arXiv subjects

Peter Harrowell

Publications and source records attributed to Peter Harrowell.

At least 19 recordsLinked to original sources

arXiv2026

What Lies Between Crystal and Randomly Packed Structures? A General Characterization of Non-Periodic Order

In this paper we address the characterization of the structure of condensed materials, periodic and non-periodic. Carrying out an extensive study of over 7000 different groundstate structures of a 2D lattice model of binary packing, we find a predominance of non-periodic structures (over 96%) that extend across the entire range of possible diversities. These non-periodic structures are resolved by establishing whether a structure will accommodate or reject additional local structures. This property, structural selectivity, is treated as a signature of an underlying ordering principle. The major result of the paper is the determination that roughly 35% of the non-periodic structures are selective and, hence, ordered in some way. This selectivity extends up to a diversity of ~ 9, well beyond the upper threshold for diversity in periodically ordered states.

cond-mat.soft↗

arXiv2024

Structural Diversity in Condensed Matter: A General Characterization of Crystals, Amorphous Solids and the Structures Between

A definition of structural diversity, adapted from the biodiversity literature, is introduced to provide a general characterization of structures of condensed matter. Using the Favored Local Structure (FLS) lattice model as a testbed, the diversity measure is found to effectively filter extrinsic noise and to provide a useful differentiation between crystal and amorphous structures. We identify an interesting class of structures intermediate between crystals and glasses that are characterised by a complex combination of short-range ordering and long-range disorder. We demonstrate how the diversity can be used as an order parameter to organise various scenarios by structure change in response to increasing diversity.

cond-mat.mtrl-sci↗

arXiv2023

The Structural Difference Between Strong and Fragile Liquids

A structural order parameter for disordered configurations is defined, based, not on local topologies, but on the degree of local restraint imposed on each atom. This restraint parameter provides a clear distinction between a strong liquid (SiO2) and a fragile liquid (a binary Lennard-Jones mixture) without reference to dynamics. Where the fragile liquid exhibits an abrupt transition from unrestrained to restrain below the melting point, molten silica is highly constrained even at equilibrium. We determine the temperature dependence of the average number of particles restrained per pinned particle and consider the feature of the potential energy landscape responsible for determining the fragility.

cond-mat.soft↗

arXiv2023

Translation-Rotation Coupling and the Kinematics of Non-Slip Boundary Conditions: A Rough Sphere between Two Sliding Walls

A non-slip constraint between a particle and a wall is applied at the microscopic level of collision dynamics using the rough sphere model. We analyse the consequences of the translation-rotation coupling of the rough sphere confined between two parallel planar walls and establish that shearing the walls past each other i) preferentially deposits energy into the rotational degree of freedom and ii) results in a bounded oscillation of the energy of the confined particle.

physics.chem-ph↗

arXiv2023

Amorphous Solidification of a Supercooled Liquid in the Limit of Rapid Cooling

We monitor the transformation of a liquid into an amorphous solid in simulations of a glass forming liquid by measuring the variation of a structural order parameter with either changing temperature or potential energy to establish the influence of the cooling rate on amorphous solidification. We show that the latter representation, unlike the former, exhibits no significant dependence on cooling rate. This independence extends to the limit of instantaneous quenches which we find can accurately reproduce the solidification observed during slow cooling. We conclude that amorphous solidification is an expression of the topography of the energy landscape and present the relevant topographic measures.

cond-mat.soft↗

arXiv2023

Crystal Growth Rates from Molecular Liquids: The Kinetics of Entropy Loss

It has been established empirically that the rate of addition of molecules to the crystal during crystal growth from the melt is proportional to exp(-|{\Delta}S_fus|/R) where {\Delta}S_fus is the entropy of fusion. Here we show that this entropic slowdown arises directly from the separation of the entropy loss and energy loss processes associated with the freezing of the liquid. We present a theoretical treatment of the kinetics based on a model flat energy landscape and derive an explicit expression for the coupling magnitude in terms of the crystal-melt interfacial free energy. The implications of our work for nucleation kinetics are also discussed.

