5,071 research outputs found
Topological interactions between ring polymers: Implications for chromatin loops
Chromatin looping is a major epigenetic regulatory mechanism in higher
eukaryotes. Besides its role in transcriptional regulation, chromatin loops
have been proposed to play a pivotal role in the segregation of entire
chromosomes. The detailed topological and entropic forces between loops still
remain elusive. Here, we quantitatively determine the potential of mean force
between the centers of mass of two ring polymers, i.e. loops. We find that the
transition from a linear to a ring polymer induces a strong increase in the
entropic repulsion between these two polymers. On top, topological interactions
such as the non-catenation constraint further reduce the number of accessible
conformations of close-by ring polymers by about 50%, resulting in an
additional effective repulsion. Furthermore, the transition from linear to ring
polymers displays changes in the conformational and structural properties of
the system. In fact, ring polymers adopt a markedly more ordered and aligned
state than linear ones. The forces and accompanying changes in shape and
alignment between ring polymers suggest an important regulatory function of
such a topology in biopolymers. We conjecture that dynamic loop formation in
chromatin might act as a versatile control mechanism regulating and maintaining
different local states of compaction and order.Comment: 12 pages, 11 figures. The article has been accepted by The Journal Of
Chemical Physics. After it is published, it will be found at
http://jcp.aip.or
Effect of flow on the acoustic reflection coefficient at a duct inlet
The effect of duct Mach number upon the acoustic reflection coefficient at the inlet of a duct with mean
flow is investigated. An analysis, which models the duct inlet as a very short, one-dimensional nozzle over
which the mean flow is accelerated from rest, gives good agreement with some recent experimental results.
Discrepancies between the analysis and the experimental results are discussed in terms of radiation losses at
the inlet and real fluid-flow effects within the duct
Dynamics of ultracold molecules in confined geometry and electric field
We present a time-independent quantum formalism to describe the dynamics of
molecules with permanent electric dipole moments in a two-dimensional confined
geometry such as a one-dimensional optical lattice, in the presence of an
electric field. Bose/Fermi statistics and selection rules play a crucial role
in the dynamics. As examples, we compare the dynamics of confined fermionic and
bosonic polar KRb molecules under different confinements and electric fields.
We show how chemical reactions can be suppressed, either by a "statistical
suppression" which applies for fermions at small electric fields and
confinements, or by a "potential energy suppression", which applies for both
fermions and bosons at high electric fields and confinements. We also explore
collisions that transfer molecules from one state of the confining potential to
another. Although these collisions can be significant, we show that they do not
play a role in the loss of the total number of molecules in the gas.Comment: 13 pages, 6 figure
Critical superfluid velocity in a trapped dipolar gas
We investigate the superfluid properties of a dipolar Bose-Einstein
condensate (BEC) in a fully three-dimensional trap. Specifically, we calculate
a superfluid critical velocity for this system by applying the Landau criterion
to its discrete quasiparticle spectrum. We test this critical velocity by
direct numerical simulation of condensate depletion as a blue-detuned laser
moves through the condensate. In both cases, the presence of the roton in the
spectrum serves to lower the critical velocity beyond a critical particle
number. Since the shape of the dispersion, and hence the roton minimum, is
tunable as a function of particle number, we thereby propose an experiment that
can simultaneously measure the Landau critical velocity of a dipolar BEC and
demonstrate the presence of the roton in this system.Comment: 5 pages, 4 figures, version accepted to PR
Chaotic Orbits in Thermal-Equilibrium Beams: Existence and Dynamical Implications
Phase mixing of chaotic orbits exponentially distributes these orbits through
their accessible phase space. This phenomenon, commonly called ``chaotic
mixing'', stands in marked contrast to phase mixing of regular orbits which
proceeds as a power law in time. It is operationally irreversible; hence, its
associated e-folding time scale sets a condition on any process envisioned for
emittance compensation. A key question is whether beams can support chaotic
orbits, and if so, under what conditions? We numerically investigate the
parameter space of three-dimensional thermal-equilibrium beams with space
charge, confined by linear external focusing forces, to determine whether the
associated potentials support chaotic orbits. We find that a large subset of
the parameter space does support chaos and, in turn, chaotic mixing. Details
and implications are enumerated.Comment: 39 pages, including 14 figure
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