Glass Transition of the Monodisperse Gaussian Core Model

Abstract

We numerically investigate the dynamical properties of the one-component Gaussian core model in supercooled states. We find that nucleation is increasingly suppressed with increasing density. The system concomitantly exhibits glassy, slow dynamics characterized by the two-step stretched exponential relaxation of the density correlation and a drastic increase of the relaxation time. We also find a weaker violation of the Stokes-Einstein relation and a smaller non-Gaussian parameter than in typical model glass formers, implying weaker dynamic heterogeneities. Additionally, the agreement of the simulation data with the prediction of mode-coupling theory is exceptionally good, indicating that the nature of the slow dynamics of this ultra-soft particle fluid is mean-field-like. This fact may be understood as a consequence of the long-range nature of the interaction. The nature of the glass transition is surrounded by controversy. Several scenarios have been proposed to explain the drastic slowing down of dynamics of supercooled fluids near the glass transition point [1][2][3]. Numerical simulation of simple model fluids is an ideal tool to test these competing scenarios. However, the typical model fluids studied so far, such as Lennard-Jones, soft-core, and hard-sphere mixtures, have short-ranged, strong repulsive interactions in common, which dictate their thermodynamic, structural, and dynamical properties and render the results of these models qualitatively similar [4]. A new class of model glass formers is desirable to diversify our pictures and perspectives on the glass transition within the limited accessible time windows of the simulations. Recently, ultra-soft particle fluids have attracted particular attention in soft-materials science [5]. They are systems composed of spherical particles interacting with bounded and weak repulsions and are a good model for various soft materials, such as star-polymers and dendrimers. The absence of the hard-core-like repulsion makes the thermodynamic and dynamic behaviors of this class of systems extremely rich compared with standard molecular systems. Their phase diagrams exhibit exotic and counterintuitive properties, including a stable fluid phase at high temperatures for arbitrary densities, re-melting of solids at higher densities, and complex crystalline phases at low temperatures [5]. The dynamics of the ultra-soft particles fluids also exhibits rich and nontrivial behaviors [6][7][8][9].In this Letter, we consider the simplest version of ultrasoft particles, i.e., the Gaussian core model (GCM) fluid originally introduced by Stillinger [10]. The GCM interaction is given bywhere ǫ and σ characterize the energy and length scales, respectively. The GCM is an ideal model to study glassy dynamics because its thermodynamic phase diagram is relatively simple. Other ultra-soft particles, such as Hertzian spheres and star-polymers, exhibit complex crystalline phases, which may affect the dynamics in the supercooled state [6,11]. We numerically study the monodisperse GCM in th...

What Is a Simple Liquid?

“…F. Gaussian core model The Gaussian core model (GCM) [53,54], which is not strongly correlating, is defined by a Gaussian pair potential and thus has a finite potential energy at zero separation. The high-density regime of the GCM model (ρ > 1.5) has recently received attention as a singlecomponent model glass former [55] because it is not prone to crystallization and shows the characteristic features of glass-forming liquids (large viscosity, two-step relaxation, etc). Figure 15 shows the RDF and ISF for the GCM liquid.…”

High temperature breakdown of the Stokes-Einstein relation in a computer simulated Cu-Zr melt

“…Therefore, especially in theoretical work, the SE relation is often written in terms of the α-relaxation time, τ α , instead of the viscosity. [11][12][13][14] The SE relation was derived for a mesoscopic ball suspended in a continuum fluid; however, many experiments have demonstrated that in many liquids, Eq. (1) holds at a microscopic level over a large temperature range for the self-diffusion coefficient.…”

“…21,22 In the past several decades, the SE relation, especially its breakdown, has attracted considerable research interest of scientists in many fields. [7][8][9][10][11][12][13][14][15][16][17][23][24][25][26][27][28][29] There are three unsolved, open questions regarding the breakdown of SE relation, namely, how far can one cool down the liquid before the SE relation breaks down, what mechanism is responsible for the breakdown, and how does one predict the onset of the breakdown quantitatively. The answers to these three interrelated questions are still discussed controversially.…”

Communication: Generalizing Rosenfeld's excess-entropy scaling to predict long-time diffusivity in dense fluids of Brownian particles: From hard to ultrasoft interactions

“…For example, it has recently been shown that generalized Langevin theories 24,33,34 can qualitatively (but not yet quantitatively) predict the anomalous density dependence of the long-time self-diffusivity observed in Brownian dynamics simulations of the Gaussian-core model. Likewise, mode-coupling theory has been able to capture some of the nonmonotonic dynamic trends displayed by fluids of particles that interact via starpolymer-like, 23 harmonic, 35 Hertzian, 29 or Gaussian-core 27,36 pair potentials. Kinetic theory has also been used to gain insights into the nontrivial temperature-and density-dependent a) Author to whom correspondence should be addressed.…”