Characterization of the dynamics of glass-forming liquids from the properties of the potential energy landscape (original) (raw)
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Physica A: Statistical Mechanics and its Applications, 2016
The systematic method to explore how the dynamics of strong liquids (S) is different from that of fragile liquids (F) near the glass transition is proposed from a unified point of view based on the mean-field theory discussed recently by Tokuyama. The extensive molecular-dynamics simulations are performed on different glass-forming materials. The simulation results for the mean-nth displacement M n (t) are then analyzed from the unified point of view, where n is an even number. Thus, it is first shown that in each type of liquids there exists a master curve H (i) n as M n (t) = R n H (i) n (v th t/R; D/Rv th) onto which any simulation results collapse at the same value of D/Rv th , where R is a characteristic length such as an interatomic distance, D a long-time selfdiffusion coefficient, v th a thermal velocity, and i =F and S. The master curves H (F) n and H (S) n are then shown not to coincide with each other in the so-called cage region even at the same value of D/Rv th. Thus, it is emphasized that the dynamics of strong liquids is quite different from that of fragile liquids. A new type of strong liquids recently proposed is also tested systematically from this unified point of view. The dynamics of a new type is then shown to be different from that of well-known network glass formers in the cage region, although both liquids are classified as a strong liquid. Thus, it is suggested that a smaller grouping is further needed in strong liquids, depending on whether they have a network or not.
Dynamical singularities near the liquid-glass transition: Theory and molecular dynamics study
Solid State Ionics, 1991
This review article discusses recent developments in the study of the dynamical property of supercooled liquids close to the liquid-glass transition, in particular the dynamical singularity accompanied by the transition. The glass transition is a dynamical transition in the sense that when a liquid is rapidly cooled or compressed beyond the freezing point, the system can go into a quasi-equilibrium, metastable or nonequilibrium state, characterized by long structural relaxation times. Therefore, investigations on the dynamical properties of the supercooled liquids are essential to understanding the nature of the liquid-to-glass transition, which are the main issues of this article. The article reviews recent molecular dynamics studies on the dynamical properties of highly supercooled liquids and glasses and the theoretical developments about the liquid-glass transition based on modecoupling approximations, generalized nonlinear hydrodynamic equations and a trapping diffusion model which have extensively been studied over the past decade. The molecular dynamics simulations prove to be an extremely useful and increasingly valuable aid to the elucidation of glass transition phenomena and the study of amorphous structures, but care is needed in the interpretation of the simulation results on the dynamical properties of such highly supercooled liquids and glasses, in which dynamical slowing down is essential, and consequently the relaxation time exceeds far over the order of the time investigated in computer experiments. We examine a variety of results on the dynamical behaviors obtained by molecular dynamics simulations and theoretical works from different points of view, and elucidate the dynamical singularity near the transition, which we call "quasi critical phenomena", as in the second-order phase transition.
Dynamics and thermodynamics of supercooled liquids and glasses from a model energy landscape
Physical Review B, 2001
The dynamics and thermodynamics of a model potential-energy surface are analyzed with regard to supercooling and glass formation. Relaxation is assumed to be mediated by pathways that connect groups of local minima. The dynamics between these groups is treated via transition state theory using appropriate densities of states consistent with the thermodynamics of the model, with a general expression for the free energy barrier. Nonergodicity is admitted by successive disconnection of regions that no longer contribute to the partition function as a function of the observation time scale. The model exhibits properties typical of supercooled liquids and glasses spanning the whole range of ''fragile'' and ''strong'' behavior. Non-Arrhenius dynamics, characteristic of ''fragile'' glass formers, are observed when the barriers to relaxation increase as the potential energy decreases, but only if the observation time scale is long enough. For a fixed observation time, fragility generally increases as the free energy barriers decrease and vibrational frequencies increase. We associate higher vibrational frequencies with systems that have more local minima, and hence when the model exhibits dynamic fragility we usually see a large change in the heat capacity at the glass transition. However, in some regions of parameter space the expected correlations between dynamics and thermodynamics are not present.
Journal of Physics: Condensed Matter, 2009
We report Molecular Dynamics simulations for a new model of tetrahedral network glass-former, based on short-range, spherical potentials. Despite the simplicity of the forcefield employed, our model reproduces some essential physical properties of silica, an archetypal network-forming material. Structural and dynamical properties, including dynamic heterogeneities and the nature of local rearrangements, are investigated in detail and a direct comparison with models of close-packed, fragile glass-formers is performed. The outcome of this comparison is rationalized in terms of the properties of the Potential Energy Surface, focusing on the unstable modes of the stationary points. Our results indicate that the weak degree of dynamic heterogeneity observed in network glass-formers may be attributed to an excess of localized unstable modes, associated to elementary dynamical events such as bond breaking and reformation. On the contrary, the more fragile Lennard-Jones mixtures are characterized by a larger fraction of extended unstable modes, which lead to a more cooperative and heterogeneous dynamics.
