6,603 research outputs found
Slow crack propagation through a disordered medium: Critical transition and dissipation
We show that the intermittent and self-similar fluctuations displayed by a
slow crack during the propagation in a heterogeneous medium can be
quantitatively described by an extension of a classical statistical model for
fracture. The model yields the correct dynamical and morphological scaling, and
allows to demonstrate that the scale invariance originates from the presence of
a non-equilibrium, reversible, critical transition which in the presence of
dissipation gives rise to self organized critical behaviour.Comment: 16 pages, 4 figures, to be published on EPL
(http://epljournal.edpsciences.org/
Random sequential adsorption and diffusion of dimers and k-mers on a square lattice
We have performed extensive simulations of random sequential adsorption and
diffusion of -mers, up to in two dimensions with particular attention
to the case . We focus on the behavior of the coverage and of vacancy
dynamics as a function of time. We observe that for a complete coverage
of the lattice is never reached, because of the existence of frozen
configurations that prevent isolated vacancies in the lattice to join. From
this result we argue that complete coverage is never attained for any value of
. The long time behavior of the coverage is not mean field and nonanalytic,
with as leading term. Long time coverage regimes are independent of
the initial conditions while strongly depend on the diffusion probability and
deposition rate and, in particular, different values of these parameters lead
to different final values of the coverage. The geometrical complexity of these
systems is also highlighted through an investigation of the vacancy population
dynamics.Comment: 9 pages, 9 figures, to be published in the Journal of Chemical
Physic
Ultracold polarized Fermi gas at intermediate temperatures
We consider non-zero temperature properties of the polarized two-component
Fermi gas. We point out that stable polarized paired states which are more
stable than their phase separated counterparts with unpolarized superfluid
region can exist below the critical temperature. We also solve the system
behavior in a trap using the local density approximation and find gradually
increasing polarization in the center of the system as the temperature is
increased. However, in the strongly interacting region the central polarization
increases most rapidly close to the mean-field critical temperature, which is
known to be substantially higher than the critical temperature for
superfluidity. This indicates that most of the phase separation occurs in the
fluctuation region prior to superfluidity and that the polarization in the
actual superfluid is modest.Comment: Final published versio
Coupling internal atomic states in a two-component Bose-Einstein condensate via an optical lattice: Extended Mott-superfluid transitions
An ultracold gas of coupled two-component atoms in an optical field is
studied. Due to the internal two-level structure of the atoms, three competing
energy terms exist; atomic kinetic, atomic internal, and atom-atom interaction
energies. A novel outcome of this interplay, not present in the regular
Bose-Hubbard model, is that in the single band and tight binding approximations
four different phases appear: two superfluid and two Mott phases. When passing
through the critical point between the two superfluid or the two Mott phases, a
swapping of the internal atomic populations takes place. By means of the strong
coupling expansion, we find the full phase diagram for the four different
phases.Comment: 9 pages, 7 figure
Brownian ratchet in a thermal bath driven by Coulomb friction
The rectification of unbiased fluctuations, also known as the ratchet effect,
is normally obtained under statistical non-equilibrium conditions. Here we
propose a new ratchet mechanism where a thermal bath solicits the random
rotation of an asymmetric wheel, which is also subject to Coulomb friction due
to solid-on-solid contacts. Numerical simulations and analytical calculations
demonstrate a net drift induced by friction. If the thermal bath is replaced by
a granular gas, the well known granular ratchet effect also intervenes,
becoming dominant at high collision rates. For our chosen wheel shape the
granular effect acts in the opposite direction with respect to the
friction-induced torque, resulting in the inversion of the ratchet direction as
the collision rate increases. We have realized a new granular ratchet
experiment where both these ratchet effects are observed, as well as the
predicted inversion at their crossover. Our discovery paves the way to the
realization of micro and sub-micrometer Brownian motors in an equilibrium
fluid, based purely upon nano-friction.Comment: main paper: 4 pages and 4 figures; supplemental material joined at
the end of the paper; a movie of the experiment can be viewed
http://www.youtube.com/watch?v=aHrdY4BC71k ; all the material has been
submitted for publication [new version with substantial changes in the order
of the presentation of the results; differences with previous works have been
put in evidence
Relaxed Operational Semantics of Concurrent Programming Languages
We propose a novel, operational framework to formally describe the semantics
of concurrent programs running within the context of a relaxed memory model.
Our framework features a "temporary store" where the memory operations issued
by the threads are recorded, in program order. A memory model then specifies
the conditions under which a pending operation from this sequence is allowed to
be globally performed, possibly out of order. The memory model also involves a
"write grain," accounting for architectures where a thread may read a write
that is not yet globally visible. Our formal model is supported by a software
simulator, allowing us to run litmus tests in our semantics.Comment: In Proceedings EXPRESS/SOS 2012, arXiv:1208.244
- …