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Sep 18, 2013
09/13
by
G. J. Ackland
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This paper reviews the status of molecular dynamics as a method in describing solid-solid phase transitions, and its relationship to continuum approaches. Simulation work done in NiTi and Zr using first principles and semi-empirical potentials is presented. This shows failures of extending equilibrium thermodynamics to the nanoscale, and the crucial importance of system-specific details to the dynamics of martensite formation. The inconsistency between experimental and theoretical crystal...
Source: http://arxiv.org/abs/cond-mat/0509456v1
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50
Sep 18, 2013
09/13
by
G. J. Ackland
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I discuss the behaviour of Group 1 and 2 elements under pressure, describing crystallography based on diamond anvil pressure cell experiments and electronic structure calculations. In addition to these precise methods, we discuss more intuitive pictures of both the electronic driving force for the transitions, and the structural motifs which optimise the electronic energy by opening pseudogaps at the Fermi energy.
Source: http://arxiv.org/abs/cond-mat/0509454v1
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7.0
Jun 29, 2018
06/18
by
G. J. Ackland
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Long wavelength acoustic phonons are normally weakly coupled to other vibrational modes in a crystalline system. This is particularly problematic in molecular dynamics calculations where vibrations at the system size scale are typically excited at initiation. The equilibration time for these vibrations depends on the strength of coupling to other modes, so is typically very long. A very simple deterministic method which avoids the use of is presented which removes this problem.
Topics: Soft Condensed Matter, Condensed Matter
Source: http://arxiv.org/abs/1609.01980
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42
Sep 22, 2013
09/13
by
G. J. Ackland
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The devil's staircase is a term used to describe surface or an equilibrium phase diagram in which various ordered facets or phases are infinitely closely packed as a function of some model parameter. A classic example is a 1-D Ising model [bak] wherein long-range and short range forces compete, and the periodicity of the gaps between minority species covers all rational values. In many physical cases, crystal growth proceeds by adding surface layers which have the lowest energy, but are then...
Source: http://arxiv.org/abs/cond-mat/0205102v1
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70
Sep 22, 2013
09/13
by
G. J. Ackland
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We investigate whether the equilibrium time averaged state of a self-organising system with many internal degrees of freedom, 2D- daisyworld, can be described by optimising a single quantity. Unlike physical systems where a principle of maximum energy production has been observed, Daisyworld follows evolutionary dynamics rather than Hamiltonian dynamics. We find that this is sufficient to invalidate the MEP principle, finding instead a different principle, that the system self-organises to a...
Source: http://arxiv.org/abs/cond-mat/0307567v1
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Sep 20, 2013
09/13
by
G. J. Ackland
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The process of deriving an interatomic potentials represents an attempt to integrate out the electronic degrees of freedom from the full quantum description of a condensed matter system. In practice it is the derivatives of the interatomic potentials which are used in molecular dynamics, as a model for the forces on a system. These forces should be the derivative of the free energy of the electronic system, which includes contributions from the entropy of the electronic states. This free energy...
Source: http://arxiv.org/abs/1205.6684v1
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Sep 19, 2013
09/13
by
L. Mitchell; G. J. Ackland
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We present a simple one-parameter model for spatially localised evolving agents competing for spatially localised resources. The model considers selling agents able to evolve their pricing strategy in competition for a fixed market. Despite its simplicity, the model displays extraordinarily rich behavior. In addition to ``cheap'' sellers pricing to cover their costs, ``expensive'' sellers spontaneously appear to exploit short-term favorable situations. These expensive sellers ``speciate'' into...
Source: http://arxiv.org/abs/physics/0703156v2
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47
Sep 23, 2013
09/13
by
Lawrence Mitchell; G. J. Ackland
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We show that a simple model of a spatially resolved evolving economic system, which has a steady state under simultaneous updating, shows stable oscillations in price when updated asynchronously. The oscillations arise from a gradual decline of the mean price due to competition among sellers competing for the same resource. This lowers profitability and hence population but is followed by a sharp rise as speculative sellers invade the large un-inhabited areas. This cycle then begins again.
Source: http://arxiv.org/abs/0801.3973v2
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Sep 23, 2013
09/13
by
G. J. Ackland; H. Fox
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We present calculation of the high pressure crystal structures in selenium, including rational approximants to the recently reported incommensurate phases. We show how the incommensurate phases can be intuitively explained in terms of imaginary phonon frequencies arising from Kohn anomalies in the putative undistorted phase. We also find inconsistencies between the calculated and experimental Se-II phase - the calculations show it to be a metastable metal while the experiment finds a stable...
Source: http://arxiv.org/abs/cond-mat/0408539v1
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Sep 20, 2013
09/13
by
N. D. Drummond; G. J. Ackland
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We introduce a method for treating soft modes within the analytical framework of the quasiharmonic equation of state. The corresponding double-well energy-displacement relation is fitted to a functional form that is harmonic in both the low- and high-energy limits. Using density-functional calculations and statistical physics, we apply the quasiharmonic methodology to solid periclase. We predict the existence of a B1--B2 phase transition at high pressures and temperatures.
