CiteULike: waitonhill's library [138 articles]

Relaxation in a glassy binary mixture: Mode-coupling-like power laws, dynamic heterogeneity, and a new non-Gaussian parameter

Elijah Flenner — 2008-05-07T07:18:31-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 72, No. 1. (2005)

We examine the relaxation of the Kob-Andersen Lennard-Jones binary mixture using Brownian dynamics computer simulations. We find that in accordance with mode-coupling theory the self-diffusion coefficient and the relaxation time show power-law dependence on temperature. However, different mode-coupling temperatures and power laws can be obtained from the simulation data depending on the range of temperatures chosen for the power-law fits. The temperature that is commonly reported as this system's mode-coupling transition temperature, in addition to being obtained from a power law fit, is a crossover temperature at which there is a change in the dynamics from the high-temperature homogeneous, diffusive relaxation to a heterogeneous, hopping-like motion. The hopping-like motion is evident in the probability distributions of the logarithm of single-particle displacements: approaching the commonly reported mode-coupling temperature these distributions start exhibiting two peaks. Notably, the temperature at which the hopping-like motion appears for the smaller particles is slightly higher than that at which the hopping-like motion appears for the larger ones. We define and calculate a new non-Gaussian parameter whose maximum occurs approximately at the time at which the two peaks in the probability distribution of the logarithm of displacements are most evident.

Molecular dynamics simulation of viscosity in supercooled liquid and glassy AgCu alloy

NP Lazarev — 2008-05-02T02:18:04-00:00

Journal of Non-Crystalline Solids, Vol. 353, No. 32-40. (15 October 2007), pp. 3332-3337.

The viscosity of the model AgCu alloy is simulated by several methods using (i) correlation functions through the Green-Kubo formalism, (ii) a non-equilibrium molecular dynamics approach and (iii) creep tests under constant stress. Temperature dependences of the shear viscosity and the diffusion coefficient show the breakdown of the Stokes-Einstein relation well above the glass-transition temperature Tg. This observation is interpreted as a manifestation of the development of heterogeneities in the supercooled liquid approaching Tg. Based on a generalized Einstein formula for the viscosity of liquid, a temperature dependence of heterogeneity degree of supercooled liquid is estimated. Using the dependence of the deformation rate on external stress, the activation volume is evaluated to be four atomic volumes in liquid state. However, below the mode-coupling temperature Tc the activation volume increases by several times.

Dynamical fluctuation effects in glassy colloidal suspensions

Kenneth Schweizer — 2008-03-01T22:36:00-00:00

Current Opinion in Colloid & Interface Science, Vol. 12, No. 6. (December 2007), pp. 297-306.

Fundamental understanding of heterogeneous dynamics in concentrated glassy hard sphere fluids and colloidal suspensions, even at the single particle level, requires major theoretical advances. Recent simulations and confocal microscopy experiments suggest strong nongaussian dynamical fluctuation effects and activated transport emerge well before an apparent kinetic glass transition is reached. New theoretical approaches that can predict the observable signatures of intermittent large amplitude motions and the associated fluctuation phenomena are discussed. Comparisons are made with experiments, computer simulations, and prior theory for average dynamical properties.

Elastic consequences of a single plastic event: A step towards the microscopic modeling of the flow of yield stress fluids

G Picard — 2008-05-02T02:01:12-00:00

The European Physical Journal E - Soft Matter, Vol. 15, No. 4. (1 December 2004), pp. 371-381.

With the eventual aim of describing flowing elasto-plastic materials, we focus here on the elementary process of such a flow, a plastic event, and compute the long-range perturbation it elastically induces in a medium submitted to a global shear strain. We characterize the effect of a nearby wall on this perturbation, and quantify the importance of finite-size effects. Although most of our explicit formulae refer to 2D situations, our statements hold for 3D situations as well.

Visualizing dislocation nucleation by indenting colloidal crystals

Peter Schall — 2006-03-15T23:36:35-00:00

Nature, Vol. 440, No. 7082., pp. 319-323.

Gravity-Induced Aging in Glasses of Colloidal Hard Spheres

Nikoleta Simeonova — 2008-05-02T01:45:14-00:00

Physical Review Letters, Vol. 93, No. 3. (15 July 2004), 035701.

Dynamics of viscoplastic deformation in amorphous solids

ML Falk — 2007-09-27T03:35:32-00:00

Physical Review E, Vol. 57, No. 6. (June 1998), 7192.