Journal of Statistical Mechanics: Theory and Experiment, 2016
The sluggish and heterogeneous dynamics of glass forming liquids is frequently associated to the transient coexistence of two phases of particles, respectively with an high and low mobility. In the absence of a dynamical order parameter that acquires a transient bimodal shape, these phases are commonly identified empirically, which makes difficult investigating their relation with the structural properties of the system. Here we show that the distribution of single particle diffusivities can be accessed within a Continuous Time Random Walk description of the intermittent motion, and that this distribution acquires a transient bimodal shape in the deeply supercooled regime, thus allowing for a clear identification of the two coexisting phase. In a simple two-dimensional glass forming model, the dynamic phase coexistence is accompanied by a striking structural counterpart: the distribution of the crystalline-like order parameter becomes also bimodal on cooling, with increasing overlap between ordered and immobile particles. This simple structural signature is absent in other models, such as the three-dimesional Kob-Andersen Lennard-Jones mixture, where more sophisticated order parameter might be relevant. In this perspective, the identification of the two dynamical coexisting phases opens the way to deeper investigations of structure-dynamics correlations.
Potential energy landscape and long-time dynamics in a simple model glass
Physical Review E, 2000
We analyze the properties of a Lennard-Jones system at the level of the potential energy landscape. After an exhaustive investigation of the topological features of the landscape of the systems, obtained studying small size sample, we describe the dynamics of the systems in the multi-dimensional configurational space by a simple model. This consider the configurational space as a connected network of minima where the dynamics proceeds by jumps described by an appropriate master equation. Using this model we are able to reproduce the long time dynamics and the low temperature regime. We investigate both the equilibrium regime and the off-equilibrium one, finding those typical glassy behavior usually observed in the experiments such as: i) stretched exponential relaxation, ii) temperature-dependent stretching parameter, iii) breakdown of the Stokes-Einstein relation, and iv) appearance of a critical temperature below which one observes deviation from the fluctuation-dissipation relation as consequence of the lack of equilibrium in the system.
Long-Lived Structures in Fragile Glass-Forming Liquids
Physical Review Letters, 1995
We present molecular dynamics results for the existence of long-lived clusters near the glass transition in a two component, two-dimensional Lennard-Jones supercooled liquid. Several properties of this system are similar to a mean-field glass-forming liquid near the spinodal. This similarity suggests that the glass "transition" in the supercooled liquid is associated with an incipient thermodynamic instability. Our results also suggest that single particle properties are not relevant for characterizing the instability, but are relevant to the kinetic transition that occurs at a lower temperature than the glass transition.
Orientational relaxation (OR) in a viscous, glassy liquid is investigated by carrying out extensive NPT molecular dynamics simulations of isolated ellipsoids in a glass forming binary mixture. Near the glass transition, the OR occurs mainly via correlated hopping, sometimes involving participation of several neighboring atoms, placed in a ring like tunnel. In the glassy state, hopping is found to be accompanied by larger fluctuations in the total energy and the volume of the system. Both orientational and translational hopping are found to be gated, restricted primarily by the entropic bottlenecks, with orientation becoming increasingly slower than translation as the pressure is increased. OR is heterogeneous, with a wide distribution of decay times.
The phase behavior of multicomponent metallic liquids is exceedingly complex because of the convoluted many-body and many-elemental interactions. Herein, we present systematic studies of the dynamical aspects of a model ternary metallic liquid Cu 40 Zr 51 Al 9 using molecular dynamics simulations with embedded atom method. We observed a dynamical crossover from Arrhenius to super-Arrhenius behavior in the transport properties (self diffusion coefficient, self relaxation time, and shear viscosity) bordered at T x ∼ 1300 K. Unlike in many molecular and macromolecular liquids, this crossover phenomenon occurs well above the melting point of the system (T m ∼ 900 K) in the equilibrium liquid state; and the crossover temperature T x is roughly twice of the glass-transition temperature of the system (T g ). Below T x , we found the elemental dynamics decoupled and the Stokes-Einstein relation broke down, indicating the onset of heterogeneous spatially correlated dynamics in the system mediated by dynamic communications among local configurational excitations. To directly characterize and visualize the correlated dynamics, we employed a nonparametric, unsupervised machine learning technique and identified dynamical clusters of atoms with similar atomic mobility. The revealed average dynamical cluster size shows an accelerated increase below T x and mimics the trend observed in other ensemble averaged quantities that are commonly used to quantify the spatially heterogeneous dynamics such as the non-Gaussian parameter α 2 and the four-point correlation function χ 4 .