296 research outputs found
Evolution of the universality class in slightly diluted (1>p>0.8) Ising systems
The crossover of a pure (undiluted) Ising system (spin per site probability
p=1) to a diluted Ising system (spin per site probability p<0.8) is studied by
means of Monte Carlo calculations with p ranging between 1 and 0.8 at intervals
of 0.025. The evolution of the self averaging is analyzed by direct
determination of the normalized square widths for magnetization and
susceptibility as a function of p. We find a monotonous and smooth evolution
from the pure to the randomly diluted universality class. The p-dependent
transition is found to be independent of the size (L). This property is very
convenient for extrapolation towards the randomly diluted universality class
avoiding complications resulting from finite size effects.Comment: 15 pages, 6 figures, RevTe
The Structure of TGB Phases
We study the transition from the cholesteric phase to two TGB phases near
the upper critical twist : the Renn-Lubensky TGB phase, with layer
normal rotating in a plane perpendicular to the pitch axis, and the Bordeaux
TGB phase, with the layer normal rotating on a cone parallel to the pitch
axis. We calculate properties, including order-parameter profiles, of both
phases.Comment: 4 pages, 4 figures, Submitted to Physical Review E, Rapid
Communications, September 5, 2003; Revised manuscript (to the paper submitted
on March 18, 2003, cond-mat/0303365)that includes an important missing
reference and presents an improved analysis of a generalized mode
Unzipping Kinetics of Double-Stranded DNA in a Nanopore
We studied the unzipping kinetics of single molecules of double-stranded DNA
by pulling one of their two strands through a narrow protein pore. PCR analysis
yielded the first direct proof of DNA unzipping in such a system. The time to
unzip each molecule was inferred from the ionic current signature of DNA
traversal. The distribution of times to unzip under various experimental
conditions fit a simple kinetic model. Using this model, we estimated the
enthalpy barriers to unzipping and the effective charge of a nucleotide in the
pore, which was considerably smaller than previously assumed.Comment: 10 pages, 5 figures, Accepted: Physics Review Letter
Phase diagram for unzipping DNA with long-range interactions
We present a critique and extension of the mean-field approach to the
mechanical pulling transition in bound polymer systems. Our model is motivated
by the theoretically and experimentally important examples of adsorbed polymers
and double-stranded DNA, and we focus on the case in which quenched disorder in
the sequence of monomers is unimportant for the statistical mechanics. We show
how including excluded volume interactions in the model affects the phase
diagram for the critical pulling force, and we predict a re-entrancy phase at
low temperatures which has not been previously discussed. We also consider the
case of non-equilibrium pulling, in which the external force probes the local,
rather than the global structure of the dsDNA or adsorbed polymer. The dynamics
of the pulling transition in such experiments could illuminate the polymer's
loop structure, which depends on the nature of excluded volume interactions.Comment: 4 pages, 2 figures; this version clarifies Eq. 8, and corrects errors
in Fig.
Simulations and electrical conductivity of percolated networks of finite rods with various degrees of axial alignment
We present a three-dimensional simulation and calculation of electrical conductivity above the filler percolation threshold for networks containing finite, conductive cylinders as a function of axial orientation (S) and aspect ratio (L/D). At a fixed volume fraction and L/D, the simulations exhibit a critical degree of orientation, S-c, above which the electrical conductivity decreases dramatically. With increasing filler concentration and aspect ratio, this critical orientation shifts to higher degrees of alignment. Additionally, at a fixed volume fraction and L/D, the simulated electrical conductivity displays a maximum at slight uniaxial orientation, which is less pronounced at higher volume fractions and aspect ratios. Our approach can be used as a predictive tool to design the optimal filler concentration and degree of orientation required to maximize electrical conductivity in polymer nanocomposites with conductive cylindrical fillers of finite dimension
Effects of mechanical strain on thermal denaturation of DNA
As sections of a strand duplexed DNA denature when exposed to high
temperature, the excess linking number is taken up by the undenatured portions
of the molecule. The mechanical energy that arises because of the overwinding
of the undenatured sections can, in principle, alter the nature of the thermal
denaturation process. Assuming that the strains associated with this
overwinding are not relieved, we find that a simple model of strain-altered
melting leads to a suppression of the melting transition when the unaltered
transition is continuous. When the melting transition is first order in the
absence of strain associated with overwinding, the modification is to a third
order phase transition.Comment: 4 pages, 5 figures, RevTe
Self-averaging of random and thermally disordered diluted Ising systems
Self-averaging of singular thermodynamic quantities at criticality for
randomly and thermally diluted three dimensional Ising systems has been studied
by the Monte Carlo approach. Substantially improved self-averaging is obtained
for critically clustered (critically thermally diluted) vacancy distributions
in comparison with the observed self-averaging for purely random diluted
distributions. Critically thermal dilution, leading to maximum relative
self-averaging, corresponds to the case when the characteristic vacancy
ordering temperature is made equal to the magnetic critical temperature for the
pure 3D Ising systems. For the case of a high ordering temperature, the
self-averaging obtained is comparable to that in a randomly diluted system.Comment: 4 pages, 4figures, RevTe
Unzipping Vortices in Type-II Superconductors
The unzipping of vortex lines using magnetic-force microscopy from extended
defects is studied theoretically. We study both the unzipping isolated vortex
from common defects, such as columnar pins and twin-planes, and the unzipping
of a vortex from a plane in the presence of other vortices. We show, using
analytic and numerical methods, that the universal properties of the unzipping
transition of a single vortex depend only on the dimensionality of the defect
in the presence and absence of disorder. For the unzipping of a vortex from a
plane populated with many vortices is shown to be very sensitive to the
properties of the vortices in the two-dimensional plane. In particular such
unzipping experiments can be used to measure the ``Luttinger liquid parameter''
of the vortices in the plane. In addition we suggest a method for measuring the
line tension of the vortex directly using the experiments.Comment: 19 pages 15 figure
Critical Fluctuations and Disorder at the Vortex Liquid to Crystal Transition in Type-II Superconductors
We present a functional renormalization group (FRG) analysis of a
Landau-Ginzburg model of type-II superconductors (generalized to complex
fields) in a magnetic field, both for a pure system, and in the presence of
quenched random impurities. Our analysis is based on a previous FRG treatment
of the pure case [E.Br\'ezin et. al., Phys. Rev. B, {\bf 31}, 7124 (1985)]
which is an expansion in . If the coupling functions are
restricted to the space of functions with non-zero support only at reciprocal
lattice vectors corresponding to the Abrikosov lattice, we find a stable FRG
fixed point in the presence of disorder for , identical to that of the
disordered model in dimensions. The pure system has a stable fixed
point only for and so the physical case () is likely to have a
first order transition. We speculate that the recent experimental findings that
disorder removes the apparent first order transition are consistent with these
calculations.Comment: 4 pages, no figures, typeset using revtex (v3.0
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