40 research outputs found
Statistical Field Theory and Effective Action Method for scalar Active Matter
We employ Statistical Field Theory techniques for coarse-graining the
steady-state properties of Active Ornstein-Uhlenbeck particles. The computation
is carried on in the framework of the Unified Colored Noise approximation that
allows an effective equilibrium picture. We thus develop a mean-field theory
that allows to describe in a unified framework the phenomenology of scalar
Active Matter. In particular, we are able to describe through spontaneous
symmetry breaking mechanism two peculiar features of Active Systems that are
(i) The accumulation of active particles at the boundaries of a confining
container, and (ii) Motility-Induced Phase Separation (MIPS).
\textcolor{black}{We develop a mean-field theory for steric interacting active
particles undergoing to MIPS and for Active Lennard-Jones (ALJ) fluids.}
\textcolor{black}{Within this framework}, we discuss the universality class of
MIPS and ALJ \textcolor{black}{showing that it falls into Ising universality
class.} We \textcolor{black}{thus} compute analytically the critical line
for both models. In the case of MIPS, gives rise to a
reentrant phase diagram compatible with an inverse transition from liquid to
gas as the strength of the noise decreases. \textcolor{black}{However, in the
case of particles interacting through anisotropic potentials, } the field
theory acquires a term that, \textcolor{black}{in general, cannot
be canceled performing the expansion around the critical point.} In this case,
the \textcolor{black}{Ising} critical point might \textcolor{black}{be
replaced} by a first-order phase transition \textcolor{black}{region}
Thermodynamic first order transition and inverse freezing in a 3D spin-glass
We present a numerical study of the random Blume-Capel model in three
dimension. The phase diagram is characterized by spin-glass/paramagnet phase
transitions both of first and second order in the thermodynamic sense.
Numerical simulations are performed using the Exchange-Monte Carlo algorithm,
providing clear evidence for inverse freezing. The main features at criticality
and in the phase coexistence region are investigated. We are not privy to other
3D short-range systems with quenched disorder undergoing inverse freezing.Comment: 4 pages, 3 figures
Effective equilibrium picture in model with exponentially correlated noise
We study the effect of exponentially correlated noise on model in the
limit of small correlation time discussing the order-disorder transition in
mean-field and the topological transition in two dimensions. We map the steady
states of the non-equilibrium dynamics into an effective equilibrium theory. In
mean-field, the critical temperature increases with the noise correlation time
indicating that memory effects promote ordering. This finding is
confirmed by numerical simulations. The topological transition temperature in
two dimensions remains untouched. However, finite size effects induce a
crossover in the vortices proliferation that is confirmed by numerical
simulations
Probing the Debye spectrum in glasses using small system sizes
The work aims to show that small system sizes in numerical simulations turns out to be useful for investigating the Debye spectrum in glasses
The most probable path of Active Ornstein-Uhlenbeck particles
Using the path integral representation of the non-equilibrium dynamics, we
compute the most probable path between arbitrary starting and final points,
followed by an active particle driven by persistent noise. We focus our
attention on the case of active particles immersed in harmonic potentials,
where the trajectory can be computed analytically. Once we consider the
extended Markovian dynamics where the self-propulsive drive evolves according
to an Ornstein-Uhlenbeck process, we can compute the trajectory analytically
with arbitrary conditions on position and self-propulsion velocity. We test the
analytical predictions against numerical simulations and we compare the
analytical results with those obtained within approximated equilibrium-like
dynamics
Effective potential method for active particles
We investigate the steady state properties of an active fluid modeled as an
assembly of soft repulsive spheres subjected to Gaussian colored noise. Such a
noise captures one of the salient aspects of active particles, namely the
persistence of their motion and determines a variety of novel features with
respect to familiar passive fluids. We show that within the so-called
multidimensional unified colored noise approximation, recently introduced in
the field of active matter, the model can be treated by methods similar to
those employed in the study of standard molecular fluids. The system shows a
tendency of the particles to aggregate even in the presence of purely repulsive
forces because the combined action of colored noise and interactions enhances
the the effective friction between nearby particles. We also discuss whether an
effective two-body potential approach, which would allow to employ methods
similar to those of density functional theory, is appropriate. The limits of
such an approximation are discussed.Comment: 14 pages, 6 figures in Molecular Physics, 11 march 2016. arXiv admin
note: text overlap with arXiv:cond-mat/0605094 by other author