arXiv subjects

Peter Harrowell

Publications and source records attributed to Peter Harrowell.

54 records · Page 3Linked to original sources

arXiv2015

Favoured Local Structures in Liquids and Solids: a 3D Lattice Model

We investigate the connection between the geometry of Favoured Local Structures (FLS) in liquids and the associated liquid and solid properties. We introduce a lattice spin model - the FLS model on a face-centered cubic lattice - where this geometry can be arbitrarily chosen among a discrete set of 115 possible FLS. We find crystalline groundstates for all choices of a single FLS. Sampling all possible FLS's, we identify the following trends: i) low symmetry FLS's produce larger crystal unit cells but not necessarily higher energy groundstates, ii) chiral FLS's exhibit to peculiarly poor packing properties, iii) accumulation of FLS's in supercooled liquids is linked to large crystal unit cells, and iv) low symmetry FLS's tend to find metastable structures on cooling.

cond-mat.soft↗

arXiv2014

Defect-Mediated Relaxation in the Random Tiling Phase of a Binary Mixture: Birth, Death and Mobility of an Atomic Zipper

This paper describes the mechanism of defect-mediated relaxation in a dodecagonal square-triangle random tiling phase exhibited by a simulated binary mixture of soft discs in 2D. We examine the internal transitions within the elementary mobile defect (christened the 'zipper') that allow it to move, as well as the mechanisms by which the zipper is created and annihilated. The structural relaxation of the random tiling phase is quantified and we show that this relaxation is well described by a model based on the distribution of waiting times for each atom to be visited by the diffusing zipper. This system, representing one of the few instances where a well defined mobile defect is capable of structural relaxation, can provide a valuable test case for general theories of relaxation in complex and disordered materials.

cond-mat.soft↗

arXiv2013

Multiple ordering transitions in a chiral liquid

We present here a numerical study of a lattice model of a chiral liquid. The low symmetry of the favoured local structure depresses the freezing point to reveal an exotic liquid-liquid transition characterised by the appearance of an extended chirality,prior to freezing. What mechanisms impede crystallisation in liquids with low molecular symmetry ? The ordered liquid can be readily supercooled to zero temperature, as the combination of critical slowing down and competing crystal polymorphs results in a dramatically slow crystallisation process.

cond-mat.soft↗

arXiv2013

Freezing Transitions in Liquids Characterized by a Favoured Local Structure

The ordering transitions of a 2D lattice liquid characterized by a single favoured local structure (FLS) are studied using Monte Carlo simulations. All eight distinct geometries for the FLS are considered and we find a variety of ordering transitions - first order, continuous and multi-step transitions. Using a microcanonical representation of the freezing transition we resolve the dual influence of the local structure on the ordering transition, i.e. via the energy of the crystal and the entropy cost of structure in the liquid.

cond-mat.stat-mech↗

arXiv2012

The Origin of Persistent Shear Stress in Supercooled Liquids

We show that the long time tail of the shear stress autocorrelation, whose growth at large supercooling is responsible for the apparent divergence of the shear viscosity, is a direct result of a residual shear stress in the structures associated with the local potential minima. We argue that the essential mechanical transition experienced by a liquid on cooling occurs at a temperature well above the glass transition temperature and corresponds to the crossover from the high temperature liquid to the viscous liquid, the latter characterised by stress relaxation dominated by the residual stress. Following on from this observation, as the density is decreased, the local potential minima become unable to sustain any persistent stress (and, hence, support a glass transition), in a manner that can be explicitly connected to the interactions between atoms. The reported crossover implies an associated change in the mechanism of dissipation in liquids and, hence, raises the prospect of a coherent microscopic treatment of nonlinear rheology and the relationship between self diffusion and viscosity in supercooled liquids.

cond-mat.soft↗

arXiv2011

Structural Phases in Non-Additive Soft-Disk Mixtures: Glasses, Substitutional Order, and Random Tilings

