502 research outputs found
Complexity and line of critical points in a short-range spin-glass model
We investigate the critical behavior of a three-dimensional short-range spin
glass model in the presence of an external field \eps conjugated to the
Edwards-Anderson order parameter. In the mean-field approximation this model is
described by the Adam-Gibbs-DiMarzio approach for the glass transition. By
Monte Carlo numerical simulations we find indications for the existence of a
line of critical points in the plane (\eps,T) which separates two
paramagnetic phases and terminates in a critical endpoint. This line of
critical points appears due to the large degeneracy of metastable states
present in the system (configurational entropy) and is reminiscent of the
first-order phase transition present in the mean-field limit. We propose a
scenario for the spin-glass transition at \eps=0, driven by a spinodal point
present above , which induces strong metastability through Griffiths
singularities effects and induces the absence of a two-step shape relaxation
curve characteristic of glasses.Comment: 5 pages, 4 postscript figure, revte
Off-equilibrium confined dynamics in a glassy system with level-crossing states
We study analytically the dynamics of a generalized p-spin model, starting
with a thermalized initial condition. The model presents birth and death of
states, hence the dynamics (even starting at equilibrium) may go out of
equilibrium when the temperature is varied. We give a full description of this
constrained out of equilibrium behavior and we clarify the connection to the
thermodynamics by computing (sub-dominant) TAP states, constrained to the
starting equilibrium configuration.Comment: 10 pages, 3 figures; longer version with appendi
Simulation of magnetic active polymers for versatile microfluidic devices
We propose to use a compound of magnetic nanoparticles (20-100 nm) embedded
in a flexible polymer (Polydimethylsiloxane PDMS) to filter circulating tumor
cells (CTCs). The analysis of CTCs is an emerging tool for cancer biology
research and clinical cancer management including the detection, diagnosis and
monitoring of cancer. The combination of experiments and simulations lead to a
versatile microfluidic lab-on-chip device. Simulations are essential to
understand the influence of the embedded nanoparticles in the elastic PDMS when
applying a magnetic gradient field. It combines finite element calculations of
the polymer, magnetic simulations of the embedded nanoparticles and the fluid
dynamic calculations of blood plasma and blood cells. With the use of magnetic
active polymers a wide range of tunable microfluidic structures can be created.
The method can help to increase the yield of needed isolated CTCs
Monte-Carlo simulations of the violation of the fluctuation-dissipation theorem in domain growth processes
Numerical simulations of various domain growth systems are reported, in order
to compute the parameter describing the violation of fluctuation dissipation
theorem (FDT) in aging phenomena. We compute two-times correlation and response
functions and find that, as expected from the exact solution of a certain
mean-field model (equivalent to the O(N) model in three dimensions, in the
limit of N going to infinity), this parameter is equal to one (no violation of
FDT) in the quasi-equilibrium regime (short separation of times), and zero in
the aging regime.Comment: 5 pages, 5 eps figure
Measuring equilibrium properties in aging systems
We corroborate the idea of a close connection between replica symmetry
breaking and aging in the linear response function for a large class of
finite-dimensional systems with short-range interactions. In these system,
characterized by a continuity condition with respect to weak random
perturbations of the Hamiltonian, the ``fluctuation dissipation ratio'' in
off-equilibrium dynamics should be equal to the static cumulative distribution
function of the overlaps. This allows for an experimental measurement of the
equilibrium order parameter function.Comment: 5 pages, LaTeX. The paper has been completely rewritten and shortene
On Mean Field Glassy Dynamics out of Equilibrium
We study the off equilibrium dynamics of a mean field disordered systems
which can be interpreted both as a long range interaction spin glass and as a
particle in a random potential. The statics of this problem is well known and
exhibits a low temperature spin glass phase with continuous replica symmetry
breaking. We study the equations of off equilibrium dynamics with analytical
and numerical methods. In the spin glass phase, we find that the usual
equilibrium dynamics (observed when the observation time is much smaller than
the waiting time) coexists with an aging regime. In this aging regime, we
propose a solution implying a hierarchy of crossovers between the observation
time and the waiting time.Comment: LaTeX, LPTENS preprint 94/0
Time decay of the remanent magnetization in the spin glass model at T=0
Using the zero-temperature Metropolis dynamics, the time decay of the
remanent magnetization in the Edward-Anderson spin glass model with a
uniform random distribution of ferromagnetic and antiferromagnetic interactions
has been investigated. Starting from the saturation, the magnetization per spin
reveals a slow decrease with time, which can be approximated by a power
law:, . Moreover, its
relaxation does not lead it into one of the ground states, and therefore the
system is trapped in metastable isoenergetic microstates remaining magnetized.
Such behaviour is discussed in terms of a random walk the system performs on
its available configuration space.Comment: 9 pages, 3 figure
The triangular Ising antiferromagnet in a staggered field
We study the equilibrium properties of the nearest-neighbor Ising
antiferromagnet on a triangular lattice in the presence of a staggered field
conjugate to one of the degenerate ground states. Using a mapping of the ground
states of the model without the staggered field to dimer coverings on the dual
lattice, we classify the ground states into sectors specified by the number of
``strings''. We show that the effect of the staggered field is to generate
long-range interactions between strings. In the limiting case of the
antiferromagnetic coupling constant J becoming infinitely large, we prove the
existence of a phase transition in this system and obtain a finite lower bound
for the transition temperature. For finite J, we study the equilibrium
properties of the system using Monte Carlo simulations with three different
dynamics. We find that in all the three cases, equilibration times for low
field values increase rapidly with system size at low temperatures. Due to this
difficulty in equilibrating sufficiently large systems at low temperatures, our
finite-size scaling analysis of the numerical results does not permit a
definite conclusion about the existence of a phase transition for finite values
of J. A surprising feature in the system is the fact that unlike usual glassy
systems, a zero-temperature quench almost always leads to the ground state,
while a slow cooling does not.Comment: 12 pages, 18 figures: To appear in Phys. Rev.
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