We propose a dynamical theory of low-temperature shear deformation in amorphous solids. Our analysis is based on molecular-dynamics simulations of a two-dimensional; two-component noncrystalline system. These numerical simulations reveal behavior typical of metallic glasses and other viscoplastic materials; specifically; reversible elastic deformation at small applied stresses; irreversible plastic deformation at larger stresses; a stress threshold above which unbounded plastic flow occurs; and a strong dependence of the state of the system on the history of past deformations. Microscopic observations suggest that a dynamically complete description of the macroscopic state of this deforming body requires specifying; in addition to stress and strain; certain average features of a population of two-state shear transformation zones. Our introduction of these state variables into the constitutive equations for this system is an extension of earlier models of creep in metallic glasses. In the treatment presented here; we specialize to temperatures far below the glass transition and postulate that irreversible motions are governed by local entropic fluctuations in the volumes of the transformation zones. In most respects; our theory is in good quantitative agreement with the rich variety of phenomena seen in the simulations.

Microscopic particle motions and topological defects in two-dimensional hexatics and dense fluids

CA Murray — 2008-04-23T09:49:20-00:00

Physical Review Letters, Vol. 62, No. 14. (3 April 1989), 1643.

Plastic response of a 2D Lennard-Jones amorphous solid: Detailed analysis of the local rearrangements at very slow strain rate

A Tanguy — 2008-04-23T09:48:17-00:00

The European Physical Journal E - Soft Matter, Vol. 20, No. 3. (1 July 2006), pp. 355-364.

Abstract. We analyze in detail the atomistic response of a model amorphous material submitted to plastic shear in the athermal, quasi-static limit. After a linear stress-strain behavior, the system undergoes a noisy plastic flow. We show that the plastic flow is spatially heterogeneous. Two kinds of plastic events occur in the system: quadrupolar localized rearrangements, and shear bands. The analysis of the individual motion of a particle shows also two regimes: a hyper-diffusive regime followed by a diffusive regime, even at zero temperature.

Elementary triangles in a 2D binary colloidal glass former

H König — 2008-04-23T09:30:37-00:00

EPL (Europhysics Letters), Vol. 71, No. 5. (2005), pp. 838-844.

Particle positions of a two-dimensional (2D) binary colloidal glass former were measured video-microscopically. Local density-optimized structures of triangles of nearest-neighboring particles (TNNP) were found from the shortest pair-distances. These are referred to as elementary triangles (ET)--exactly one for each 3-particle combination of the two kinds of colloids. Clustering of ET-like TNNP implies larger distances between two particles, which generate the near-zone maxima in the pair-distribution functions. Tiling mismatches of different kinds of ET create structural frustrations. Increasing combination possibilities for the tiling of the different ET lead to the loss of long-range order for larger pair-distances. All features of the pair-distribution functions are qualitatively described.

Experimental realization of a model glass former in 2D

H König — 2007-12-19T15:56:52-00:00

The European Physical Journal E - Soft Matter, Vol. 18, No. 3. (4 November 2005), pp. 287-293.

We have studied binary two-dimensional (2D) mixtures of superparamagnetic colloidal particles interacting through magnetic dipole moments, which were induced by an external magnetic field B. By tuning B the effective system temperature could be widely adjusted. Time-dependent particle coordinates measured by video-microscopy provide radial pair-distribution functions, mean-square displacements as well as evidence for heterogeneous dynamics. Characteristic features of 3D glass formers are observed experimentally in 2D for the first time.

Slow dynamics in glassy soft matter

Laurence Ramos — 2005-02-05T15:06:02-00:00

Journal of Physics: Condensed Matter, Vol. 17, No. 6. (16 February 2005), R253.

Hopping in a supercooled binary Lennard-Jones liquid

Thomas Schroder — 2008-03-12T04:37:17-00:00

Journal of Non-Crystalline Solids, Vol. 235-237 (2 August 1998), pp. 331-334.

A binary Lennard-Jones liquid has been investigated by molecular dynamics at equilibrium supercooled conditions. At the lowest temperature investigated, hopping is present in the system as indicated by a secondary peak in 4[pi]r2Gs(r,t), where Gs(r,t) is the van Hove self correlation function. To examine the dynamics of the system, we consider transitions between the inherent structures (local minima in the potential energy) along the trajectory. We conclude that the plateau in the mean square displacement found at lower temperatures is indeed a result of particles being trapped in local "cages", as often argued, and that the system has a single-peaked distribution of hopping-distances centered around the characteristic intermolecular distance.

