21 research outputs found
Geometrical properties of the potential energy of the soft-sphere binary mixture
We report a detailed study of the stationary points (zero-force points) of
the potential energy surface (PES) of a model structural glassformer. We
compare stationary points found with two different algorithms (eigenvector
following and square gradient minimization), and show that the mapping between
instantaneous configuration and stationary points defined by those algorithms
is as different as to strongly influence the instability index K vs.
temperature plot, which relevance in analyzing the liquid dynamics is thus
questioned. On the other hand, the plot of K vs. energy is much less sensitive
to the algorithm employed, showing that the energy is the good variable to
discuss geometric properties of the PES. We find new evidence of a geometric
transition between a minima-dominated phase and a saddle-point-dominated one.
We analyze the distances between instantaneous configurations and stationary
points, and find that above the glass transition, the system is closer to
saddle points than to minima
A new early warning indicator of tree species crashes from effective intraspecific interactions in tropical forests
The vulnerability of species richness to several factors like, climate change, habitat fragmentation, resource exploitation, etc., poses a challenge to conservation biologists and agencies working to sustain the ecosystem services. Hence, there is a clear need for early warning indicators of species loss generated from empirical data.
The tree community of the long-term 50-hectare plot on Barro Colorado Island (BCI), Panama, is one of the most intensively studied in the world. This plot was established in 1981 and fully censused in 1982 then every 5 years from 1985 through 2015. This extensive dataset reveals that some tree species suffered steep population declines.
Here we propose an early warning indicator of such tree population crashes and test it against the BCI dataset. The spatial covariance matrices, Cij, of the 20 most abundant tree species in BCI allow us to compute, via MaxEnt, the effective interaction matrices, Jij, among these species for the eight censuses available from 1982 to 2015. For each species i and each census c, the absolute value of the intraspecific competition coefficients Jii(c) are much larger than those of the interspecific interaction coefficients Jij(c) with i ≠j. We show that this result can be derived from a similar empirical relationship observed for the matrices Cii(c). Our main finding is that for those tree species that suffered steep population declines (of at least 50%), across the eight tree censuses, the drop of Jii is always steeper and occurs before the drop of the corresponding species abundance Ni. Indeed, such sharp declines in Jii occur between 5 and 15 years in advance than comparable declines for Ni, and thus they serve as early warnings of impending population busts. Furthermore, this drop of Jii is linked to the anomalous variance, which is a known early warning of incoming catastrophic shifts
Dynamic relaxation of a liquid cavity under amorphous boundary conditions
The growth of cooperatively rearranging regions was invoked long ago by Adam
and Gibbs to explain the slowing down of glass-forming liquids. The lack of
knowledge about the nature of the growing order, though, complicates the
definition of an appropriate correlation function. One option is the
point-to-set correlation function, which measures the spatial span of the
influence of amorphous boundary conditions on a confined system. By using a
swap Monte Carlo algorithm we measure the equilibration time of a liquid
droplet bounded by amorphous boundary conditions in a model glass-former at low
temperature, and we show that the cavity relaxation time increases with the
size of the droplet, saturating to the bulk value when the droplet outgrows the
point-to-set correlation length. This fact supports the idea that the
point-to-set correlation length is the natural size of the cooperatively
rearranging regions. On the other hand, the cavity relaxation time computed by
a standard, nonswap dynamics, has the opposite behavior, showing a very steep
increase when the cavity size is decreased. We try to reconcile this difference
by discussing the possible hybridization between MCT and activated processes,
and by introducing a new kind of amorphous boundary conditions, inspired by the
concept of frozen external state as an alternative to the commonly used frozen
external configuration.Comment: Completely rewritten version. After the first submission it was
realized that swap and nonswap dynamics results are qualitatively different.
This version reports the results of both dynamics and discusses the different
behaviors. 17 pages, 18 figure
Mosaic multi-state scenario vs. one-state description of supercooled liquids
According to the mosaic scenario, relaxation in supercooled liquids is ruled
by two competing mechanisms: surface tension, opposing the creation of local
excitations, and entropy, providing the drive to the configurational
rearrangement of a given region. We test this scenario through numerical
simulations well below the Mode Coupling temperature. For an equilibrated
configuration, we freeze all the particles outside a sphere and study the
thermodynamics of this sphere. The frozen environment acts as a pinning field.
