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PubMed2006

Thermodynamic and dynamic anomalies for dumbbell molecules interacting with a repulsive ramplike potential.

Using collision driven discrete molecular dynamics, we investigate the thermodynamics and dynamics of systems of 500 dumbbell molecules interacting by a purely repulsive ramplike discretized potential, consisting of n steps of equal size. We compare the behavior of the two systems, with n=18 and n=144 steps. Each system exhibits both thermodynamic and dynamic anomalies, a density maximum and the translational and rotational mobilities show anomalous behavior. Starting with very dense systems and decreasing the density, both mobilities first increase, reach a maximum, then decrease, reach a minimum, and finally increase; this behavior is similar to the behavior of SPC/E (Simple Point Charge-Extended) water. The regions in the pressure-temperature plane of translational and rotational mobility anomalies depend strongly on n. The product of the translational diffusion coefficient and the orientational correlation time increases with temperature, in contrast with the behavior of most liquids.

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PubMed2006

Self-consistent quasiparticle model results for ultrarelativistic electron-positron thermodynamic plasma.

Relativistic plasma with two charge species and radiation at thermodynamic equilibrium is a general system of interest in astrophysics and high-energy physics. We develop a self-consistent quasiparticle model for such a system to take account of the collective behavior of plasma, and thermodynamic properties are derived. It is applied to the ultrarelativistic electron-positron plasma and compared with previous results.

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PubMed2006

Correlations between isobaric and isochoric fragilities and thermodynamical scaling exponent for glass-forming liquids.

Two correlations concerning the isobaric and isochoric fragilities, m(P) and m(V), as well as the scaling exponent gamma reported by Casalini and Roland [Phys. Rev. E 72, 031503 (2005)] have been examined for several van der Waals and hydrogen-bonded glass formers. It has been pointed out that the correlations lead to some serious inconsistency with the exponent gamma, which is expected to be a constant dependent only on material, but varies also on pressure if experimentally found pressure dependences of m(P) are taken into account. This problem could be solved in the case of van der Waals liquids, but then at least one of the correlations becomes dependent on thermodynamic conditions, and consequently, loses its universality. However, some H-bonded systems, due to properties of hydrogen bonding, have been well argued not to be included to determine the correlations independently of thermodynamic conditions. Furthermore, it has been noticed that another correlation concerning the fragility, between m(P) and the structural relaxation peak breadth, yields discrepancies in comparison with results of experiments under elevated pressure.

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PubMed2001

Quantum approach to a derivation of the second law of thermodynamics.

We reinterpret the microcanonical conditions in the quantum domain as constraints for the interaction of the "gas subsystem" under consideration and its environment ("container"). The time average of a purity measure is found to equal the average over the respective path in Hilbert space. We then show that for typical (degenerate or nondegenerate) thermodynamical systems almost all states within the allowed region of Hilbert space have a local von Neumann entropy S close to the maximum and a purity P close to its minimum, respectively. Typically, thermodynamical systems should obey the second law.

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PubMed2001

Thermodynamic and transport properties of superconducting Mg10B2.

Transport and thermodynamic properties of a sintered pellet of the newly discovered MgB2 superconductor have been measured to determine the characteristic critical magnetic fields and critical current densities. Both resistive transition and magnetization data give similar values of the upper critical field, Hc2, with magnetization data giving dHc2/dT = 0.44 T/K at the transition temperature of Tc = 40.2 K. Close to the transition temperature, magnetization curves are thermodynamically reversible, but at low temperatures the trapped flux can be on the order of 1 T. The value of dHc/dT at Tc is estimated to be about 12 mT/K, a value similar to classical superconductors like Sn. Hence, the Ginzburg-Landau parameter kappa approximately 26. Estimates of the critical supercurrent density, Jc, using hysteresis loops and the Bean model, give critical current densities on the order of 10(5) A/cm2. Hence the supercurrent coupling through the grain boundaries is comparable to intermetallics like Nb3Sn.

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PubMed2001

Thermodynamics of C incorporation on Si(100) from ab initio calculations.

We study the thermodynamics of C incorporation on Si(100), a system where strain and chemical effects are both important. Our analysis is based on first-principles atomistic calculations to obtain the important lowest-energy structures, and a classical effective Hamiltonian which is employed to represent the long-range strain effects and incorporate the thermodynamic aspects. We determine the equilibrium phase diagram in temperature and C chemical potential, which allows us to predict the mesoscopic structure of the system that should be observed under experimentally relevant conditions.

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PubMed2001

Self-assembled quantum dots: crossover from kinetically controlled to thermodynamically limited growth.

By means of kinetic Monte Carlo simulations of the self-organized growth of quantum dots in strained semiconductor systems we resolve the seemingly contradictory features of kinetic versus thermodynamic behavior, e.g., with respect to the temperature dependence of the average dot size and their dispersion. We show that the size distribution immediately after deposition is kinetically controlled, with smaller islands for lower temperatures and larger islands for higher temperatures. For longer simulation times the kinetics leads to equilibration, and a crossover effect between the size distributions occurs, which is in good agreement with the predictions of thermodynamics.

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PubMed2001

Is Tsallis thermodynamics nonextensive?

Within the Tsallis thermodynamics framework, and using scaling properties of the entropy, we derive a generalization of the Gibbs-Duhem equation. The analysis suggests a transformation of variables that allows standard thermodynamics to be recovered. Moreover, we also generalize Einstein's formula for the probability of a fluctuation to occur by means of the maximum statistical entropy method. The use of the proposed transformation of variables also shows that fluctuations within Tsallis statistics can be mapped to those of standard statistical mechanics.

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PubMed2002

Uniqueness of the entanglement measure for bipartite pure states and thermodynamics.

