2,220 research outputs found

### Heat fluctuations in Ising models coupled with two different heat baths

Monte Carlo simulations of Ising models coupled to heat baths at two
different temperatures are used to study a fluctuation relation for the heat
exchanged between the two thermostats in a time $\tau$. Different kinetics
(single--spin--flip or spin--exchange Kawasaki dynamics), transition rates
(Glauber or Metropolis), and couplings between the system and the thermostats
have been considered. In every case the fluctuation relation is verified in the
large $\tau$ limit, both in the disordered and in the low temperature phase.
Finite-$\tau$ corrections are shown to obey a scaling behavior.Comment: 5 pages, 2 figures. To be published in Journal of Physics A:
Mathematical and Theoretical as fast track communicatio

### Six vertex model with domain-wall boundary conditions in the Bethe-Peierls approximation

We use the Bethe-Peierls method combined with the belief propagation
algorithm to study the arctic curves in the six vertex model on a square
lattice with domain-wall boundary conditions, and the six vertex model on a
rectangular lattice with partial domain-wall boundary conditions. We show that
this rather simple approximation yields results that are remarkably close to
the exact ones when these are known, and allows one to estimate the location of
the phase boundaries with relative little effort in cases in which exact
results are not available.Comment: 19 pages, 14 figure

### Fluctuations of rotational and translational degrees of freedom in an interacting active dumbbell system

We study the dynamical properties of a two-dimensional ensemble of
self-propelled dumbbells with only repulsive interactions. After summarizing
the behavior of the translational and rotational mean-square displacements in
the homogeneous phase that we established in a previous study, we analyze their
fluctuations. We study the dependence of the probability distribution functions
in terms of the P\'eclet number, describing the relative role of active forces
and thermal fluctuations, and of particle density.Comment: arXiv admin note: text overlap with arXiv:1501.0405

### Energy and Heat Fluctuations in a Temperature Quench

Fluctuations of energy and heat are investigated during the relaxation
following the instantaneous temperature quench of an extended system. Results
are obtained analytically for the Gaussian model and for the large $N$ model
quenched below the critical temperature $T_C$. The main finding is that
fluctuations exceeding a critical threshold do condense. Though driven by a
mechanism similar to that of Bose-Einstein condensation, this phenomenon is an
out-of-equilibrium feature produced by the breaking of energy equipartition
occurring in the transient regime. The dynamical nature of the transition is
illustrated by phase diagrams extending in the time direction.Comment: To be published in the Proceedings of the Research Program "Small
system non equilibrium fluctuations, dynamics and stochastics, and anomalous
behavior", Kavli Institute for Theoretical Physics China, July 2013. 40
pages, 9 figure

### Lattice Boltzmann study of chemically-driven self-propelled droplets

We numerically study the behavior of self-propelled liquid droplets whose
motion is triggered by a Marangoni-like flow. This latter is generated by
variations of surfactant concentration which affect the droplet surface tension
promoting its motion. In the present paper a model for droplets with a third
amphiphilic component is adopted. The dynamics is described by Navier-Stokes
and convection-diffusion equations, solved by lattice Boltzmann method coupled
with finite-difference schemes. We focus on two cases. First the study of
self-propulsion of an isolated droplet is carried on and, then, the interaction
of two self-propelled droplets is investigated. In both cases, when the
surfactant migrates towards the interface, a quadrupolar vortex of the velocity
field forms inside the droplet and causes the motion. A weaker dipolar field
emerges instead when the surfactant is mainly diluted in the bulk. The dynamics
of two interacting droplets is more complex and strongly depends on their
reciprocal distance. If, in a head-on collision, droplets are close enough, the
velocity field initially attracts them until a motionless steady state is
achieved. If the droplets are vertically shifted, the hydrodynamic field leads
to an initial reciprocal attraction followed by a scattering along opposite
directions. This hydrodynamic interaction acts on a separation of some droplet
radii otherwise it becomes negligible and droplets motion is only driven by
Marangoni effect. Finally, if one of the droplets is passive, this latter is
generally advected by the fluid flow generated by the active one.Comment: 14 pages, 9 figures. In press on EPJ

### Enhancement of structural rearrangement in glassy systems under shear flow

We extend the analysis of the mean field schematic model recently introduced
for the description of glass forming liquids to the case of a supercooled fluid
subjected to a shear flow of rate $\gamma$. After quenching the system to a low
temperature $T$, a slow glassy regime is observed before stationarity is
achieved at the characteristic time $\tau_g$. $\tau_g$ is of the order of the
usual equilibration time without shear $\tau_g^o$ for weak shear, $\gamma
\tau_g ^o1$, local rearrangement of
dense regions is instead enhanced by the flow, and $\tau_g \simeq 1/(T\gamma)$.Comment: 8 pages, 3 figures, changed content Pacs{64}{70.Pf}{Glass
transitions} Pacs{05}{70.Ln}{Non-equilibrium thermodynamics, irreversible
processes} Pacs{83}{50.Ax}{Steady shear flows

### Study of the performance of the NA62 Small-Angle Calorimeter at the DA$\Phi$NE Linac

The measurement of $BR(K^+\to\pi^+\nu\bar{\nu})$ with 10% precision by the
NA62 experiment requires extreme background suppression. The Small Angle
Calorimeter aims to provide an efficient veto for photons flying at angles down
to zero with respect to the kaon flight direction. The initial prototype was
upgraded and tested at the Beam Test Facility of the DA$\Phi$NE Linac at
Frascati. The energy resolution and the efficiency were measured and are
presented.Comment: 5 pages, 7 figure

### Full phase diagram of active Brownian disks: from melting to motility-induced phase separation

We establish the complete phase diagram of self-propelled hard disks in two
spatial dimensions from the analysis of the equation of state and the
statistics of local order parameters. The equilibrium melting scenario is
maintained at small activities, with coexistence between active liquid and
hexatic order, followed by a proper hexatic phase and a further transition to
an active solid. As activity increases, the emergence of hexatic and solid
order is shifted towards higher densities. Above a critical activity and for a
certain range of packing fractions, the system undergoes MIPS and demixes into
low and high density phases; the latter can be either disordered (liquid) or
ordered (hexatic or solid) depending on activity

### Bistable defect structures in blue phase devices

Blue phases (BPs) are liquid crystals made up by networks of defects, or
disclination lines. While existing phase diagrams show a striking variety of
competing metastable topologies for these networks, very little is known as to
how to kinetically reach a target structure, or how to switch from one to the
other, which is of paramount importance for devices. We theoretically identify
two confined blue phase I systems in which by applying an appropriate series of
electric field it is possible to select one of two bistable defect patterns.
Our results may be used to realise new generation and fast switching
energy-saving bistable devices in ultrathin surface treated BPI wafers.Comment: 4 pages, 3 figures. Accepted for publication in Phys. Rev. Let

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