343 research outputs found
Reunion of random walkers with a long range interaction: applications to polymers and quantum mechanics
We use renormalization group to calculate the reunion and survival exponents
of a set of random walkers interacting with a long range and a short
range interaction. These exponents are used to study the binding-unbinding
transition of polymers and the behavior of several quantum problems.Comment: Revtex 3.1, 9 pages (two-column format), 3 figures. Published version
(PRE 63, 051103 (2001)). Reference corrections incorporated (PRE 64, 059902
(2001) (E
Phase transitions in a network with range dependent connection probability
We consider a one-dimensional network in which the nodes at Euclidean
distance can have long range connections with a probabilty in addition to nearest neighbour connections. This system has been
shown to exhibit small world behaviour for above which its
behaviour is like a regular lattice. From the study of the clustering
coefficients, we show that there is a transition to a random network at . The finite size scaling analysis of the clustering coefficients obtained
from numerical simulations indicate that a continuous phase transition occurs
at this point. Using these results, we find that the two transitions occurring
in this network can be detected in any dimension by the behaviour of a single
quantity, the average bond length. The phase transitions in all dimensions are
non-trivial in nature.Comment: 4 pages, revtex4, submitted to Physical Review
Alternative Technique for "Complex" Spectra Analysis
. The choice of a suitable random matrix model of a complex system is very
sensitive to the nature of its complexity. The statistical spectral analysis of
various complex systems requires, therefore, a thorough probing of a wide range
of random matrix ensembles which is not an easy task. It is highly desirable,
if possible, to identify a common mathematcal structure among all the ensembles
and analyze it to gain information about the ensemble- properties. Our
successful search in this direction leads to Calogero Hamiltonian, a
one-dimensional quantum hamiltonian with inverse-square interaction, as the
common base. This is because both, the eigenvalues of the ensembles, and, a
general state of Calogero Hamiltonian, evolve in an analogous way for arbitrary
initial conditions. The varying nature of the complexity is reflected in the
different form of the evolution parameter in each case. A complete
investigation of Calogero Hamiltonian can then help us in the spectral analysis
of complex systems.Comment: 20 pages, No figures, Revised Version (Minor Changes
Scaling in DNA unzipping models: denaturated loops and end-segments as branches of a block copolymer network
For a model of DNA denaturation, exponents describing the distributions of
denaturated loops and unzipped end-segments are determined by exact enumeration
and by Monte Carlo simulations in two and three dimensions. The loop
distributions are consistent with first order thermal denaturation in both
cases. Results for end-segments show a coexistence of two distinct power laws
in the relative distributions, which is not foreseen by a recent approach in
which DNA is treated as a homogeneous network of linear polymer segments. This
unexpected feature, and the discrepancies with such an approach, are explained
in terms of a refined scaling picture in which a precise distinction is made
between network branches representing single stranded and effective double
stranded segments.Comment: 8 pages, 8 figure
Atomic-scale perspective on the origin of attractive step interactions on Si(113)
Recent experiments have shown that steps on Si(113) surfaces self-organize
into bunches due to a competition between long-range repulsive and short-range
attractive interactions. Using empirical and tight-binding interatomic
potentials, we investigate the physical origin of the short-range attraction,
and report the formation and interaction energies of steps. We find that the
short-range attraction between steps is due to the annihilation of force
monopoles at their edges as they combine to form bunches. Our results for the
strengths of the attractive interactions are consistent with the values
determined from experimental studies on kinetics of faceting.Comment: 4 pages, 3 figures, to appear in Phys. Rev B, Rapid Communication
Unzipping Dynamics of Long DNAs
The two strands of the DNA double helix can be `unzipped' by application of
15 pN force. We analyze the dynamics of unzipping and rezipping, for the case
where the molecule ends are separated and re-approached at constant velocity.