A two dimensional glass: microstructure and dynamics of a 2D binary mixture

Donna Perera — 2008-03-12T04:32:24-00:00

Journal of Non-Crystalline Solids, Vol. 235-237 (2 August 1998), pp. 314-319.

A 2D equimolar binary mixture of r-12 soft disks with a diameter ratio of 1.4 is shown, via constant pressure-temperature molecular dynamics simulations, to exhibit, on cooling, a wide range of properties of glass-forming liquids. At all temperatures studied, the liquid is found to be kinetically stable with respect to crystallisation over time scales many times that of the structural relaxation. The temperature at which the `step' in the incoherent scattering function first appears is identified as an important crossover in relaxation mechanisms. Cooling below this temperature results in the development of kinetic heterogeneity and the breakdown in scaling between the structural relaxation time and the diffusion constant. Direct visualisation provides a measure of increasing cooperativity and insight into the nature of the collective motions.

Amorphous ground states and collective dynamics in a 2D glass-forming mixture

Herb Fynewever — 2008-03-12T04:15:36-00:00

Journal of Physics: Condensed Matter, Vol. 12, No. 8A. (2000), pp. A399-A402.

On cooling, many supercooled liquids arrive at a rigid disordered state, metastable to the crystal. In this paper, we characterize such amorphous ground states in a binary mixture of soft discs in 2D using molecular dynamics simulations. We find these ground states exhibit very strong correlations between local fivefold and sevenfold sites and an inherent stress heterogeneity. The heat capacity Cp is found to undergo an abrupt increase on heating through a glass transition temperature. We show that this increase is largely due to volume fluctuations.

Predicting the Long-Time Dynamic Heterogeneity in a Supercooled Liquid on the Basis of Short-Time Heterogeneities

Asaph Cooper — 2008-03-12T04:13:03-00:00

Physical Review Letters, Vol. 96, No. 18. (2006)

We report that the local Debye-Waller factor in a simulated 2D glass-forming mixture exhibits significant spatial heterogeneities and that these short-time fluctuations provide an excellent predictor of the spatial distribution of the long-time dynamic propensities [Phys. Rev. Lett. 93, 135701 (2004)]. In contrast, the potential energy per particle of the inherent structure does not correlate well with the spatially distributed dynamics.

Free volume cannot explain the spatial heterogeneity of Debye-Waller factors in a glass-forming binary alloy

Asaph Widmer-Cooper — 2008-03-12T04:08:42-00:00

Journal of Non-Crystalline Solids, Vol. 352, No. 42-49. (15 November 2006), pp. 5098-5102.

We examine the relation between the free volume per particle and the variance of the particle position, equivalent to a local Debye-Waller (DW) factor for a 2D glass-forming alloy using molecular dynamics simulations. We find that the latter quantity exhibits significant spatial heterogeneity despite involving trajectories two orders of magnitude shorter than those typically used to measure such heterogeneities. We find that the free volume exhibits no significant spatial correlation with the local DW factor. We conclude that the spatial variation in local free volume is not the cause of the short time dynamic heterogeneity.

Icosahedral Bond Orientational Order in Supercooled Liquids

Paul Steinhardt — 2008-02-12T04:30:02-00:00

Physical Review Letters, Vol. 47, No. 18. (2 November 1981), 1297.

Three-dimensional bond orientational order is studied via computer simulations of 864 particles interacting through a Lennard-Jones pair potential. Long-range orientational fluctuations appear upon supercooling about ten percent below the equilibrium melting temperature. The fluctuations suggest a broken icosahedral symmetry with extended correlations in the orientations of local icosahedral packing units.

Bond-orientational order in liquids and glasses

Paul Steinhardt — 2007-06-12T19:53:03-00:00

Physical Review B, Vol. 28, No. 2. (15 July 1983), 784.

Bond-orientational order in molecular-dynamics simulations of supercooled liquids and in models of metallic glasses is studied. Quadratic and third-order invariants formed from bond spherical harmonics allow quantitative measures of cluster symmetries in these systems. A state with short-range translational order; but extended correlations in the orientations of particle clusters; starts to develop about 10% below the equilibrium melting temperature in a supercooled Lennard-Jones liquid. The order is predominantly icosahedral; although there is also a cubic component which we attribute to the periodic boundary conditions. Results are obtained for liquids cooled in an icosahedral pair potential as well. Only a modest amount of orientational order appears in a relaxed Finney dense-random-packing model. In contrast; we find essentially perfect icosahedral bond correlations in alternative "amorphon" cluster models of glass structure.

