Temperature-dependent Potentials Research Articles

Robustness of the quintessence scenario in particle cosmologies

We study the robustness of the quintessence tracking scenario in the context of more general cosmological models that derive from high-energy physics. We consider the effects of the inclusion of multiple scalar fields, corrections to the Hubble expansion law (such as those that arise in brane cosmological models), and potentials that decay with expansion of the universe. We find that in a successful tracking quintessence model the average equation of state must remain nearly constant. Overall, the conditions for successful tracking become more complex in these more general settings. Tracking can become more fragile in the presence of multiple scalar fields, and more stable when temperature-dependent potentials are present. Interestingly, though, most of the cases where tracking is disrupted are those in which the cosmological model is itself nonviable due to other constraints. In this sense tracking remains robust in models that are cosmologically viable.

On the prosthetic groups of the NiFe sulfhydrogenase from Pyrococcus furiosus: topology, structure, and temperature-dependent redox chemistry.

The sulfhydrogenase complex of Pyrococcus furiosus is an alpha beta gamma delta heterotetramer with both hydrogenase activity (borne by the alpha delta subunits) and sulfur reductase activity (carried by the beta gamma subunits). The beta-subunit contains at least two [4Fe-4S] cubanes and the gamma-subunit contains one [2Fe-2S] cluster and one FAD molecule. The delta-subunit contains three [4Fe-4S] cubanes and the alpha-subunit carries the NiFe dinuclear center. Only three Fe/S signals are observed in EPR-monitored reduction by dithionite, NADPH, or internal substrate upon heating. All other clusters presumably have reduction potentials well below that of the H+/H2 couple. Heat-induced reduction by internal substrate allows, for the first time, EPR monitoring of the NiFe center in a hyperthermophilic hydrogenase, which passes through a number of states, some of which are similar to states previously defined for mesophilic hydrogenases. The complexity of the observed transitions reflects a combination of temperature-dependent activation and temperature-dependent reduction potentials.

Temperature-dependent surface potentials of fluorinated alkanethiolate self-assembled monolayers

Self-assembled monolayers (SAMs) consisting of twenty-two carbon, methyl-terminated alkanethiolates adsorbed on vapor-deposited gold have been fluorinated in vacuum using an effusive F atom source. The reactive uptake of fluorine as a function of F atom exposure was calibrated using X-ray photoelectron spectroscopy. The surface potentials ( V s) of SAMs that were fluorinated to different degrees were measured as a function of temperature using a high-sensitivity vibrating probe electrostatic voltmeter. The surface potential grew increasingly negative with increasing fluorine uptake, reflecting the charge asymmetry that is induced in the alkanethiolate chains as a result of the substitution of fluorine for hydrogen. The V s of the most highly fluorinated SAMs displayed a negative temperature dependence. This observation may be indicative of a pyroelectric effect in these monolayers although a definitive conclusion awaits further measurements.

A new approach to calculating the thermophysical properties of binary mixtures containing heavy globular molecules

A new method for calculating equilibrium and transport properties of mixtures containing heavy globular molecules is proposed. The interaction between equal and unequal molecules is described by means of isotropic temperature-dependent potentials (ITDP). The deviations of the calculated mixed and interaction second virial coefficients from the corresponding experimental values are 3 - 5 times lower than those obtained when the traditional extended law of corresponding states is used. The most accurate measured mixture viscosity data have been reproduced by means of the same potential parameters within an accuracy of better than 1%. The effective parameters of the isotropic potentials for the interactions between equal and unequal molecules are given, to enable accurate calculations at arbitrary temperatures and proportions of the constituents. and are plotted as functions of the temperature and the mole fractions of the components and the graphs can be used for approximate evaluations.

The isotropic temperature-dependent potential describing the binary interactions in gaseous and

The parameters of the temperature-dependent isotropic potentials of binary - and - interactions are obtained from the experimentally measured second virial coefficient B and viscosity of these gases. The parameters of the ground state (equilibrium distance and the potential depth well at , together with the repulsive parameter n and the temperature dependence ) are defined and can be used to calculate any potential-dependent property of the two gases. The measured viscosities of the - and - mixtures at T = 354 K are reproduced within the experimental error using earlier defined potential parameters for .

A Pressure- and Temperature-Controlled Electrochemical Cell for Investigating Biological Electron Transfer Reactions

The construction and operation of a cell for pressure- and temperature-controlled direct electrochemical studies of oxygen-sensitive biological materials is described. The stainless steel electrochemical cell, which contains a pyrolytic graphite working electrode, and Ag/AgCl counter and reference electrodes in an isolated compartment, easily maintains pressures between 1 and 2000 atm, and temperatures between 4°C and at least 100°C. The utility of the cell was demonstrated by determining the reduction potential of a redox dye, phenol red, by square wave voltammetry as a function of pressure and temperature. Thermodynamic parameters associated with electron transfer (Δ V°, Δβ, Δ S°, and Δ H°) were determined from the pressure and temperature/reduction potential profiles. Both pressure-and temperature-dependent reduction potentials of phenol red were attributed to differences in solvent interactions between its oxidation states. A discussion regarding the analysis of electrostatic interaction energies of biomolecules through pressure- and temperature-controlled electrochemical studies is presented.