Source: http://arxiv.org/abs/cond-mat/0204005v1
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50
Sep 22, 2013
09/13
by
M. C. Warren; G. J. Ackland
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Density-functional simulations are used to calculate structural properties and high-symmetry phonons of the hypothetical cubic phase, the stable orthorhombic phase and an intermediate tetragonal phase of magnesium silicate perovskite. We show that the structure of the stable phase is well described by freezing in a small number of unstable phonons into the cubic phase. We use the frequencies of these unstable modes to estimate transition temperatures for cubic--tetragonal and...
Source: http://arxiv.org/abs/mtrl-th/9503001v1
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51
Sep 19, 2013
09/13
by
A. N. Jackson; G. J. Ackland
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We show how to generalize the Lattice Switch Monte Carlo method to calculate the phase diagram of a binary system. A global coordinate transformation is combined with a modification of particle diameters, enabling the multi-component system in question to be explored and directly compared to a suitable reference state in a single Monte Carlo simulation. We use the method to evaluate the free energies of binary hard sphere crystals. Calculations at moderate size ratios, \alpha=0.58 and...
Source: http://arxiv.org/abs/0708.0497v1
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4.0
Jun 29, 2018
06/18
by
T. L. Underwood; G. J. Ackland
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Lattice-switch Monte Carlo (LSMC) is a method for evaluating the free energy between two given solid phases. LSMC is a general method, being applicable to a wide range of problems and interatomic potentials. Furthermore it is extremely efficient, ostensibly more efficient than other existing general methods. Here we introduce a package, monteswitch, which can be used to perform LSMC simulations. The package can be used to evaluate the free energy differences between pairs of solid phases,...
Topics: Computational Physics, Physics
Source: http://arxiv.org/abs/1609.04329
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69
Sep 23, 2013
09/13
by
G. J. Galloway; G. J. Ackland
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We show that voids adjacent to radiation damage cascades can be moved in their entirety by several lattice spacings. This is done using molecular dynamics cascade simulations in iron at energies of 1-5 keV. The effect of this mechanism is studied further using an OKMC code and shows enhancement of void diffusion by 2 orders of magnitude from 1x10^-22 cm^2/s to 3x10^-20 cm^2/s. Repeating the study on He bubbles shows that the movement is damped by the presence of helium in the void.
Source: http://arxiv.org/abs/1212.1652v2
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10.0
Jun 30, 2018
06/18
by
T. L. Underwood; G. J. Ackland
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Lattice-switch Monte Carlo is an efficient method for calculating the free energy difference between two solid phases, or a solid and a fluid phase. Here, we provide a brief introduction to the method, and list its applications since its inception. We then describe a lattice switch for the fcc and bcc phases based on the Bain orientation relationship. Finally, we present preliminary results regarding our application of the method to the fcc and bcc phases in the Lennard-Jones system. Our...
Topics: Statistical Mechanics, Condensed Matter
Source: http://arxiv.org/abs/1412.1450
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46
Sep 21, 2013
09/13
by
G. J. Ackland; E. S. Tweedie
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A microscopic model of the effect of unbinding in diffusion limited aggregation based on a cellular automata approach is presented. The geometry resembles electrochemical deposition - ``ions'' diffuse at random from the top of a container until encountering a cluster in contact with the bottom, to which they stick. The model exhibits dendritic (fractal) growth in the diffusion limited case. The addition of a field eliminates the fractal nature but the density remains low. The addition of...
Source: http://arxiv.org/abs/cond-mat/0508675v1
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6.0
Jun 30, 2018
06/18
by
G. J. Ackland; I. B. Magdau
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We present and evaluate an efficient method for simulating Raman spectra from molecular dynamics (MD) calculations {\it without} defining normal modes. We apply the method to high pressure hydrogen in the high-temperature "Phase IV": a plastic crystal in which the conventional picture of fixed phonon eigenmodes breaks down. Projecting trajectories onto in-phase molecular stretches is shown to be many orders of magnitude faster than polarisability calculations, allowing statistical...
Topics: Materials Science, Condensed Matter
Source: http://arxiv.org/abs/1408.3972
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54
Sep 23, 2013
09/13
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I. Pagonabarraga; M. E. Cates; G. J. Ackland
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The structure of polydisperse hard sphere fluids, in the presence of a wall, is studied by the Rosenfeld density functional theory. Within this approach, the local excess free energy depends on only four combinations of the full set of density fields. The case of continuous polydispersity thereby becomes tractable. We predict, generically, an oscillatory size segregation close to the wall, and connect this, by a perturbation theory for narrow distributions, with the reversible work for changing...
