arXiv subjects

Francesco Sciortino

Publications and source records attributed to Francesco Sciortino.

160 records · Page 9Linked to original sources

arXiv2000

The Debye-Waller factor of liquid silica: Theory and simulation

We show that the prediction of mode-coupling theory for a model of a network-forming strong glass-former correctly describes the wave-vector dependence of the Debye-Waller factor. To obtain a good description it is important to take into account the triplet correlation function c_3, which we evaluate from a computer simulation. Our results support the possibility that this theory is able to accurately describe the non-ergodicity parameters of simple as well as of network-forming liquids.

cond-mat.stat-mech↗

arXiv2000

Thermodynamic and structural aspects of the potential energy surface of simulated water

Relations between the thermodynamics and dynamics of supercooled liquids approaching a glass transition have been proposed over many years. The potential energy surface of model liquids has been increasingly studied since it provides a connection between the configurational component of the partition function on one hand, and the system dynamics on the other. This connection is most obvious at low temperatures, where the motion of the system can be partitioned into vibrations within a basin of attraction and infrequent inter-basin transitions. In this work, we present a description of the potential energy surface properties of supercooled liquid water. The dynamics of this model has been studied in great details in the last years. Specifically, we locate the minima sampled by the liquid by ``quenches'' from equilibrium configurations generated via molecular dynamics simulations. We calculate the temperature and density dependence of the basin energy, degeneracy, and shape. The temperature dependence of the energy of the minima is qualitatively similar to simple liquids, but has anomalous density dependence. The unusual density dependence is also reflected in the configurational entropy, the thermodynamic measure of degeneracy. Finally, we study the structure of simulated water at the minima, which provides insight on the progressive tetrahedral ordering of the liquid on cooling.

cond-mat.soft↗

arXiv2000

Computer simulations of liquid silica: equation of state and liquid-liquid phase transition

We conduct extensive molecular dynamics computer simulations of two models for liquid silica [the model of Woodcock, Angell and Cheeseman, J. Phys. Chem. 65, 1565 (1976); and that of van Beest, Kramer and van Santen, Phys. Rev. Lett. 64, 1955, (1990)] to determine their thermodynamic properties at low temperature T across a wide density range. We find for both models a wide range of states in which isochores of the potential energy U are a linear function of T^(3/5), as recently proposed for simple liquids [Rosenfeld and P. Tarazona, Mol. Phys. 95, 141 (1998)]. We exploit this behavior to fit an accurate equation of state to our thermodynamic data. Extrapolation of this equation of state to low T predicts the occurrence of a liquid-liquid phase transition for both models. We conduct simulations in the region of the predicted phase transition, and confirm its existence by direct observation of phase separating droplets of atoms with distinct local density and coordination environments.

cond-mat.stat-mech↗

arXiv2000

Role of the unstable directions in the equilibrium and aging dynamics of supercooled liquids

The connectivity of the potential energy landscape in supercooled atomic liquids is investigated through the calculation of the instantaneous normal modes spectrum and a detailed analysis of the unstable directions in configuration space. We confirm the hypothesis that the mode-coupling critical temperature is the $T$ at which the dynamics crosses over from free to activated exploration of configuration space. We also report the observed changes in the local connectivity of configuration space sampled during aging, following a temperature jump from a liquid to a glassy state.

cond-mat.soft↗

arXiv2000

Extension of the Fluctuation-Dissipation theorem to the physical aging of a model glass-forming liquid

We present evidence in favor of the possibility of treating an out-of-equilibrium supercooled simple liquid as a system in quasi-equilibrium. Two different temperatures, one controlled by the external bath and one internally selected by the system characterize the quasi-equilibrium state. The value of the internal temperature is explicitly calculated within the inherent structure thermodynamic formalism. We find that the internal temperature controls the relation between the response to an external perturbation and the long-time decay of fluctuations in the liquid.

cond-mat.soft↗

arXiv2000

Supercooled Water: Dynamics, Structure and Thermodynamics

The anomalous properties of water in the supercooled state are numerous and well-known. Particularly striking are the strong changes in dynamic properties that appear to display divergences at temperatures close to -- but beyond -- the lowest temperatures attainable either experimentally or in computer simulations. Recent work on slow or glassy dynamics in water suggests analogies with simple liquids not previously appreciated, and at the same time highlights some aspects that remain peculiar to water. A comparison of the behavior of water with normal liquids, with respect to its dynamic, thermodynamic and structural changes in the supercooled regime is made by analyzing, via computer simulations, the properties of local potential energy minima sampled by water in supercooled temperatures and pressures.

cond-mat.stat-mech↗

arXiv1999

Aging in a simple glassformer

Using molecular dynamics computer simulations we investigate the out-of-equilibrium dynamics of a Lennard-Jones system after a quench from a high temperature to one below the glass transition temperature. By studying the radial distribution function we give evidence that during the aging the system is very close to the critical surface of mode-coupling theory. Furthermore we show that two-time correlation functions show a strong dependence on the waiting time since the quench and that their shape is very different from the one in equilibrium. By investigating the temperature and time dependence of the frequency distribution of the normal modes we show that the energy of the inherent structures can be used to define an effective (time dependent) temperature of the aging system.

cond-mat.stat-mech↗

arXiv1999

Configurational Entropy and Diffusivity of Supercooled Water

We calculate the configurational entropy S_conf for the SPC/E model of water for state points covering a large region of the (T,rho) plane. We find that (i) the (T,rho) dependence of S_conf correlates with the diffusion constant and (ii) that the line of maxima in S_conf tracks the line of density maxima. Our simulation data indicate that the dynamics are strongly influenced by S_conf even above the mode-coupling temperature T_MCT(rho).