Relaxation of the additivity condition on the interaction length between unlike species in a binary mixture of soft disks opens up a rich variety of structures in both crystal and amorphous states with an associated diverse range of relaxation dynamics. We report on MD simulation studies of binary soft disks with negative deviations from additivity that include evidence of accumulation of crystal-like structures in metastable liquids prior to crystallization and the occurrence of a liquid to random-tiling transition.

cond-mat.soft↗

arXiv2010

Crystallization of the Lewis-Wahnström ortho-terphenyl model

Crystallization is observed during long molecular dynamics simulations of bent trimers, a molecular model proposed by Lewis and Wahnström for ortho-terphenyl. In the crystal, the three spheres that make up the rigid molecule sit near sites of a body centered cubic lattice, the trimer bond angle being almost optimal for this structure. The crystal exhibits orientational disorder with the molecules aligned randomly along the three Cartesian axis (an example of cubatic orientational order). The rotational and translational mobilities exhibit only modest decreases on crystallization, by factors of 10 and 3 respectively. The rotational relaxation does change from Debye-like in the liquid to large angle jumps in the crystal. We consider the origin of the superior glass forming ability of the the trimer over the monatomic liquid.

cond-mat.soft↗

arXiv2010

The Length Scales of Dynamic Heterogeneity: Results from Molecular Dynamics Simulations

Over times shorter than that required for relaxation of enthalpy, a liquid can exhibit striking heterogeneities. The picture of these heterogeneities is complex with transient patches of rigidity, irregular yet persistent, intersected by tendrils of mobile particles, flickering intermittently into new spatial patterns of motion and arrest. The study of these dynamic heterogeneities has, over the last 20 years, allowed us to characterize cooperative dynamics, to identify new strategies in controlling kinetics in glass-forming liquids and to begin to systematically explore the relationship between dynamics and structure that underpins the behaviour of amorphous materials. Computer simulations of the dynamics in atomic and molecular liquids have played a dominant role in all of this progress. While some may be uneasy about this reliance on modelling, it is unavoidable, given the amount of microscopic detail needed to characterize the dynamic heterogeneities. The complexities revealed by these simulations have called for new conceptual tools. In this essay, I have tried to provide the reader with a clear and complete account of how these tools have been developed in terms of the literature on kinetic length scales in molecular dynamics simulations. Through the `prism' of these length scales, this essay addresses the question what have we learnt about dynamic heterogeneities from computer simulations?

cond-mat.soft↗

arXiv2010

Macroscopic Facilitation of Glassy Relaxation Kinetics: Ultra Stable Glass Films with Front-Like Thermal Response

The recent experimental fabrication of ultra stable glass films via vapour deposition [Science 315, 353 (2007)] and the observation of front-like response to the annealing of these films [Phys.Rev.Lett. 102, 065503 (2009)], have raised important questions about the possibility of manipulating the properties of glass films and addressing fundamental questions about kinetics and thermodynamics of amorphous materials. Central to both of these issues is the need to establish the essential physics that governs the formation of the ultra-stable films and their subsequent response. In this paper we demonstrate that a generic model of glassy dynamics can account for the formation of ultra-stable films, the associated enhancement of relaxation rates by a factor of 10^5, the observation of front-like response to temperature jumps and the temperature dependence of the front velocity

cond-mat.soft↗

arXiv2009

Time Scales in the Theory of Elasto-Plasticity of Amorphous Solids

Developing a macroscopic theory of elasto-plasticity in amorphous solids calls for (i) identifying the relevant macro state-variables and (ii) discriminating the different time-scales which characterize these variables. In current theories it is assumed that the stress reaches its elasto-plastic steady state value on the same time-scale as the configurational variables (be they the configurational energy, configurational entropy or the effective temperature). By examining numerical simulations in two and three dimensions we show that this is generally not the case, the configurational degrees of freedom may reach the elasto-plastic steady state on the time scales which can be very different from the time scale of the stress. We provide a physical discussion to rationalize these findings.