Source: http://arxiv.org/abs/cond-mat/9909339v1
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51
Sep 21, 2013
09/13
by
D. J. Hepburn; G. J. Ackland; P. Olsson
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We present empirical potentials for dilute transition metal solutes in a-iron. It is in the Finnis-Sinclair form and is therefore suitable for billion atom molecular dynamics simulations. First principles calculation shows that there are clear trends across the transition metal series which enable us to relate the rescaling parameters to principal quantum number and number of d electrons. The potential has been developed using a rescaling technique to provide solute-iron and solute-solute...
Source: http://arxiv.org/abs/1008.0508v1
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3.0
Jun 30, 2018
06/18
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D. J. Hepburn; E. MacLeod; G. J. Ackland
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We present a comprehensive set of first principles electronic structure calculations to study transition metal solutes and their interactions with point defects in austenite. Clear trends were observed across the series. Solute-defect interactions were strongly correlated to the solute size factors, consistent with local strain field effects. Strong correlations with results in ferrite show insensitivity to the underlying crystal structure in Fe. Oversized solutes act as strong traps for...
Topics: Materials Science, Condensed Matter
Source: http://arxiv.org/abs/1407.7996
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47
Sep 17, 2013
09/13
by
G. J. Ackland; D. J. Hepburn; J. Wallenius
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}We present a formalism for extending the second moment tight-binding model, incorporating ferro- and anti-ferromagn etic interaction terms which are needed for the FeCr system. For antiferromagnetic and paramagnetic materials, an explicit additional variable representing the spin is required. In a mean-field approximation this spin can be eliminated, and the potential becomes explicitly temperature dependent. For ferromagnetic interactions, this degree of freedom can be eliminated, and the...
Source: http://arxiv.org/abs/0912.3439v1
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Sep 22, 2013
09/13
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G. J. Ackland; I. R. Macleod; O. Degtyareva
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We present a unifying theory for the observed complex structures of the sp-bonded elements under pressure based on nearly free electron picture (NFE). In the intermediate pressure regime the dominant contribution to crystal structure arises from Fermi-surface Brillouin zone (FSBZ) interactions - structures which allow this are favoured. This simple theory explains the observed crystal structures, transport properties, the evolution of internal and unit cell parameters with pressure. We...
Source: http://arxiv.org/abs/cond-mat/0310380v1
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4.0
Jun 30, 2018
06/18
by
T. L. Underwood; G. J. Ackland; R. J. Cole
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In alloys exhibiting substitutional disorder, the variety of atomic environments manifests itself as a `disorder broadening' in their core level binding energy spectra. Disorder broadening can be measured experimentally, and in principle can be used to deduce information about specific atomic environments within a sample. However, progress in this endeavor is hampered by the lack of a model for this phenomenon which can treat complex systems. In this work we describe such a model. The model is...
Topics: Materials Science, Condensed Matter
Source: http://arxiv.org/abs/1402.2201
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47
Sep 21, 2013
09/13
by
D. J. Hepburn; D. Ferguson; S. Gardner; G. J. Ackland
texts
eye 47
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An extensive set of first-principles density functional theory calculations have been performed to study the behaviour of He, C and N solutes in austenite, dilute Fe-Cr-Ni austenitic alloys and Ni in order to investigate their influence on the microstructural evolution of austenitic steel alloys under irradiation.
Source: http://arxiv.org/abs/1301.5317v1
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105
Jul 20, 2013
07/13
by
T. P. C. Klaver; D. J. Hepburn; G. J. Ackland
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We present results of an extensive set of first-principles density functional theory calculations of point defect formation, binding and clustering energies in austenitic Fe with dilute concentrations of Cr and Ni solutes.
Source: http://arxiv.org/abs/1107.0260v2
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Jul 20, 2013
07/13
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A. D. Bruce; A. N. Jackson; G. J. Ackland; N. B. Wilding
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We present a Monte Carlo method for the direct evaluation of the difference between the free energies of two crystal structures. The method is built on a lattice-switch transformation that maps a configuration of one structure onto a candidate configuration of the other by `switching' one set of lattice vectors for the other, while keeping the displacements with respect to the lattice sites constant. The sampling of the displacement configurations is biased, multicanonically, to favor paths...
Source: http://arxiv.org/abs/cond-mat/9910330v1
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169
Jul 20, 2013
07/13
by
G. J. Ackland; M. I. Mendelev; D. Srolovitz; S. Han; A. V. Barashev
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We present the derivation of an interatomic potential for the iron phosphorus system based primarily on {\it ab initio} data. Transferrability in this system is extremely problematic, and the potential is intended specifically to address the problem of radiation damage and point defects in iron containing low concentrations of phosphorus atoms. Some preliminary molecular dynamics calculations show that P strongly affects point defect migration.
Source: http://arxiv.org/abs/cond-mat/0406356v1
