13 research outputs found
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
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
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
Transition-metal interactions in aluminum-rich intermetallics
The extension of the first-principles generalized pseudopotential theory
(GPT) to transition-metal (TM) aluminides produces pair and many-body
interactions that allow efficient calculations of total energies. In
aluminum-rich systems treated at the pair-potential level, one practical
limitation is a transition-metal over-binding that creates an unrealistic TM-TM
attraction at short separations in the absence of balancing many-body
contributions. Even with this limitation, the GPT pair potentials have been
used effectively in total-energy calculations for Al-TM systems with TM atoms
at separations greater than 4 AA. An additional potential term may be added for
systems with shorter TM atom separations, formally folding repulsive
contributions of the three- and higher-body interactions into the pair
potentials, resulting in structure-dependent TM-TM potentials. Towards this
end, we have performed numerical ab-initio total-energy calculations using VASP
(Vienna Ab Initio Simulation Package) for an Al-Co-Ni compound in a particular
quasicrystalline approximant structure. The results allow us to fit a
short-ranged, many-body correction of the form a(r_0/r)^{b} to the GPT pair
potentials for Co-Co, Co-Ni, and Ni-Ni interactions.Comment: 18 pages, 5 figures, submitted to PR
Total-energy-based prediction of a quasicrystal structure
Consiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7 Rome / CNR - Consiglio Nazionale delle RichercheSIGLEITItal