cond-mat.stat-mech↗

arXiv2009

The Geometry of Slow Structural Fluctuations in a Supercooled Binary Alloy

The liquid structure of a glass-forming binary alloy is studied using molecular dynamics simulations. The analysis combines common neighbour analysis with the geometrical approach of Frank and Kasper to establish that the supercooled liquid contains extended clusters characterised by the same short range order as the crystal. Fluctuations in these clusters exhibit strong correlations with fluctuations in the inherent structure energy. The steep increase in the heat capacity on cooling is, thus, directly coupled to the growing fluctuations of the Frank-Kasper clusters. The relaxation of particles in the clusters dominates the slow tail of the self-intermediate scattering function.

cond-mat.soft↗

arXiv2009

The central role of thermal collective strain in the relaxation of structure in a supercooled liquid

The spatial distribution of structural relaxation in a supercooled liquid is studied using molecular dynamics simulations of a 2D binary mixture. It is shown that the spatial heterogeneity of the relaxation along with the time scale of the relaxation is determined, not by the frequency with which particles move a distance pi/2kBragg, but by the frequency with which particles can achieve persistent displacements. We show that these persistent displacements are achieved through the coupled action of local reorganizations and unrecoverable thermal strains.

cond-mat.soft↗

arXiv2009

Localised soft modes and the supercooled liquid's irreversible passage through its configuration space

Using computer simulations, we show that the localized low frequency normal modes of a configuration in a supercooled liquid are strongly correlated with the irreversible structural reorganization of the particles within that configuration. Establishing this correlation constitutes the identification of the aspect of a configuration that determines the heterogeneity of the subsequent motion. We demonstrate that the spatial distribution of the summation over the soft local modes can persist in spite of particle reorganization that produces significant changes in individual modes. Along with spatial localization, the persistent influence of soft modes in particle relaxation results in anisotropy in the displacements of mobile particles over the timescale referred to as beta relaxation.

cond-mat.soft↗

arXiv2009

On the relationship between structure and dynamics in a supercooled liquid

We present the dynamic propensity distribution as an explicit measure of the degree to which the dynamics in a liquid over the time scale of structural relaxation is determined by the initial configuration. We then examine, for a binary mixture of soft discs in two dimensions, the correlation between the spatial distribution of propensity and that of two localmeasures of configuration structure: the local composition and local free volume. While the small particles dominate the high propensity population,we find no strong correlation between either the local composition or the local free volume and the propensity. It is argued that this is a generic failure of purely local structural measures to capture the inherently non-local character of collective behaviour.

cond-mat.stat-mech↗

arXiv2009

On the Study of Collective Dynamics in Supercooled Liquids through the Statistics of the Iso-Configurational Ensemble

The use of the isoconfigurational ensemble to explore structure-dynamic correlations in supercooled liquids is examined. The statistical error of the dynamic propensity and its spatial distribution are determined. The authors present the spatial distribution of the particle non-Gaussian parameter as a measure of the intermittency with which particles exhibit their propensity for motion. The ensemble average of the direction of particle motion is introduced to establish the anisotropy of the dynamic propensity.

cond-mat.stat-mech↗

arXiv2009

Irreversible reorganization in a supercooled liquid originates from localised soft modes

The transition of a fluid to a rigid glass upon cooling is a common route of transformation from liquid to solid that embodies the most poorly understood features of both phases1,2,3. From the liquid perspective, the puzzle is to understand stress relaxation in the disordered state. From the perspective of solids, the challenge is to extend our description of structure and its mechanical consequences to materials without long range order. Using computer simulations, we show that the localized low frequency normal modes of a configuration in a supercooled liquid are causally correlated to the irreversible structural reorganization of the particles within that configuration. We also demonstrate that the spatial distribution of these soft local modes can persist in spite of significant particle reorganization. The consequence of these two results is that it is now feasible to construct a theory of relaxation length scales in glass-forming liquids without recourse to dynamics and to explicitly relate molecular properties to their collective relaxation.

cond-mat.soft↗

arXiv2005

Predicting the long time dynamic heterogeneity in a supercooled liquid on the basis of short time heterogeneities

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.

cond-mat.stat-mech↗

arXiv2005

Free Volume cannot Explain the Spatial Heterogeneity of Debye-Waller factors in a Glass-Forming Binary Alloy

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.

cond-mat.dis-nn↗