For unzipping of 50 kilobase DNAs at less than about 1000 bases per second,
thermal equilibrium-based theory applies. However, for higher unzipping
velocities, rotational viscous drag creates a buildup of elastic torque to
levels above kBT in the dsDNA region, causing the unzipping force to be well
above or well below the equilibrium unzipping force during respectively
unzipping and rezipping, in accord with recent experimental results of Thomen
et al. [Phys. Rev. Lett. 88, 248102 (2002)]. Our analysis includes the effect
of sequence on unzipping and rezipping, and the transient delay in buildup of
the unzipping force due to the approach to the steady state.Comment: 15 pages Revtex file including 9 figure
A pseudo-spectral method for the Kardar-Parisi-Zhang equation
We discuss a numerical scheme to solve the continuum Kardar-Parisi-Zhang
equation in generic spatial dimensions. It is based on a momentum-space
discretization of the continuum equation and on a pseudo-spectral approximation
of the non-linear term. The method is tested in (1+1)- and (2+1)- dimensions,
where it is shown to reproduce the current most reliable estimates of the
critical exponents based on Restricted Solid-on-Solid simulations. In
particular it allows the computations of various correlation and structure
functions with high degree of numerical accuracy. Some deficiencies which are
common to all previously used finite-difference schemes are pointed out and the
usefulness of the present approach in this respect is discussed.Comment: 12 pages, 13 .eps figures, revetx4. A few equations have been
corrected. Erratum sent to Phys. Rev.
Soliton Lattices in the Incommensurate Spin-Peierls Phase: Local Distortions and Magnetizations
It is shown that nonadiabatic fluctuations of the soliton lattice in the
spin-Peierls system CuGeO_3 lead to an important reduction of the NMR line
widths. These fluctuations are the zero-point motion of the massless phasonic
excitations. Furthermore, we show that the discrepancy of X-ray and NMR soliton
widths can be understood as the difference between a distortive and a magnetic
width. Their ratio is controlled by the frustration of the spin system. By this
work, theoretical and experimental results can be reconciled in two important
points.Comment: 9 pages, 5 figures included, Revtex submitted to Physical Review
Random walks and polymers in the presence of quenched disorder
After a general introduction to the field, we describe some recent results
concerning disorder effects on both `random walk models', where the random walk
is a dynamical process generated by local transition rules, and on `polymer
models', where each random walk trajectory representing the configuration of a
polymer chain is associated to a global Boltzmann weight. For random walk
models, we explain, on the specific examples of the Sinai model and of the trap
model, how disorder induces anomalous diffusion, aging behaviours and Golosov
localization, and how these properties can be understood via a strong disorder
renormalization approach. For polymer models, we discuss the critical
properties of various delocalization transitions involving random polymers. We
first summarize some recent progresses in the general theory of random critical
points : thermodynamic observables are not self-averaging at criticality
whenever disorder is relevant, and this lack of self-averaging is directly
related to the probability distribution of pseudo-critical temperatures
over the ensemble of samples of size . We describe the
results of this analysis for the bidimensional wetting and for the
Poland-Scheraga model of DNA denaturation.Comment: 17 pages, Conference Proceedings "Mathematics and Physics", I.H.E.S.,
France, November 200
Nearby quasar remnants and ultra-high energy cosmic rays
As recently suggested, nearby quasar remnants are plausible sites of
black-hole based compact dynamos that could be capable of accelerating
ultra-high energy cosmic rays (UHECRs). In such a model, UHECRs would originate
at the nuclei of nearby dead quasars, those in which the putative underlying
supermassive black holes are suitably spun-up. Based on galactic optical
luminosity, morphological type, and redshift, we have compiled a small sample
of nearby objects selected to be highly luminous, bulge-dominated galaxies,
likely quasar remnants. The sky coordinates of these galaxies were then
correlated with the arrival directions of cosmic rays detected at energies EeV. An apparently significant correlation appears in our data. This
correlation appears at closer angular scales than those expected when taking
into account the deflection caused by typically assumed IGM or galactic
magnetic fields over a charged particle trajectory. Possible scenarios
producing this effect are discussed, as is the astrophysics of the quasar
remnant candidates. We suggest that quasar remnants be also taken into account
in the forthcoming detailed search for correlations using data from the Auger
Observatory.Comment: 2 figures, 4 tables, 11 pages. Final version to appear in Physical
Review
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