cond-mat.soft↗

arXiv2023

Direct Measurement of the Structural Change Associated with Amorphous Solidification using Static Scattering of Coherent Radiation

In this paper we demonstrate that the weak temperature dependence of structure factor of supercooled liquids, a defining feature of the glass transition, is a consequence of the averaging of the scattering intensity either due to the use of an incoherent radiation source or explicit angular averaging. We show that the speckle scattering at individual wavevectors, calculated from a simulated glass former, exhibits a Debye-Waller factor with a sufficiently large temperature dependence to represent a structural order parameter capable of distinguishing liquid from glass. We also extract from the speckle intensities a quantity proportional to the variance of the local restraint, i.e. a direct experimental measure of the amplitude of structural heterogeneity.

cond-mat.mtrl-sci↗

arXiv2021

A General Structural Order Parameter for the Amorphous Solidification of a Supercooled Liquid

The persistent problem posed by the glass transition is to develop a general atomic level description of a solidification process that is not associated with any change in the symmetry of the atomic structure. The answer proposed in this paper is t measure a configuration's capacity to restrain the motion of the constituent atoms. Here we show that the instantaneous normal modes can be used to define a measure of atomic restraint that accounts for the difference between fragile and strong liquids and the collective length scale of the supercooled liquid. These results represent a significant simplification of the description of amorphous solidification and provide a powerful systematic treatment of the influence of microscopic factors on the formation of the amorphous solid.

cond-mat.soft↗

arXiv2021

The Influence on Crystal Nucleation of an Order-Disorder Transition among the Subcritical Clusters

Studies of nucleation generally focus on the properties of the critical cluster, but the presence of defects within the crystal lattice means that the population of nuclei necessarily evolve through a distribution of pre-critical clusters with varying degrees of structural disorder on their way to forming a growing stable crystal. To investigate the role pre-critical clusters play in nucleation, we develop a simple thermodynamic model for crystal nucleation in terms of cluster size and the degree of cluster order that allows us to alter the work of forming the pre-critical clusters without effecting the properties of the critical cluster. The steady state and transient nucleation behaviour of the system are then studied numerically, for different microscopic ordering kinetics. We find that the models exhibits a generic order-disorder transition in the pre-critical clusters. Independent of the type of ordering kinetics, increasing the accessibility of disordered pre-critical clusters decreases both the steady state nucleation rate and the nucleation lag time. Furthermore, the interplay between the free energy surface and the microscopic ordering kinetics leads to three distinct nucleation pathways.

cond-mat.soft↗

arXiv2021

The Structure and Thermodynamic Stability of Reverse Micelles in Dry AOT/Alkane Mixtures

Monte Carlo simulation studies of reverse micelles of an anionic surfactant, sodium AOT, in a non-polar solvent provide strong evidence that, in the absence of water, these clusters are charge ordered polyhedral shells. The stabilizing energy of these clusters is so large that the entropy of mixing is, in comparison, inconsequential and we predict that, if all waters of hydration could be removed (something not yet accomplished for the sodium salt) then AOT would be insoluble in nonpolar solvents.

cond-mat.soft↗

arXiv2021

Translational-Rotational Coupling during the Scattering of a Frictional Sphere from a Flat Surface

At a macroscopic level, concepts such as top spin, back spin and rolling are commonly used to describe the collision of balls and surfaces. Each term refers to an aspect of the coupling of rotational motion during the collision of a spherical particle with a planar surface. In this paper we explore the mechanisms of energy transfer involving the collision of a rotating sphere and a surface using a model of frictional interactions developed for granular material. We present explicit analytical treatments for the scattering and derive expressions for two important limiting classes: energy conserving collisions and collisions subject to rapid transverse dissipation.

