11,592 research outputs found
The looping probability of random heteropolymers helps to understand the scaling properties of biopolymers
Random heteropolymers are a minimal description of biopolymers and can
provide a theoretical framework to the investigate the formation of loops in
biophysical experiments. A two--state model provides a consistent and robust
way to study the scaling properties of loop formation in polymers of the size
of typical biological systems. Combining it with self--adjusting
simulated--tempering simulations, we can calculate numerically the looping
properties of several realizations of the random interactions within the chain.
Differently from homopolymers, random heteropolymers display at different
temperatures a continuous set of scaling exponents. The necessity of using
self--averaging quantities makes finite--size effects dominant at low
temperatures even for long polymers, shadowing the length--independent
character of looping probability expected in analogy with homopolymeric
globules. This could provide a simple explanation for the small scaling
exponents found in experiments, for example in chromosome folding
Shuttling heat across 1D homogenous nonlinear lattices with a Brownian heat motor
We investigate directed thermal heat flux across 1D homogenous nonlinear
lattices when no net thermal bias is present on average. A nonlinear lattice of
Fermi-Pasta-Ulam-type or Lennard-Jones-type system is connected at both ends to
thermal baths which are held at the same temperature on temporal average. We
study two different modulations of the heat bath temperatures, namely: (i) a
symmetric, harmonic ac-driving of temperature of one heat bath only and (ii) a
harmonic mixing drive of temperature acting on both heat baths. While for case
(i) an adiabatic result for the net heat transport can be derived in terms of
the temperature dependent heat conductivity of the nonlinear lattice a similar
such transport approach fails for the harmonic mixing case (ii). Then, for case
(ii), not even the sign of the resulting Brownian motion induced heat flux can
be predicted a priori. A non-vanishing heat flux (including a non-adiabatic
reversal of flux) is detected which is the result of an induced dynamical
symmetry breaking mechanism in conjunction with the nonlinearity of the lattice
dynamics. Computer simulations demonstrate that the heat flux is robust against
an increase of lattice sizes. The observed ratchet effect for such directed
heat currents is quite sizable for our studied class of homogenous nonlinear
lattice structures, thereby making this setup accessible for experimental
implementation and verification.Comment: 9 pages, 10 figures. Phys. Rev. E (in press
The Multiphase Intracluster Medium in Galaxy Groups Probed by the Lyman Alpha Forest
The case is made that the intracluster medium (ICM) in spiral-rich galaxy
groups today probably has undergone much slower evolution than that in
elliptical-rich groups and clusters. The environments of proto-clusters and
proto-groups at z > 2 are likely similar to spiral-rich group environments at
lower redshift. Therefore, like the ICM in spiral-rich groups today, the ICM in
proto-groups and proto-clusters at z > 2 is predicted to be significantly
multiphased. The QSO Lyman alpha forest in the vicinity of galaxies is an
effective probe of the ICM at a wide range of redshift. Two recent observations
of Lyman alpha absorption around galaxies by Adelberger et al. and by
Pascarelle et al are reconciled, and it is shown that observations support the
multiphase ICM scenario. Galaxy redshifts must be very accurate for such
studies to succeed. This scenario can also explain the lower metallicity and
lower hot gas fraction in groups.Comment: 4 pages, 1 figure, replaced with the version after proo
Molecular wires acting as quantum heat ratchets
We explore heat transfer in molecular junctions between two leads in the
absence of a finite net thermal bias. The application of an unbiased,
time-periodic temperature modulation of the leads entails a dynamical breaking
of reflection symmetry, such that a directed heat current may emerge (ratchet
effect). In particular, we consider two cases of adiabatically slow driving,
namely (i) periodic temperature modulation of only one lead and (ii)
temperature modulation of both leads with an ac driving that contains a second
harmonic, thus generating harmonic mixing. Both scenarios yield sizeable
directed heat currents which should be detectable with present techniques.
Adding a static thermal bias, allows one to compute the heat current-thermal
load characteristics which includes the ratchet effect of negative thermal bias
with positive-valued heat flow against the thermal bias, up to the thermal
stop-load. The ratchet heat flow in turn generates also an electric current. An
applied electric stop-voltage, yielding effective zero electric current flow,
then mimics a solely heat-ratchet-induced thermopower (``ratchet Seebeck
effect''), although no net thermal bias is acting. Moreover, we find that the
relative phase between the two harmonics in scenario (ii) enables steering the
net heat current into a direction of choice.Comment: 9 pages, 8 figure
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