Orientational order parameter in the ordered phase of solid deuterium from neutron-diffraction data

VV Danchuk — 2008-02-12T04:23:43-00:00

Low Temperature Physics, Vol. 30, No. 2. (2004), pp. 118-121.

Particle rearrangements during transitions between local minima of the potential energy landscape of a binary Lennard-Jones liquid

Michael Vogel — 2008-01-10T10:19:30-00:00

The Journal of Chemical Physics, Vol. 120, No. 9. (2004), pp. 4404-4414.

Fluctuating Topological Defects in 2D Liquids: Heterogeneous Motion and Noise

C Reichhardt — 2007-12-26T13:20:02-00:00

Physical Review Letters, Vol. 90, No. 9. (5 March 2003), 095504.

We measure the defect density as a function of time at different temperatures in simulations of a two-dimensional system of interacting particles. Just above the solid to liquid transition temperature; the power spectrum of the defect fluctuations shows a 1/ f signature; which crosses over to a white noise signature at higher temperatures. When 1/ f noise is present; the 5–7 defects predominantly form stringlike structures; and the particle trajectories show a 1D correlated motion that follows the defect strings. At higher temperatures this heterogeneous motion is lost.

Facilitated kinetic Ising models and the glass transition

Glenn Fredrickson — 2007-12-26T13:13:59-00:00

The Journal of Chemical Physics, Vol. 83, No. 11. (1985), pp. 5822-5831.

Kinetic Ising Model of the Glass Transition

Glenn Fredrickson — 2007-12-26T13:11:36-00:00

Physical Review Letters, Vol. 53, No. 13. (1984), 1244.

A graph theory of single-spin-flip kinetic Ising models is developed and applied to a class of spin models with strongly cooperative dynamics. Self-consistent approximations for the spin time correlation function are presented. One of the dynamical models exhibits a glass transition with no underlying thermodynamic singularity. The approximation for the time correlation function predicts a critical temperature; below which small fluctuations from equilibrium in the thermodynamic limit cannot relax in a finite amount of time.

Nontopographic description of inherent structure dynamics in glassformers

Ludovic Berthier — 2007-12-26T13:10:37-00:00

The Journal of Chemical Physics, Vol. 119, No. 8. (2003), pp. 4367-4371.

Coarse-grained microscopic model of glass formers

Juan Garrahan — 2007-09-27T14:39:35-00:00

Proceedings of the National Academy of Sciences, Vol. 100, No. 17. (19 August 2003), pp. 9710-9714.

We introduce a coarse-grained model for atomic glass formers. Its elements are physically motivated local microscopic dynamical rules parameterized by observables. Results of the model are established and used to interpret the measured behaviors of supercooled fluids approaching glass transitions. The model predicts the presence of a crossover from hierarchical super-Arrhenius dynamics at short length scales to diffusive Arrhenius dynamics at large length scales. This prediction distinguishes our model from other theories of glass formers and can be tested by experiment. 10.1073/pnas.1233719100

Geometrical Explanation and Scaling of Dynamical Heterogeneities in Glass Forming Systems

Juan Garrahan — 2007-12-26T13:02:01-00:00

Physical Review Letters, Vol. 89, No. 3. (1 July 2002), 035704.

We show how dynamical heterogeneities in glass forming systems emerge as a consequence of the existence of dynamical constraints; and we offer an interpretation of the glass transition as an entropy crisis in trajectory space (space-time) rather than in configuration space. To illustrate our general ideas; we analyze the one-dimensional ( d = 1) Fredrickson-Andersen and East models. Dynamics of such dynamically constrained systems are shown to be isomorphic to the statics of ( d +1)-dimensional dense mixtures of polydisperse noninterpenetrating domains. The domains coincide with arrested regions in trajectory space.

Dynamical heterogeneity in a dense quasi-two-dimensional colloidal liquid

Bianxiao Cui — 2007-12-26T12:56:04-00:00

The Journal of Chemical Physics, Vol. 114, No. 20. (2001), pp. 9142-9155.