cond-mat.soft↗

arXiv2021

Deposition control of model glasses with surface-mediated orientational order

We introduce a minimal model of solid-forming anisotropic molecules that displays, in thermal equilibrium, surface orientational order without bulk orientational order. The model reproduces the nonequilibrium behavior of recent experiments in that a bulk nonequilibrium structure grown by deposition contains regions of orientational order characteristic of the surface equilibrium. This order is deposited in general in a nonuniform way, because of the emergence of a growth-poisoning mechanism that causes equilibrated surfaces to grow slower than non-equilibrated surfaces. We use evolutionary methods to design oscillatory protocols able to grow nonequilibrium structures with uniform order, demonstrating the potential of protocol design for the fabrication of this class of materials.

cond-mat.stat-mech↗

arXiv2021

How Real are Liquid Groundstates? Ultra-Fast Crystal Growth and the Susceptibility of Energy Minima in Liquids

We calculate the degree to which the final structure of the local groundstate in a liquid is a function of the strength of a perturbing potential applied during energy minimization. This structural susceptibility is shown to correlate well with the observed tendency of liquid adjacent to a crystal interface to exhibit a crystalline groundstate, a feature that has been strongly linked to the observation of ultra-fast crystal growth in pure metals and ionic melts. It is shown that the structural susceptibility increases dramatically as the interaction potential between atoms is softened.

cond-mat.soft↗

arXiv2020

How a Supercooled Liquid Borrows Structure from the Crystal

Using computer simulations, we establish that the structure of a supercooled binary atomic liquid mixture consists of common neighbour structures similar to those found in the equilibrium crystal phase, a Laves structure. Despite the large accumulation of crystal-like structure, we establish that the supercooled liquid represents a true metastable liquid and that liquid can borrow crystal structure without being destabilized. We consider whether this feature might be the origin of all instances of liquids of a strongly favoured local structure.

cond-mat.soft↗

arXiv2020

The Displacement Field Associated with the Freezing of a Melt and its Role in Determining Crystal Growth Kinetics

The atomic displacements associated with the freezing of metals and salts are calculated by treating crystal growth as an assignment problem through the use of an optimal transport algorithm. Converting these displacements into time scales based on the dynamics of the bulk liquid, we show that we can predict the activation energy for crystal growth rates, including activation energies significantly smaller than those for atomic diffusion in the liquid. The exception to this success, pure metals that freeze into face centred cubic crystals with little to no activation energy, are discussed. The atomic displacements generated by the assignment algorithm allows us to quantify the key roles of crystal structure and liquid caging length in determining the temperature dependence of crystal growth kinetics.

cond-mat.mtrl-sci↗

arXiv2019

What Stabilizes the Intermediate Structure of an Amorphous Alloy?

We present the results of simulation studies of a model binary metal-metalloid alloy in which we characterize and explain the local coordination structure, the intermediate structure associated with the packing of these coordination polyhedra and the thermal stability of the various structural elements of this model amorphous solid.

cond-mat.soft↗

arXiv2019

The Formation of Ultra-Stable Glasses via Precipitation: a Modelling Study

The precipitation of a glass forming solute from solution is modelled using a lattice model previously introduced to study dissolution kinetics of amorphous materials. The model includes the enhancement of kinetics at the surface of a glass in contact with a plasticizing solvent. We demonstrate that precipitation can produce a glass substantially more stable than that produced by very long time annealing of the bulk glass former. The energy of these ultra-stable amorphous precipitates is found to be dominated by residual solvent rather than high energy glass configurations.

cond-mat.soft↗

arXiv2019

The Role of Interfacial Inherent Structures in the Fast Crystal Growth from Molten Salts and Metals

Molecular dynamics simulations of the temperature dependent crystal growth rates of the salts, NaCl and ZnS, from their melts are reported, along with those of a number of pure metals. The growth rate of NaCl and the FCC-forming metals show little evidence of activated control, while that of ZnS and Fe, a BCC forming metal, exhibit activation barriers similar to those observed for diffusion in the melt. Unlike ZnS and Fe, the interfacial inherent structures of NaCl and Cu and Ag are found to be crystalline. We calculate the median displacement between the interfacial liquid and crystalline states and show that this distance is smaller than the cage length, demonstrating that crystal growth in the fast crystallizers can occur via local vibrations and so largely avoid the activated kinetics associated with the larger displacements associated with particle transport.

cond-mat.mtrl-sci↗