199 research outputs found
Energy Dissipation and Fluctuation-Response in Driven Quantum Langevin Dynamics
Energy dissipation in a nonequilibrium steady state is studied in driven
quantum Langevin systems. We study energy dissipation flow to thermal
environment, and obtain a general formula for the average rate of energy
dissipation using an autocorrelation function for the system variable. This
leads to a general expression of the equality that connects the violation of
the fluctuation-response relation to the rate of energy dissipation, the
classical version of which was first studied by Harada and Sasa. We also point
out that the expression depends on coupling form between system and reservoir.Comment: 4 pages, 1 figur
Activated dynamics and effective temperature in a steady state sheared glass
We conduct nonequilibrium molecular dynamics simulations to measure the shear
stress, the average inherent structure energy, and the effective temperature
of a sheared model glass as a function of bath temperature and
shear strain rate. For above the glass transition temperature , the
rheology approaches a Newtonian limit and approaches as the
strain rate approaches zero, while for , the shear stress approaches a
yield stress and approaches a limiting value near . In the
shear-dominated regime at high , high strain rate or at low , we find
that the shear stress and the average inherent structure energy each collapse
onto a single curve as a function of . This indicates that
is controlling behavior in this regime.Comment: 4 pages, 2 figures. Revised to include additional data. Inherent
structure energy results were included, and much of the shear transformation
zone discussion was remove
Density of states of colloidal glasses
Glasses are structurally liquid-like, but mechanically solid-like. Most
attempts to understand glasses start from liquid state theory. Here we take the
opposite point of view, and use concepts from solid state physics. We determine
the vibrational modes of a colloidal glass experimentally, and find soft
low-frequency modes that are very different in nature from the usual acoustic
vibrations of ordinary solids. These modes extend over surprisingly large
length scales
Fluctuation-Dissipation Theorem in Nonequilibrium Steady States
In equilibrium, the fluctuation-dissipation theorem (FDT) expresses the
response of an observable to a small perturbation by a correlation function of
this variable with another one that is conjugate to the perturbation with
respect to \emph{energy}. For a nonequilibrium steady state (NESS), the
corresponding FDT is shown to involve in the correlation function a variable
that is conjugate with respect to \emph{entropy}. By splitting up entropy
production into one of the system and one of the medium, it is shown that for
systems with a genuine equilibrium state the FDT of the NESS differs from its
equilibrium form by an additive term involving \emph{total} entropy production.
A related variant of the FDT not requiring explicit knowledge of the stationary
state is particularly useful for coupled Langevin systems. The \emph{a priori}
surprising freedom apparently involved in different forms of the FDT in a NESS
is clarified.Comment: 6 pages; EPL, in pres
Exact mean field inference in asymmetric kinetic Ising systems
We develop an elementary mean field approach for fully asymmetric kinetic
Ising models, which can be applied to a single instance of the problem. In the
case of the asymmetric SK model this method gives the exact values of the local
magnetizations and the exact relation between equal-time and time-delayed
correlations. It can also be used to solve efficiently the inverse problem,
i.e. determine the couplings and local fields from a set of patterns, also in
cases where the fields and couplings are time-dependent. This approach
generalizes some recent attempts to solve this dynamical inference problem,
which were valid in the limit of weak coupling. It provides the exact solution
to the problem also in strongly coupled problems. This mean field inference can
also be used as an efficient approximate method to infer the couplings and
fields in problems which are not infinite range, for instance in diluted
asymmetric spin glasses.Comment: 10 pages, 7 figure
Glassy behaviour in disordered systems with non-relaxational dynamics
We show that a family of disordered systems with non-relaxational dynamics
may exhibit ``glassy'' behavior at nonzero temperature, although such a
behavior appears to be ruled out by a face-value application of mean-field
theory. Nevertheless, the roots of this behavior can be understood within
mean-field theory itself, properly interpreted. Finite systems belonging to
this family have a dynamical regime with a self-similar pattern of alternating
periods of fast motion and trapping.Comment: 4 pages, 4 figure
Sampling rare fluctuations of height in the Oslo ricepile model
We have studied large deviations of the height of the pile from its mean
value in the Oslo ricepile model. We sampled these very rare events with
probabilities of order by Monte Carlo simulations using importance
sampling. These simulations check our qualitative arguement [Phys. Rev. E, {\bf
73}, 021303, 2006] that in steady state of the Oslo ricepile model, the
probability of large negative height fluctuations about
the mean varies as as with
held fixed, and .Comment: 7 pages, 8 figure
Role of saddles in mean-field dynamics above the glass transition
Recent numerical developments in the study of glassy systems have shown that
it is possible to give a purely geometric interpretation of the dynamic glass
transition by considering the properties of unstable saddle points of the
energy. Here we further develop this program in the context of a mean-field
model, by analytically studying the properties of the closest saddle point to
an equilibrium configuration of the system. We prove that when the glass
transition is approached the energy of the closest saddle goes to the threshold
energy, defined as the energy level below which the degree of instability of
the typical stationary points vanishes. Moreover, we show that the distance
between a typical equilibrium configuration and the closest saddle is always
very small and that, surprisingly, it is almost independent of the temperature
Fluctuation formula for nonreversible dynamics in the thermostated Lorentz gas
We investigate numerically the validity of the Gallavotti-Cohen fluctuation
formula in the two and three dimensional periodic Lorentz gas subjected to
constant electric and magnetic fields and thermostated by the Gaussian
isokinetic thermostat. The magnetic field breaks the time reversal symmetry,
and by choosing its orientation with respect to the lattice one can have either
a generalized reversing symmetry or no reversibility at all. Our results
indicate that the scaling property described by the fluctuation formula may be
approximately valid for large fluctuations even in the absence of
reversibility.Comment: 6 pages, 6 figure
Dynamics and geometric properties of the k-Trigonometric model
We analyze the dynamics and the geometric properties of the Potential Energy
Surfaces (PES) of the k-Trigonometric Model (kTM), defined by a fully-connected
k-body interaction. This model has no thermodynamic transition for k=1, a
second order one for k=2, and a first order one for k>2. In this paper we i)
show that the single particle dynamics can be traced back to an effective
dynamical system (with only one degree of freedom); ii) compute the diffusion
constant analytically; iii) determine analytically several properties of the
self correlation functions apart from the relaxation times which we calculate
numerically; iv) relate the collective correlation functions to the ones of the
effective degree of freedom using an exact Dyson-like equation; v) using two
analytical methods, calculate the saddles of the PES that are visited by the
system evolving at fixed temperature. On the one hand we minimize |grad V|^2,
as usually done in the numerical study of supercooled liquids and, on the other
hand, we compute the saddles with minimum distance (in configuration space)
from initial equilibrium configurations. We find the same result from the two
calculations and we speculate that the coincidence might go beyond the specific
model investigated here.Comment: 36 pages, 13 figure
- …