487 research outputs found
Phase transition of one dimensional bosons with strong disorder
We study one dimensional disordered bosons at large commensurate filling.
Using a real space renormalization group approach we find a new random fixed
point which controls a phase transition from a superfluid to an incompressible
Mott-glass. The transition can be tuned by changing the disorder distribution
even with vanishing interactions. We derive the properties of the transition,
which suggest that it is in the Kosterlitz-Thouless universality class.Comment: 4 pages 3 embedded eps figure
Phase transition in a non-conserving driven diffusive system
An asymmetric exclusion process comprising positive particles, negative
particles and vacancies is introduced. The model is defined on a ring and the
dynamics does not conserve the number of particles. We solve the steady state
exactly and show that it can exhibit a continuous phase transition in which the
density of vacancies decreases to zero. The model has no absorbing state and
furnishes an example of a one-dimensional phase transition in a homogeneous
non-conserving system which does not belong to the absorbing state universality
classes
Sequence heterogeneity and the dynamics of molecular motors
The effect of sequence heterogeneity on the dynamics of molecular motors is
reviewed and analyzed using a set of recently introduced lattice models. First,
we review results for the influence of heterogenous tracks such as a
single-strand of DNA or RNA on the dynamics of the motors. We stress how the
predicted behavior might be observed experimentally in anomalous drift and
diffusion of motors over a wide range of parameters near the stall force and
discuss the extreme limit of strongly biased motors with one-way hopping. We
then consider the dynamics in an environment containing a variety of different
fuels which supply chemical energy for the motor motion, either on a
heterogeneous or on a periodic track. The results for motion along a periodic
track are relevant to kinesin motors in a solution with a mixture of different
nucleotide triphosphate fuel sources.Comment: To appear in a JPhys special issue on molecular motor
Localization of Denaturation Bubbles in Random DNA Sequences
We study the thermodynamic and dynamic behaviors of twist-induced
denaturation bubbles in a long, stretched random sequence of DNA. The small
bubbles associated with weak twist are delocalized. Above a threshold torque,
the bubbles of several tens of bases or larger become preferentially localized
to \AT-rich segments. In the localized regime, the bubbles exhibit ``aging''
and move around sub-diffusively with continuously varying dynamic exponents.
These properties are derived using results of large-deviation theory together
with scaling arguments, and are verified by Monte-Carlo simulations.Comment: TeX file with postscript figure
Phase Transition in Two Species Zero-Range Process
We study a zero-range process with two species of interacting particles. We
show that the steady state assumes a simple factorised form, provided the
dynamics satisfy certain conditions, which we derive. The steady state exhibits
a new mechanism of condensation transition wherein one species induces the
condensation of the other. We study this mechanism for a specific choice of
dynamics.Comment: 8 pages, 3 figure
Vortex pinning by a columnar defect in planar superconductors with point disorder
We study the effect of a single columnar pin on a dimensional array
of vortex lines in planar type II superconductors in the presence of point
disorder. In large samples, the pinning is most effective right at the
temperature of the vortex glass transition. In particular, there is a
pronounced maximum in the number of vortices which are prevented from tilting
by the columnar defect in a weak transverse magnetic field. Using
renormalization group techniques we show that the columnar pin is irrelevant at
long length scales both above and below the transition, but due to very
different mechanisms. This behavior differs from the disorder-free case, where
the pin is relevant in the low temperature phase. Solutions of the
renormalization equations in the different regimes allow a discussion of the
crossover between the pure and disordered cases. We also compute density
oscillations around the columnar pin and the response of these oscillations to
a weak transverse magnetic field.Comment: 12 pages, 5 figures, minor typos corrected, a new reference adde
Will jams get worse when slow cars move over?
Motivated by an analogy with traffic, we simulate two species of particles
(`vehicles'), moving stochastically in opposite directions on a two-lane ring
road. Each species prefers one lane over the other, controlled by a parameter
such that corresponds to random lane choice and
to perfect `laning'. We find that the system displays one large cluster (`jam')
whose size increases with , contrary to intuition. Even more remarkably, the
lane `charge' (a measure for the number of particles in their preferred lane)
exhibits a region of negative response: even though vehicles experience a
stronger preference for the `right' lane, more of them find themselves in the
`wrong' one! For very close to 1, a sharp transition restores a homogeneous
state. Various characteristics of the system are computed analytically, in good
agreement with simulation data.Comment: 7 pages, 3 figures; to appear in Europhysics Letters (2005
Bubble dynamics in DNA
The formation of local denaturation zones (bubbles) in double-stranded DNA is
an important example for conformational changes of biological macromolecules.
We study the dynamics of bubble formation in terms of a Fokker-Planck equation
for the probability density to find a bubble of size n base pairs at time t, on
the basis of the free energy in the Poland-Scheraga model. Characteristic
bubble closing and opening times can be determined from the corresponding first
passage time problem, and are sensitive to the specific parameters entering the
model. A multistate unzipping model with constant rates recently applied to DNA
breathing dynamics [G. Altan-Bonnet et al, Phys. Rev. Lett. 90, 138101 (2003)]
emerges as a limiting case.Comment: 9 pages, 2 figure
Two-dimensional wetting with binary disorder: a numerical study of the loop statistics
We numerically study the wetting (adsorption) transition of a polymer chain
on a disordered substrate in 1+1 dimension.Following the Poland-Scheraga model
of DNA denaturation, we use a Fixman-Freire scheme for the entropy of loops.
This allows us to consider chain lengths of order to ,
with disorder realizations. Our study is based on the statistics of
loops between two contacts with the substrate, from which we define Binder-like
parameters: their crossings for various sizes allow a precise determination
of the critical temperature, and their finite size properties yields a
crossover exponent .We then analyse at
criticality the distribution of loop length in both regimes
and , as well as the finite-size properties of the contact
density and energy. Our conclusion is that the critical exponents for the
thermodynamics are the same as those of the pure case, except for strong
logarithmic corrections to scaling. The presence of these logarithmic
corrections in the thermodynamics is related to a disorder-dependent
logarithmic singularity that appears in the critical loop distribution in the
rescaled variable as .Comment: 12 pages, 13 figure
Stochastic Ballistic Annihilation and Coalescence
We study a class of stochastic ballistic annihilation and coalescence models
with a binary velocity distribution in one dimension. We obtain an exact
solution for the density which reveals a universal phase diagram for the
asymptotic density decay. By universal we mean that all models in the class are
described by a single phase diagram spanned by two reduced parameters. The
phase diagram reveals four regimes, two of which contain the previously studied
cases of ballistic annihilation. The two new phases are a direct consequence of
the stochasticity. The solution is obtained through a matrix product approach
and builds on properties of a q-deformed harmonic oscillator algebra.Comment: 4 pages RevTeX, 3 figures; revised version with some corrections,
additional discussion and in RevTeX forma
- …