Heterogeneous dynamics in liquids: fluctuations in space and time

Ranko Richert — 2007-12-26T12:51:28-00:00

Journal of Physics: Condensed Matter, Vol. 14, No. 23. (2002), pp. R703-R738.

The disordered nature of glass-forming melts gives rise to non-Arrhenius and non-exponential behaviour of their dynamics. With respect to the microscopic details involved in the structural relaxation, these materials have remained an unsolved puzzle for over a century. The observation of spatial heterogeneity regarding the dynamics provides an important step towards understanding the relation between the macroscopic properties of soft condensed matter and the molecular mechanisms involved. On the other hand, dynamic heterogeneity is the source of several new questions: What is the length scale and persistence time associated with such clusters of relaxation time? What is the signature of heterogeneity at high temperatures and in the glassy state? How do these features depend on the particular material and on the correlation function used for probing these heterogeneities? This work attempts to review the various approaches to heterogeneous dynamics and the generally accepted results, as well as some controversial issues. Undoubtedly, heterogeneity has provoked a number of novel experimental techniques targeted at studying glass-forming liquids at the molecular level. It will be emphasized that the picture of heterogeneity is a requirement for rationalizing an increasing number of experimental observations rather than just an alternative model for the dynamics of molecules.

Experimental Evidence for Fast Heterogeneous Collective Structural Relaxation in a Supercooled Liquid near the Glass Transition

M Russina — 2007-12-26T12:41:39-00:00

Physical Review Letters, Vol. 84, No. 16. (17 April 2000), 3630.

We have extended the exploration of microscopic dynamics of supercooled liquids to small wave numbers Q corresponding to the scale of intermediate range order; by developing a new experimental approach for precise data correction for multiple scattering noise in inelastic coherent neutron scattering. Our results in supercooled Ca 0.4 K 0.6 (NO 3 ) 1.4 reveal the first direct experimental evidence; after a decade of controversy; that the so-called picosecond process around the glass transition corresponds to a predicted first; faster stage of the structural relaxation. In addition; they show that this process takes the spatial form of fast heterogeneous collective flow of correlated groups of atoms.

Experimental observations of non-Gaussian behavior and stringlike cooperative dynamics in concentrated quasi-two-dimensional colloidal liquids

Andrew Marcus — 2007-05-18T06:55:00-00:00

Physical Review E, Vol. 60, No. 5. (November 1999), 5725.

We report; from direct observation of particle trajectories as a function of time; the presence of stringlike cooperative motion in a quasi-two-dimensional liquid. We have used digital video microscopy to study the equilibrium dynamics of suspensions of sterically stabilized uncharged poly(methylmethacrylate) spheres confined in a thin glass cell. Our experiments reveal the existence; in semidilute and dense liquid states; of a transition in the qualitative dynamical behavior of the system. At short times particles undergo unhindered Brownian motion; at intermediate times they undergo uncorrelated binary collisions; and at long times these one-particle self-diffusive modes are coupled to collective longitudinal acoustic modes of the fluid; the signature of which is local fluctuating domains of enhanced particle mobility. We study the properties of these domains by examining the density dependence of the van Hove self-correlation function and its deviation from Gaussian behavior. We observe that periods of non-Gaussian behavior correlate precisely with the timing of events involved in the relaxation of “caged” particles and their nearest neighbors. In contrast with relaxation processes in supercooled liquids; the lifetime of dynamical heterogeneities in a dissipative colloidal suspension is found to shift towards shorter time scales with increasing particle density. During time periods for which a quasi-two-dimensional system follows Gaussian behavior; we observe that; as predicted by Cichocki and Felderhof [J. Phys. Condens. Matter 6 ; 7287 (1994)]; the time dependence of the evolution of the effective diffusion coefficient from its short time to its long time value has the form (ln t )/ t . This last finding is true for all observed particle densities. To our knowledge; these results are the first experimental verification of the existence of microscopic cooperativity and the predicted temporal evolution of the diffusion coefficient for Brownian motion in concentrated quasi-two-dimensional liquids.

Polymer-specific effects of bulk relaxation and stringlike correlated motion in the dynamics of a supercooled polymer melt

M Aichele — 2007-12-26T12:25:55-00:00

The Journal of Chemical Physics, Vol. 119, No. 10. (2003), pp. 5290-5304.

Observation of single transits in supercooled monatomic liquids

Duane Wallace — 2007-12-20T16:38:23-00:00

Physical Review E, Vol. 64, No. 1. (20 June 2001), 011205.

