1,720,963 research outputs found
Thermodynamic scaling of diffusion in supercooled Lennard-Jones liquids
No full textThe manner in which the intermolecular potential u(r) governs structural relaxation in liquids is a long standing problem in condensed matter physics. Herein, we show, in agreement with recent experimental results, that diffusion coefficients for simulated Lennard-Jones m-6 liquids (8 <= m <= 36) in normal and moderately supercooled states are a unique function of the variable p(gamma)/T, where p is density and T is temperature. The scaling exponent gamma is a material specific constant whose magnitude is related to the steepness of the repulsive part of u(r), evaluated around the distance of closest approach between particles probed in the supercooled regime. Approximations of u(r) in terms of inverse power laws are also discussed
Heterogeneous slow dynamics and the interaction potential of glass-forming liquids
No full textThe role of the intermolecular interaction potential on the dynamic and thermodynamic properties of model glass-forming mixtures is investigated through molecular dynamics simulations. Variations of the repulsive exponent m in the well-studied Lennard-Jones Kob-Andersen mixture are shown to have a negligible effect on the fragility and dynamic correlation volumes when quenches are performed at constant pressure. The number of dynamically correlated particles, estimated from the temperature derivative of a two-point dynamic correlation function, is approximately invariant to m at any fixed relaxation time. Further, the density scaling property of a model tetrahedral network glass-former, based on inverse power law and Lennard-Jones potentials, is investigated. The optimal scaling exponent gamma is close to zero and does not superpose the data well. The breakdown of density scaling is consistent with the absence of correlation between fluctuations of the virial and the potential energy. These results emphasize the crucial role of structural many-body correlations in glass-forming systems and show the need of investigations of more complex and realistic model liquids. (C) 2010 Elsevier B.V. All rights reserved
Pressure-energy correlations and thermodynamic scaling in viscous Lennard-Jones liquids
No full textWe use molecular dynamics simulation results on viscous binary Lennard-Jones mixtures to examine the correlation between the potential energy and the virial. In accord with a recent proposal [U. R. Pedersen , Phys. Rev. Lett. 100, 015701 (2008)], the fluctuations in the two quantities are found to be strongly correlated, exhibiting a proportionality constant, Gamma, numerically equal to one-third the slope of an inverse power law approximation to the intermolecular potential function. The correlation is stronger at higher densities, where interatomic separations are in the range where the inverse power law approximation is more accurate. These same liquids conform to thermodynamic scaling of their dynamics, with the scaling exponent equal to Gamma. Thus, the properties of strong correlation between energy and pressure and thermodynamic scaling both reflect the ability of an inverse power law representation of the potential to capture interesting features of the dynamics of dense, highly viscous liquids
Density scaling in viscous liquids: From relaxation times to four-point susceptibilities
No full textWe present numerical calculations of a four-point dynamic susceptibility, chi(4)(t), for the Kob-Andersen Lennard-Jones mixture as a function of temperature T and density rho. Over a relevant range of T and rho, the full t-dependence of chi(4)(t) and thus the maximum in chi(4)(t), which is proportional to the dynamic correlation volume, are invariant for state points for which the scaling variable rho(gamma)/T is constant. The value of the material constant gamma is the same as that which superposes the relaxation time tau of the system versus rho(gamma)/T. Thus, the dynamic correlation volume is a unique function of tau for any thermodynamic condition in the regime where density scaling holds. Finally, we examine the conditions under which the density scaling properties are related to the existence of strong correlations between pressure and energy fluctuations. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3250938
Do theories of the glass transition, in which the structural relaxation time does not define the dispersion of the structural relaxation, need revision?
No full textUpon decreasing temperature or increasing pressure, a noncrystallizing liquid will vitrify; that is, the structural relaxation time, becomes so long that the system cannot attain an equilibrium configuration in the available time. Theories, including the well-known free volume and configurational entropy models, explain the glass transition by invoking a single quantity that governs the structural relaxation time. The dispersion of the structural relaxation (i.e., the structural relaxation function) is either not addressed or is derived as a parallel consequence (or afterthought) and thus is independent of tau(alpha). In these models the time dependence of the relaxation bears no fundamental relationship to the value of tau(alpha) or other dynamic properties. Such approaches appear to be incompatible with a general experimental fact recently discovered in glass-formers: for a given material at a fixed value of T, the dispersion is constant, independent of thermodynamic conditions (T and P); that is, the shape of the a-relaxation function depends only on the relaxation time. If derived independently of tau(alpha), it is an unlikely result that the dispersion of the structural relaxation would be uniquely defined by tau(alpha)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Adam-Gibbs model for the supercooled dynamics in the ortho-terphenyl ortho-phenylphenol mixture
No full textDielectric measurements of the alpha-relaxation time were carried out on a mixture of ortho-terphenyl (OTP) with ortho-phenylphenol, over a range of temperatures at two pressures, 0.1 and 28.8 MPa. These are the same conditions for which heat capacity, thermal expansivity, and compressibility measurements were reported by Takahara [S. Takahara, M. Ishikawa, O. Yamamuro, and T. Matsuo, J. Phys. Chem. B 103, 3288 (1999)] for the same mixture. From the combined dynamic and thermodynamic data, we determine that density and temperature govern to an equivalent degree the variation of the relaxation times with temperature. Over the measured range, the dependence of the relaxation times on configurational entropy is in accord with the Adam-Gibbs model, and this dependence is invariant to pressure. Consistent with the implied connection between relaxation and thermodynamic properties, the kinetic and thermodynamic fragilities are found to have the same pressure independence. In comparing the relaxation properties of the mixture to those of neat OTP, density effects are stronger in the former, perhaps suggestive of less efficient packing. (C) 2004 American Institute of Physics
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