transition. We argue that the mechanism responsible for this enhancement could be common to a wider class of strongly correlated models, including those for cuprate superconductors.

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PubMed2001

Superconductivity near ferromagnetism in MgCNi3.

An unusual quasi-two-dimensional heavy band mass van Hove singularity (vHs) lies very near the Fermi energy in MgCNi3, recently reported to superconduct at 8.5 K. This compound is strongly exchange enhanced and unstable to ferromagnetism upon hole doping with approximately 12% Mg-->Na or Li (i.e., 0.04 hole/Ni). We identify an essentially infinite mass along the M-Gamma line, which accounts for the two dimensionality of this vHs. This compound provides new opportunities to probe the ferromagnetic critical point as well as introducing the novelties of 2D behavior into a 3D system.

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PubMed2001

Two-stage rotational disordering of a molecular crystal surface: C60.

We propose a two-stage mechanism for the rotational surface disordering phase transition of a molecular crystal, as realized in C60 fullerite. Our study, based on Monte Carlo simulations, uncovers the existence of a new intermediate regime, between a low-temperature ordered (2x2) state, and a high-temperature (1x1) disordered phase. In the intermediate regime there is partial disorder, strongest for a subset of particularly frustrated surface molecules. These concepts and calculations provide a coherent understanding of experimental observations, with possible extension to other molecular crystal surfaces.

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PubMed2001

Surface states and negative electron affinity in polyethylene.

First-principles calculations are used to investigate the electronic properties of the surfaces of polyethylene. The calculations support the experimental evidence of a negative electron affinity, with calculated values of -0.17 eV and -0.10 eV for surfaces with chains perpendicular and parallel to the surface normal, respectively. Both surfaces exhibit a surface state with binding energy -1.2 +/- 0.5 eV with respect to the bulk polyethylene conduction band minimum. Implications of these findings on spectroscopy, as well as on the transport and aging properties of polyethylene for high-voltage applications, are discussed.

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PubMed2001

Direct transition between a singlet Mott insulator and a superconductor.

We argue that a normal Fermi liquid and a singlet, spin-gapped Mott insulator cannot be continuously connected, and that some intermediate phase must intrude between them. By explicitly working out a case study where the singlet insulator is stabilized by orbital degeneracy and an inverted Hund's rule coupling, mimicking a Jahn-Teller effect, we find that the intermediate phase is a superconductor.

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PubMed2001

String tension and stability of magic tip-suspended nanowires.

Multishell helical gold nanowires were recently imaged by electron microscopy. We show theoretically that the contact with the gold tips at either end of the wire plays a crucial role and that local minima in the string tension rather than the total wire free energy determine the nanowire stability. Density functional electronic structure calculations of the simplest and thinnest coaxial gold and silver wires of variable radius and chirality were carried out. We found a string tension minimum for a single-tube gold nanowire that is chiral and consists of seven strands, in striking agreement with observation. In contrast, no such minimum was found for silver, where the s-d competition leading to surface reconstruction is lacking.

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