A transit is the motion of a system from one many-particle potential energy valley to another. We report the observation of transits in molecular dynamics calculations of supercooled liquid argon and sodium. Each transit is a correlated simultaneous shift in the equilibrium positions of a small local group of particles; as revealed in the fluctuating graphs of the particle coordinates versus time. To the best of our knowledge; this is the first reported direct observation of transit motion in a monatomic liquid in thermal equilibrium. We found transits involving 2–11 particles; having mean shift in equilibrium position on the order of 0.4 R 1 in argon and 0.25 R 1 in sodium; where R 1 is the nearest neighbor distance. The time it takes for a transit to occur is approximately one mean vibrational period; confirming that transits are fast.

Evidence of short-time dynamical correlations in simple liquids

T Scopigno — 2007-12-20T16:35:13-00:00

Physical Review E, Vol. 66, No. 3. (2002), 031205.

We report a molecular dynamics study of the collective dynamics of a simple monatomic liquid—interacting through a two-body potential that mimics that of lithium—across the liquid-glass transition. In the glassy phase we find evidences of a fast relaxation process similar to that recently found in Lennard-Jones glasses. The origin of this process is ascribed to the topological disorder; i.e.; to the dephasing of the different momentum Q Fourier components of the actual normal modes of vibration of the disordered structure. More important; we find that the fast relaxation persists in the liquid phase with almost no temperature dependence of its characteristic parameters (strength and relaxation time). We conclude; therefore; that in the liquid phase well above the melting point; at variance with the usual assumption of uncorrelated binary collisions; the short time particle motion is strongly correlated and can be described via a normal mode expansion of the atomic dynamics.

Glass formation in a simple monatomic liquid with icosahedral inherent local order

Mikhail Dzugutov — 2007-12-20T16:04:18-00:00

Physical Review A, Vol. 46, No. 6. (1992), R2984.

In this Rapid Communication; a novel simple monatomic liquid; possessing the distinctive icosahedral inherent local order; is reported. It has been generated by a special form of pair potential employed in a molecular dynamics system of 16 384 particles; and remained stable within a wide range of temperatures explored. Pronounced stability of this model with respect to crystalline nucleation has been tested in a very long run under supercooling which was found to enhance its icosahedral inherent structure. The inherent structure factor exhibits an anomalous long-wavelength maximum which is interpreted as being indicative of the tendency for icosahedral clustering.

Molecular-dynamics study of dynamical properties of dense soft-sphere fluids: The role of short-range repulsion of the intermolecular potential

Shaw Kambayashi — 2007-12-20T15:38:00-00:00

Physical Review E, Vol. 49, No. 2. (February 1994), 1251.

Isokinetic molecular-dynamics simulations have been performed for soft-sphere fluids of 4th-; 6th-; 9th-; and 12th-inverse-power potentials near the respective freezing point. We have investigated how various dynamical properties of these fluids are influenced by the softness of the potentials to which some groups of material are attributed. The reduced diffusion constant is found to be insensitive to the choice of the softness of the potential. On the other hand; the frequency-dependent self-motion; as characterized by the spectrum of the velocity autocorrelation function; shows a pronounced dependence on the softness of the potential. The collective dynamics has also been studied in terms of the dynamic structure factor; which has been calculated over a wide range of wave numbers; covering the wave number corresponding to the first maximum of the static structure factor. A persistent sound wave is observed for the softer potentials such as 4th- and 6th-inverse-power potentials; which is in remarkable contrast to the results obtained for the steeper potentials such as 9th- and 12th-inverse-power potentials. These significant dynamical properties; dependent on the softness parameter; are consistent with the dynamical behavior observed in liquid alkali metals and liquified inert gases.

Molecular dynamics study of the collective density excitations in monatomic glass

Mikhail Dzugutov — 2007-12-20T15:35:59-00:00

Materials Science and Engineering A, Vol. 134 (25 March 1991), pp. 921-926.

The coherent density-density correlation function F(Q,t) in a simple one-component glassy system was simulated using the molecular dynamics technique. The simulation was carried out at constant density on the system comprised of 16 384 particles. The glassy state was reached by stepwise cooling of the system from the initial liquid phase with proper equilibration. The shape of the corresponding dynamical structure factor S(Q, [omega]) as well as the related longitudinal current correlation function spectra [omega]2S(Q, [omega]) were analyzed. Well-resolved non-zero frequency maxima in S(Q, [omega]) associated with collective density modes have been found in the wavevector range far beyondthe position of the main 3eak of S(Q). The results are compared with the availale theoretical models for glass dynamics.