We apply the axiomatic approach to thermodynamics presented by Giles to derive a unique measure of entanglement for bipartite pure states. This implies that local manipulations of entanglement in quantum information theory and adiabatic transformations of states in thermodynamics have the same underlying mathematical structure. We discuss possible extensions of our results to mixed and multipartite states.

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PubMed2002

Are the laws of entanglement theory thermodynamical?

We argue that on its face, entanglement theory satisfies laws equivalent to thermodynamics if the theory can be made reversible by adding certain bound entangled states as a free resource during entanglement manipulation. Subject to plausible conjectures, we prove that this is not the case in general, and discuss the implications of this for the thermodynamics of entanglement.

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PubMed2003

Relation between thermodynamics and kinetics of glass-forming liquids.

Vitrification of a supercooled liquid is often characterized by the hypothetical kinetic instability point, the Vogel-Fulcher temperature T0, and the thermodynamic one, the Kauzmann temperature T(K). The widely believed relation T0 congruent with T(K) is regarded as the supporting evidence of a direct connection between the thermodynamics and kinetics of glass-forming liquids. Here we demonstrate that T(K)/T(0) systematically increases from unity with a decrease in the fragility, contrary to the common belief. This systematic deviation may be explained by a synergistic effect between the weaker cooperativity and the stronger tendency of short-range ordering in stronger glass formers.

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PubMed2003

Negative specific heat, the thermodynamic limit, and ergodicity.

Thermodynamic stability, in particular, the positivity of the specific heat in the microcanonical ensemble, is not an automatic consequence of the thermodynamic limit. But it holds under special circumstances such as for the most important case of quantum-mechanical Coulomb systems. Therefore, it is surprising that there are experimental indications to the contrary. In this Letter we study a simple model for which the microcanonical specific heat is positive, if the system is ergodic. However, if the system is not ergodic, the energy shell in phase space has some ergodic components with a negative specific heat. This provides another possible general pathway for a negative specific heat in addition to the commonly accepted, the small number of particles.

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PubMed2004

Reliable first-principles alloy thermodynamics via truncated cluster expansions.

In alloys cluster expansions (CE) are increasingly used to combine first-principles electronic-structure calculations and Monte Carlo methods to predict thermodynamic properties. As a basis-set expansion in terms of lattice geometrical clusters and effective cluster interactions, the CE is exact if infinite, but is tractable only if truncated. Yet until now a truncation procedure was not well defined and did not guarantee a reliable truncated CE. We present an optimal truncation procedure for CE basis sets that provides reliable thermodynamics. We then exemplify its importance in Ni3V, where the CE has failed unpredictably, and now show agreement to a range of measured values, predict new low-energy structures, and explain the cause of previous failures.

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PubMed2004

Ga sublattice defects in (Ga,Mn)As: thermodynamical and kinetic trends.

We have used positron annihilation spectroscopy and infrared absorption measurements to study the Ga sublattice defects in epitaxial Ga(1-x)MnxAs with Mn content varying from 0% to 5%. We show that the Ga vacancy concentration decreases and As antisite concentration increases with increasing Mn content. This is in agreement with thermodynamical considerations for the electronic part of the formation energy of the Ga sublattice point defects. However, the absolute defect concentrations imply that they are determined rather by the growth kinetics than by the thermodynamical equilibrium. The As antisite concentrations in the samples are large enough to be important for compensation and magnetic properites. In addition, the Ga vacancies are likely to be involved in the diffusion and clustering of Mn at low annealing temperatures.

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PubMed2004

Morphological thermodynamics of fluids: shape dependence of free energies.

We examine the dependence of a thermodynamic potential of a fluid on the geometry of its container. If motion invariance, continuity, and additivity of the potential are satisfied, only four morphometric measures are needed to describe fully the influence of an arbitrarily shaped container on the fluid. These three constraints can be understood as a more precise definition for the conventional term extensive and have as a consequence that the surface tension and other thermodynamic quantities contain, aside from a constant term, only contributions linear in the mean and Gaussian curvature of the container and not an infinite number of curvatures as generally assumed before. We verify this numerically in the entropic system of hard spheres bounded by a curved wall.

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PubMed2005

Temporal scaling at Feigenbaum points and nonextensive thermodynamics.

We show that recent claims for the nonstationary behavior of the logistic map at the Feigenbaum point based on nonextensive thermodynamics are either incorrect or can be easily deduced from well-known properties of the Feigenbaum attractor. In particular, there is no generalized Pesin identity for this system, the existing attempts at proofs being based on misconceptions about basic notions of ergodic theory. In deriving several new scaling laws of the Feigenbaum attractor, thorough use is made of its detailed structure, but there is no obvious connection to nonextensive thermodynamics.

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PubMed2006

Thermodynamics and excitations of condensed polaritons in disordered microcavities.

We study the thermodynamic condensation of microcavity polaritons using a realistic model of disorder in semiconductor quantum wells. This approach correctly describes the polariton inhomogeneous broadening in the low density limit, and treats scattering by disorder to all orders in the condensed regime. While the weak disorder changes the thermodynamic properties of the transition little, the effects of disorder in the condensed state are prominent in the excitations and can be seen in resonant Rayleigh scattering.

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PubMed2006

Thermodynamics and statistical mechanics of dense granular media.

By detailed molecular dynamics and Monte Carlo simulations of a model system we show that granular materials at rest can be described as thermodynamics systems. First, we show that granular packs can be characterized by few parameters, as much as fluids or solids. Then, in a second independent step, we demonstrate that these states can be described in terms of equilibrium distributions which coincide with the statistical mechanics of powders first proposed by Edwards. We also derive the system equation of state as a function of the "configurational temperature," its new intensive thermodynamic parameter.

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