Temperature-dependent mean field and its effect on heavy-ion reactions

Temperature-dependent mean field potentials of nucleons are obtained by solving the Bethe-Goldstone equation for a realistic force in nuclear matter at finite temperature. For a more efficient utilization of these potentials in studying the heavy-ion reactions using a transport theory, the density and temperature dependence of these potentials is parametrized in a Skyrme type form. These parametrized temperature-dependent potentials are implemented in quantum molecular dynamics. The temperature during the simulations is deduced using a hot Thomas-Fermi approach generalized for the case of two interpenetrating pieces of nuclear matter. First of all, we show that our formalism works well in the nuclear matter limit. In order to study the effect of temperature dependence in the mean-field potential in heavy-ion reactions, the reactions 40Ca+ 40Ca and 93Nb+ 93Nb are simulated using both a finite temperature-dependent potential and a temperature-independent (i.e. zero temperature) potential. Our detailed investigation shows that the temperature dependence of the mean field affects the heavy-ion reaction dynamics to a significant amount. These effects are stronger in case of heavier nuclei and are of the same order as the differences between the usual “soft” and “hard” equation of state. An analytical parametrization of the temperature dependence of the self-consistent field is given in a Skyrme type form.

SPHALERONS AT FINITE TEMPERATURE

We construct the sphaleron for several temperature-dependent effective potentials. We determine the sphaleron energy as a function of temperature and demonstrate that the sphaleron energy at a given temperature T is well approximated by the sphaleron energy at temperature zero scaled by the ratio of the vacuum expectation values of the Higgs field at temperatures T and zero. We address the cosmological upper bound on the Higgs mass.

Temperature-dependent potentials in the theory of rubber elasticity

The new theory that aims to replace the classical theory of rubber elasticity is discussed. It is pointed out that the potential used in this theory to describe the effect of a deformation of the rubber is a potential of average force. As a result, it depends on the temperature. When this is taken into account, the objections raised against the classical theory can be shown to be invalid and the results of the new theory become identical with those of the classical one

Vibrational quantum correction for the Lennard-Jones fluid: A formalism of effective intermolecular potentials depending on mass and temperature

Using the Slater sum, a method is suggested for introducing a vibrational quantum correction into an intermolecular potential function. This method is useful for qualitative and quantitative formulations of mass- and temperature-dependent effective potentials, and is tested here on a Lennard–Jones fluid. When quantum effects increase, a negative two-body entropy is obtained because the particles behave cooperatively, and not independently. We have defined a ‘‘quantum’’ material as one with a negative two-body entropy. According to this definition, argon becomes a quantum material at temperatures lower than the melting point (super-cooled liquid), and liquid water exhibits quantum effects in its normal liquid temperature range.

The influence of substrate motion on the self-diffusion of hydrogen and its isotopes on the copper (100) surface

Work is presented that examines the effect of substrate motion on the surface self-diffusion of hydrogen and its isotopes on the Cu(100) surface. Lattice motion, represented as a sum of Lennard-Jones interactions, is found to increase the diffusion constant of hydrogen and its isotopes at all temperatures examined. The increase varies from 1.3 to 4.0 over the temperature range from 1000 to 110 K. The results agree with the recent calculations of Lauderdale and Truhlar above 150 K. The quantum contribution to the isotope effect is enhanced relative to the values for the frozen substrate. These conclusions are based on approximate path integral calculations in which quantum-mechanical effects are treated in a semiclassical manner using temperature-dependent effective potentials. The differences between the present results and those of Lauderdale and Truhlar are attributed to a breakdown of these semiclassical approximations at low temperatures. In the temperature range considered, commonly accepted harmonic corrections to the classical results afford slightly poorer agreement with Lauderdale and Truhlar than the present results.

On the existence of the thermodynamic limit for classical systems with nonspherical potentials

It is shown, that the configurational partition function for a classical system of molecules interacting with nonspherical pair potential is proportionals to the configurational partition function for a system of particles interacting with temperature-dependent spherical k-body potentials (k ⩾2). Therefore, the thermodynamic limit for nonspherical molecules exists if the effective k-body interaction is stable and tempered. A number of criteria for the nonspherical potential are developed which ensure these properties. In case the nonsphericity is small in a certain sense, stability and temperedness of the angle-averaged nonspherical potential are sufficient to ensure thermodynamic behaviour.