Molecular-dynamics study of a supercooled two-component Lennard-Jones system

Göran Wahnström — 2007-12-20T01:28:41-00:00

Physical Review A, Vol. 44, No. 6. (1991), 3752.

Molecular-dynamics (MD) simulations have been carried out on a two-component Lennard-Jones system; quenched into supercooled and amorphous states. Two different regimes of viscous behavior are found in the time window accessible in MD simulation studies (of the order of nanoseconds if units appropriate for argon are used). The results for the time dependence of the self-intermediate scattering function F s ( q ; t ) show two different slow relaxation processes; where the slowest (α relaxation) can be represented by a stretched exponential; exp[-( t /τ rel ) β ]. In the frequency domain this gives rise to a quasielastic peak; and it is found that its area; the nonergodicity parameter f s ( q )== a ; shows an anomalous decrease when increasing the temperature towards a critical value T c . This happens in the supercooled-liquid regime; and it is one of the basic predictions of the recent mode-coupling theory for the liquid-glass transition problem. In the strongly supercooled-liquid regime the diffusion is of the hopping type; and it is found to be strongly cooperative in nature.

Crystallization of a Quasi-Two-Dimensional Granular Fluid

PM Reis — 2007-12-10T22:23:17-00:00

Physical Review Letters, Vol. 96, No. 25. (2006)

We experimentally investigate the crystallization of a uniformly heated quasi-2D granular fluid as a function of the filling fraction. Our experimental results for the Lindemann melting criterion, the radial distribution function, the bond order parameter, and the statistics of topological changes at the particle level are the same as those found in simulations of equilibrium hard disks. This direct mapping suggests that the study of equilibrium systems can be effectively applied to study nonequilibrium steady states such as those found in our driven and dissipative granular system.

Two-Dimensional Melting Far from Equilibrium in a Granular Monolayer

JS Olafsen — 2007-12-11T15:59:57-00:00

Physical Review Letters, Vol. 95, No. 9. (2005)

We report an experimental investigation of the transition from a hexagonally ordered solid phase to a disordered liquid in a monolayer of vibrated spheres. The transition occurs as the intensity of the vibration amplitude is increased. Measurements of the density of dislocations and the positional and orientational correlation functions show evidence for a dislocation-mediated continuous transition from a solid phase with long-range order to a liquid with only short-range order. The results show a strong similarity to simulations of melting of hard disks in equilibrium, despite the fact that the granular monolayer is far from equilibrium due to the effects of interparticle dissipation and the vibrational forcing.

The shapes of cooperatively rearranging regions in glass-forming liquids

Jacob Stevenson — 2007-12-11T15:53:07-00:00

Nat Phys, Vol. 2, No. 4. (April 2006), pp. 268-274.

Dynamic particle tracking reveals the ageing temperature of a colloidal glass

Ping Wang — 2007-12-03T07:28:19-00:00

Nat Phys, Vol. 2, No. 8. (2006), pp. 526-531.

Frustration on the way to crystallization in glass

Hiroshi Shintani — 2007-12-11T15:41:59-00:00

Nat Phys, Vol. 2, No. 3. (March 2006), pp. 200-206.

Glass transition in the hard sphere system

Chandan Dasgupta — 2007-11-30T04:27:22-00:00

Complex Behaviour of Glassy Systems (1997), pp. 100-110.

The glass transition in a hard sphere system is studied numerically, using a model free energy functional that exhibits glassy local minima at sufficiently high densities. The numerical methods used in our work include free-energy minimization, direct integration of Langevin equations and Monte Carlo simulation. At relatively low densities, the system is found to fluctuate near the uniform liquid minimum of the free energy and to exhibit mode-coupling behavior. At densities higher than a first crossover density, the dynamics is governed by thermally activated transitions between glassy free-energy minima. The typical time scale for such transitions grows very rapidly as a second crossover density is approached from below. Interpretation of existing molecular dynamics data in the light of our results is discussed.