cond-mat.soft↗

arXiv1999

Dynamics of simulated water under pressure

We present molecular dynamics simulations of the SPC/E model of water to probe the dynamic properties at temperatures from 350 K down to 190 K and pressures from 2.5GPa (25kbar) down to -300MPa (-3kbar). We compare our results with those obtained experimentally, both of which show a diffusivity maximum as a function of pressure. We find that our simulation results are consistent with the predictions of the mode-coupling theory (MCT) for the dynamics of weakly supercooled liquids -- strongly supporting the hypothesis that the apparent divergences of {\it dynamic} properties observed experimentally may be independent of a possible thermodynamic singularity at low temperature. The dramatic change in water's dynamic and structural properties as a function of pressure allows us to confirm the predictions of MCT over a much broader range of the von Schweidler exponent values than has been studied for simple atomic liquids. We also show how structural changes are reflected in the wave-vector dependence of dynamic properties of the liquid along a path of nearly constant diffusivity. For temperatures below the crossover temperature of MCT (where the predictions of MCT are expected to fail), we find tentative evidence for a crossover of the temperature dependence of the diffusivity from power-law to Arrhenius behavior, with an activation energy typical of a strong liquid.

cond-mat↗

arXiv1999

Instantaneous Normal Mode Analysis of Supercooled Water

We use the instantaneous normal mode approach to provide a description of the local curvature of the potential energy surface of a model for water. We focus on the region of the phase diagram in which the dynamics may be described by the mode-coupling theory. We find, surprisingly, that the diffusion constant depends mainly on the fraction of directions in configuration space connecting different local minima, supporting the conjecture that the dynamics are controlled by the geometric properties of configuration space. Furthermore, we find an unexpected relation between the number of basins accessed in equilibrium and the connectivity between them.

cond-mat↗

arXiv1999

Aging as dynamics in configuration space

The relaxation dynamics of many disordered systems, such as structural glasses, proteins, granular materials or spin glasses, is not completely frozen even at very low temperatures. This residual motion leads to a change of the properties of the material, a process commonly called aging. Despite recent advances in the theoretical description of such aging processes, the microscopic mechanisms leading to the aging dynamics are still a matter of dispute. In this Letter we investigate the aging dynamics of a simple glass former by means of molecular dynamics computer simulation. Using the concept of the inherent structure we give evidence that aging dynamics can be understood as a decrease of the effective configurational temperature T of the system. From our results we conclude that the equilibration process is faster when the system is quenched to T_c, the critical T of mode-coupling theory, and that thermodynamic concepts are useful to describe the out-of-equilibrium aging process.

cond-mat.soft↗

arXiv1999

Slow dynamics of water under pressure

We perform lengthy molecular dynamics simulations of the SPC/E model of water to investigate the dynamics under pressure at many temperatures and compare with experimental measurements. We calculate the isochrones of the diffusion constant $D$ and observe power-law behavior of $D$ on lowering temperature with an apparent singularity at a temperature $T_c(P)$, as observed for water. Additional calculations show that the dynamics of the SPC/E model are consistent with slowing down due to the transient caging of molecules, as described by the mode-coupling theory (MCT). This supports the hypothesis that the apparent divergences of dynamic quantities along $T_c(P)$ in water may be associated with ``slowing down'' as described by MCT.

cond-mat↗

arXiv1998

The phase space structure of supercooled liquids: Evidence of a dynamical critical temperature

We calculate local potential energy minima (inherent structures) for a simple model of orthoterphenyl (OTP), from computer simulations over a wide temperature range. We show that the results are very sensitive to the system size. We locate, from the temperature and size dependence of inherent structure energies, a well defined cross-over temperature that corresponds to the dynamical critical temperature predicted by mode coupling theory and in mean field spin glass models. Comparing the known phase space structure of mean field $p$-spin models and the present results for the OTP model, we find evidence supporting the proposed similarity between glass-forming liquids and a class of spin glass models.

cond-mat.soft↗

arXiv1998

Test of the semischematic model for a liquid of linear molecules

We apply to a liquid of linear molecules the semischematic mode-coupling model, previously introduced to describe the center of mass (COM) slow dynamics of a network-forming molecular liquid. We compare the theoretical predictions and numerical results from a molecular dynamics simulation, both for the time and the wave-vector dependence of the COM density-density correlation function. We discuss the relationship between the presented analysis and the results from an approximate solution of the equations from molecular mode-coupling theory [R. Schilling and T. Scheidsteger, Phys. Rev. E 56 2932 (1997)].

cond-mat.soft↗

arXiv1997

A semi-schematic model for the center of mass dynamics in supercooled molecular liquids

We introduce a semi-schematic mode-coupling model to describe the slow dynamics in molecular liquids, retaining explicitly only the description of the center of mass degrees of freedom. Angular degrees of freedom are condensed in a q-vector independent coupling parameter. We compare the time and q-dependence of the density fluctuation correlators with numerical data from a 250 ns long molecular dynamics simulation. Notwithstanding the choice of a network-forming liquid as a model for comparing theory and simulation, the model describes the main static and dynamic features of the relaxation in a broad q-vector range.

cond-mat.stat-mech↗

arXiv1997

Supercooled Water and the Kinetic Glass Transition II: Collective Dynamics

In this article we study in detail the Q-vector dependence of the collective dynamics in simulated deeply supercooled SPC/E water. The evolution of the system has been followed for 250 ns at low T, allowing a clear identification of a two step relaxation process. We present evidence in favor of the use of the mode coupling theory for supercooled liquid as framework for the description of the slow alpha-relaxation dynamics in SPC/E water, notwithstanding the fact that the cage formation in this system is controlled by the formation of an open network of hydrogen bonds as opposed to packing constraints, as in the case of simple liquids.

cond-mat.soft↗