29 research outputs found
Combined energy -- diffraction data refinement of decagonal AlNiCo
We incorporate realistic pair potential energies directly into a non-linear
least-square fit of diffraction data to quantitatively compare structure models
with experiment for the Ni-rich (AlNiCo) quasicrystal. The initial structure
models are derived from a few {\it a priori} assumptions (gross features of the
Patterson function) and the pair potentials. In place of the common hyperspace
approach to the structure refinement of quasicrystals, we use a real-space tile
decoration scheme, which does not rely on strict quasiperiodicity, and makes it
easy to enforce sensible local arrangements of the atoms. Inclusion of the
energies provides information complementary to the diffraction data and
protects the fit procedure from converging on spurious solutions. The method
pinpoints sites which are likely to break the symmetry of their local
environment.Comment: 7 pages, 5 figures, proceedings of the Internation Conference on
Quasicrystals, Bangalore, India, August 200
Repton model of gel electrophoresis in the long chain limit
Reptation governs motion of long polymers through a confining environment.
Slack enters at the ends and diffuses along the polymer as stored length. The
rate at which stored length diffuses limits the speed at which the chain can
drift. This paper relates the rate of stored length diffusion to the
conformation of the tube within which the polymer is confined. In the scaling
limit of long polymer chains and weak applied electric fields, holding the
product of polymer length times field finite, the tube length and stored length
density take on their zero-field values. The drift velocity then depends only
on the the polymer's end-to-end separation in the direction of the field.Comment: 14 pages,4 figure
Vitrification of a monatomic 2D simple liquid
A monatomic simple liquid in two dimensions, where atoms interact
isotropically through the Lennard-Jones-Gauss potential [M. Engel and H.-R.
Trebin, Phys. Rev. Lett. 98, 225505 (2007)], is vitrified by the use of a rapid
cooling technique in a molecular dynamics simulation. Transformation to a
crystalline state is investigated at various temperatures and the
time-temperature-transformation (TTT) curve is determined. It is found that the
transformation time to a crystalline state is the shortest at a temerature 14%
below the melting temperature Tm and that at temperatures below Tv = 0.6 Tm the
transformation time is much longer than the available CPU time. This indicates
that a long-lived glassy state is realized for T < Tv.Comment: 5pages,5figures,accepted for publication in CEJ
Clusters, phason elasticity, and entropic stabilisation: a theory perspective
Personal comments are made about the title subjects, including: the relation
of Friedel oscillations to Hume-Rothery stabilisation; how calculations may
resolve the random-tiling versus ideal pictures of quasicrystals; and the role
of entropies apart from tile-configurational.Comment: IOP macros; 8pp, 1 figure. In press, Phil. Mag. A (Proc. Intl. Conf.
on Quasicrystals 9, Ames Iowa, May 2005
Total-energy-based prediction of a quasicrystal structure
Quasicrystals are metal alloys whose noncrystallographic symmetry and lack of
structural periodicity challenge methods of experimental structure
determination. Here we employ quantum-based total-energy calculations to
predict the structure of a decagonal quasicrystal from first principles
considerations. We employ Monte Carlo simulations, taking as input the
knowledge that a decagonal phase occurs in Al-Ni-Co near a given composition,
and using a few features of the experimental Patterson function. The resulting
structure obeys a nearly deterministic decoration of tiles on a hierarchy of
length scales related by powers of , the golden mean.Comment: 9 pages, 3 figure
Coarse-grained simulation of transmembrane peptides in the gel phase
We use Dissipative Particle Dynamics simulations, combined with parallel tempering and umbrella sampling, to investigate the potential of mean force between model transmembrane peptides in the various phases of a lipid bilayer, including the low-temperature gel phase.
The observed oscillations in the effective interaction between peptides are consistent with the different structures of the surrounding lipid phases