Caging Dynamics in a Granular Fluid

PM Reis — 2007-11-30T02:40:21-00:00

Physical Review Letters, Vol. 98, No. 18. (2007)

We report an experimental investigation of the caging motion in a uniformly heated granular fluid for a wide range of filling fractions, . At low the classic diffusive behavior of a fluid is observed. However, as is increased, temporary cages develop and particles become increasingly trapped by their neighbors. We statistically analyze particle trajectories and observe a number of robust features typically associated with dense molecular liquids and colloids. Even though our monodisperse and quasi-2D system is known to not exhibit a glass transition, we still observe many of the precursors usually associated with glassy dynamics. We speculate that this is due to a process of structural arrest provided, in our case, by the presence of crystallization.

Local fluctuations and ordering in liquid and amorphous metals

SP Chen — 2007-11-29T14:33:12-00:00

Physical Review B, Vol. 37, No. 5. (15 February 1988), 2440.

A molecular-dynamics study of the structure and dynamics of monatomic liquids and glasses is presented. The local atomic structure and its development during the quenching process are analyzed in terms of fluctuations of atomic-level stresses and their correlations. This approach extends the basis for the description of the local structure from the usually employed scalar quantity; the local density fluctuation; to a tensorial quantity; the local stress fluctuation. It is shown here that the local stress fluctuations and their spatial and temporal correlations provide a detailed picture of the dynamics of the liquid and of the transition from an ideal fluid to a viscous liquid; and then to a glass. In particular; it is demonstrated that the shear stresses which are spatially uncorrelated at high temperatures become correlated below a temperature; T s ; which is about twice the glass transition temperature. At the same time the dynamic behavior of the liquid; characterized by the diffusivity; viscosity; and phonon states; changes sharply at this temperature. Implications of this apparent structural transition and its origin are then discussed.

Atomistic computer simulation for liquid-glass transition in Zr---Ni alloy

Tomoyasu Aihara — 2007-11-29T14:23:11-00:00

Materials Science and Engineering A, Vol. 179-180, No. Part 1. (1 May 1994), pp. 256-260.

The liquid-glass transition for Zr---Ni alloy is investigated using molecular dynamics simulation and the empirical many-body potential described by the embedded atom method. The structure, distributions of the atomistic local site energy and the mean square force are investigated. The amorphous structure is compared with the crystal structure at room temperature. The potential, kinetic and total site energy distributions exhibit Maxwell-Boltzmann-type profiles. The liquid-glass transition is clearly detected for the atomistic site energy, and equivalently for each specimen. The results are discussed from the viewpoint of non-equilibrium. The mean square force exhibits an exponential dependence on the temperature.

Structural Arrest and the Dynamics of the Liquid Glass Transition

W Götze — 2007-11-29T10:38:53-00:00

Physica Scripta, Vol. 34, No. 1. (1986), pp. 66-76.

Concepts, mathematical argumentation, physical picture and scope of the mode coupling theory for the liquid glass transition as developed in a paper by Bengtzelius et al. and in succeeding work are summarized. An extensive discussion of the results for the glass form factors, discontinuities of thermodynamical functions, variation of transport coefficients, non exponential structural relaxation and scaling laws for the low frequency excitation spectra is presented.

Neutron-Spin-Echo Investigation on the Dynamics of Polybutadiene near the Glass Transition

D Richter — 2007-11-28T10:35:11-00:00

Physical Review Letters, Vol. 61, No. 21. (21 November 1988), 2465.

We present neutron-spin-echo data on the structural relaxation of glass-forming polybutadiene near the glass transition. Microscopic dynamics on the scale of an interchain distance and the monomeric friction coefficient derived from macroscopic viscosity relaxation obey the time-temperature superposition principle following a common scale. The dynamic structure factor exhibits strong similarities with recent mode-coupling predictions.

Neutron spin echo study of dynamic correlations near the liquid-glass transition

F Mezei — 2007-11-28T10:34:23-00:00

Physical Review Letters, Vol. 58, No. 6. (February 1987), 571.

Neutron spin-echo measurements have been performed on the ionic system Ca 0.4 K 0.6 (NO 3 ) 1.4 around the glass transition temperature T 0 ; in order to determine the time dependence of the density correlation function φ q (t); which plays a central role in recent theories. Above T 0 the results reveal that φ q (t) contains two distinct ‘‘slow’’ components; the slower of which (a) has a stretched exponential form exp[-(t/τ) β ]; (b) shows a slowing down as T approaches T 0 which scales with the Stokes-Einstein diffusion constant; and (c) tends to become the nonergodic fraction of the structure factor at T 0 . .AE