1,779 research outputs found
Evolution in range expansions with competition at rough boundaries.
When a biological population expands into new territory, genetic drift develops an enormous influence on evolution at the propagating front. In such range expansion processes, fluctuations in allele frequencies occur through stochastic spatial wandering of both genetic lineages and the boundaries between genetically segregated sectors. Laboratory experiments on microbial range expansions have shown that this stochastic wandering, transverse to the front, is superdiffusive due to the front's growing roughness, implying much faster loss of genetic diversity than predicted by simple flat front diffusive models. We study the evolutionary consequences of this superdiffusive wandering using two complementary numerical models of range expansions: the stepping stone model, and a new interpretation of the model of directed paths in random media, in the context of a roughening population front. Through these approaches we compute statistics for the times since common ancestry for pairs of individuals with a given spatial separation at the front, and we explore how environmental heterogeneities can locally suppress these superdiffusive fluctuations
Spontaneous emission by rotating objects: A scattering approach
We study the quantum electrodynamics (QED) vacuum in the presence of a body
rotating along its axis of symmetry and show that the object spontaneously
emits energy if it is lossy. The radiated power is expressed as a general trace
formula solely in terms of the scattering matrix, making an explicit connection
to the conjecture of Zel'dovich [JETP Lett. 14, 180 (1971)] on rotating
objects. We further show that a rotating body drags along nearby objects while
making them spin parallel to its own rotation axis
Dynamics of An Underdamped Josephson Junction Ladder
We show analytically that the dynamical equations for an underdamped ladder
of coupled small Josephson junctions can be approximately reduced to the
discrete sine-Gordon equation. As numerical confirmation, we solve the coupled
Josephson equations for such a ladder in a magnetic field. We obtain
discrete-sine-Gordon-like IV characteristics, including a flux flow and a
``whirling'' regime at low and high currents, and voltage steps which represent
a lock-in between the vortex motion and linear ``phasons'', and which are
quantitatively predicted by a simple formula. At sufficiently high anisotropy,
the fluxons on the steps propagate ballistically.Comment: 11pages, latex, no figure
Energy Barriers to Motion of Flux Lines in Random Media
We propose algorithms for determining both lower and upper bounds for the
energy barriers encountered by a flux line in moving through a two-dimensional
random potential. Analytical arguments, supported by numerical simulations,
suggest that these bounds scale with the length of the line as
and , respectively. This provides the first confirmation
of the hypothesis that barriers have the same scaling as the fluctuation in the
free energy. \pacs{PACS numbers: 74.60.Ge, 05.70.Ln, 05.40.+j}Comment: 4 pages Revtex, 2 figures, to appear in PRL 75, 1170 (1995
Thermodynamic Fingerprints of Disorder in Flux Line Lattices and other Glassy Mesoscopic Systems
We examine probability distributions for thermodynamic quantities in
finite-sized random systems close to criticality. Guided by available exact
results, a general ansatz is proposed for replicated free energies, which leads
to scaling forms for cumulants of various macroscopic observables. For the
specific example of a planar flux line lattice in a two dimensional
superconducting film near H_c1, we provide detailed results for the statistics
of the magnetic flux density, susceptibility, heat capacity, and their
cross-correlations.Comment: 4 page
Free Energy Self-Averaging in Protein-Sized Random Heteropolymers
Current theories of heteropolymers are inherently macrpscopic, but are
applied to folding proteins which are only mesoscopic. In these theories, one
computes the averaged free energy over sequences, always assuming that it is
self-averaging -- a property well-established only if a system with quenched
disorder is macroscopic. By enumerating the states and energies of compact 18,
27, and 36mers on a simplified lattice model with an ensemble of random
sequences, we test the validity of the self-averaging approximation. We find
that fluctuations in the free energy between sequences are weak, and that
self-averaging is a valid approximation at the length scale of real proteins.
These results validate certain sequence design methods which can exponentially
speed up computational design and greatly simplify experimental realizations.Comment: 4 pages, 3 figure
Modified critical correlations close to modulated and rough surfaces
Correlation functions are sensitive to the presence of a boundary. Surface
modulations give rise to modified near surface correlations, which can be
measured by scattering probes. To determine these correlations, we develop a
perturbative calculation in deformations in height from a flat surface. The
results, combined with a renormalization group around four dimensions, are also
used to predict critical behavior near a self-affinely rough surface. We find
that a large enough roughness exponent can modify surface critical behavior.Comment: 4 pages, 1 figure. Revised version as published in Phys. Rev. Lett.
86, 4596 (2001
Anomalous Dynamics of Forced Translocation
We consider the passage of long polymers of length N through a hole in a
membrane. If the process is slow, it is in principle possible to focus on the
dynamics of the number of monomers s on one side of the membrane, assuming that
the two segments are in equilibrium. The dynamics of s(t) in such a limit would
be diffusive, with a mean translocation time scaling as N^2 in the absence of a
force, and proportional to N when a force is applied. We demonstrate that the
assumption of equilibrium must break down for sufficiently long polymers (more
easily when forced), and provide lower bounds for the translocation time by
comparison to unimpeded motion of the polymer. These lower bounds exceed the
time scales calculated on the basis of equilibrium, and point to anomalous
(sub-diffusive) character of translocation dynamics. This is explicitly
verified by numerical simulations of the unforced translocation of a
self-avoiding polymer. Forced translocation times are shown to strongly depend
on the method by which the force is applied. In particular, pulling the polymer
by the end leads to much longer times than when a chemical potential difference
is applied across the membrane. The bounds in these cases grow as N^2 and
N^{1+\nu}, respectively, where \nu is the exponent that relates the scaling of
the radius of gyration to N. Our simulations demonstrate that the actual
translocation times scale in the same manner as the bounds, although influenced
by strong finite size effects which persist even for the longest polymers that
we considered (N=512).Comment: 13 pages, RevTeX4, 16 eps figure
Melting of Flux Lines in an Alternating Parallel Current
We use a Langevin equation to examine the dynamics and fluctuations of a flux
line (FL) in the presence of an {\it alternating longitudinal current}
. The magnus and dissipative forces are equated to those
resulting from line tension, confinement in a harmonic cage by neighboring FLs,
parallel current, and noise. The resulting mean-square FL fluctuations are
calculated {\it exactly}, and a Lindemann criterion is then used to obtain a
nonequilibrium `phase diagram' as a function of the magnitude and frequency of
. For zero frequency, the melting temperature of the
mixed phase (a lattice, or the putative "Bose" or "Bragg Glass") vanishes at a
limiting current. However, for any finite frequency, there is a non-zero
melting temperature.Comment: 5 pages, 1 figur
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