Measuring the overlap between the unpinned and pinned equilibrium
configurations of the sphere, we can see whether it has switched to a different
state. We do not find any clear evidence of the mosaic scenario. Rather, our
results seem compatible with the existence of a single (liquid) state. However,
we find evidence of a growing static correlation length, apparently unrelated
to the mosaic one.Comment: 4 pages, 3 figures, final version accepted in PR
Renormalization group study of marginal ferromagnetism
When studying the collective motion of biological groups a useful theoretical
framework is that of ferromagnetic systems, in which the alignment interactions
are a surrogate of the effective imitation among the individuals. In this
context, the experimental discovery of scale-free correlations of speed
fluctuations in starling flocks poses a challenge to the common statistical
physics wisdom, as in the ordered phase of standard ferromagnetic models with
symmetry, the modulus of the order parameter has finite
correlation length. To make sense of this anomaly a novel ferromagnetic theory
has been proposed, where the bare confining potential has zero second
derivative (i.e.\ it is marginal) along the modulus of the order parameter. The
marginal model exhibits a zero-temperature critical point, where the modulus
correlation length diverges, hence allowing to boost both correlation and
collective order by simply reducing the temperature. Here, we derive an
effective field theory describing the marginal model close to the
critical point and calculate the renormalization group equations at one loop
within a momentum shell approach. We discover a non-trivial scenario, as the
cubic and quartic vertices do not vanish in the infrared limit, while the
coupling constants effectively regulating the exponents and have
upper critical dimension , so that in three dimensions the critical
exponents acquire their free values, and . This theoretical
scenario is verified by a Monte Carlo study of the modulus susceptibility in
three dimensions, where the standard finite-size scaling relations have to be
adapted to the case of . The numerical data fully confirm our
theoretical results.Comment: 13 pages, 6 figures. Added numerical simulation
Equilibrium to off-equilibrium crossover in homogeneous active matter
We study the crossover between equilibrium and off-equilibrium dynamical
universality classes in the Vicsek model near its ordering transition. Starting
from the incompressible hydrodynamic theory of Chen et al
\cite{chen2015critical}, we show that increasing the activity leads to a
renormalization group (RG) crossover between the equilibrium ferromagnetic
fixed point, with dynamical critical exponent , and the off-equilibrium
active fixed point, with (in ). We run simulations of the
classic Vicsek model in the near-ordering regime and find that critical slowing
down indeed changes with activity, displaying two exponents that are in
remarkable agreement with the RG prediction. The equilibrium-to-off-equilibrium
crossover is ruled by a characteristic length scale beyond which active
dynamics takes over. Such length scale is smaller the larger the activity,
suggesting the existence of a general trade-off between activity and system's
size in determining the dynamical universality class of active matter.Comment: 10 pages, 4 figure
Geometric approach to the dynamic glass transition
We numerically study the potential energy landscape of a fragile glassy
system and find that the dynamic crossover corresponding to the glass
transition is actually the effect of an underlying geometric transition caused
by a qualitative change in the topological properties of the landscape.
Furthermore, we show that the potential energy barriers connecting local glassy
minima increase with decreasing energy of the minima, and we relate this
behaviour to the fragility of the system. Finally, we analyze the real space
structure of activated processes by studying the distribution of particle
displacements for local minima connected by simple saddles
Silent flocks: Constraints on signal propagation across biological groups
Experiments find coherent information transfer through biological groups on length and time scales distinctly below those on which asymptotically correct hydrodynamic theories apply. We present here a new continuum theory of collective motion coupling the velocity and density fields of Toner and Tu to the inertial spin field recently introduced to describe information propagation in natural flocks of birds. The long-wavelength limit of the new equations reproduces the Toner-Tu theory, while at shorter wavelengths (or, equivalently, smaller damping), spin fluctuations dominate over density fluctuations, and second-sound propagation of the kind observed in real flocks emerges. We study the dispersion relation of the new theory and find that when the speed of second sound is large, a gap in momentum space sharply separates first- from second-sound modes. This gap implies the existence of silent flocks, namely, of medium-sized systems across which information cannot propagate in a linear and underdamped way, either under the form of orientational fluctuations or under that of density fluctuations, making it hard for the group to achieve coordination.Instituto de Investigaciones FisicoquÃmicas Teóricas y AplicadasFacultad de Ciencias Exacta
Non-equilibrium Characterization of Spinodal Points using Short Time Dynamics
Though intuitively appealing, the concept of spinodal is rigourously defined
only in systems with infinite range interactions (mean field systems). In
short-range systems, a pseudo-spinodal can be defined by extrapolation of
metastable measurements, but the point itself is not reachable because it lies
beyond the metastability limit. In this work we show that a sensible definition
of spinodal points can be obtained through the short time dynamical behavior of
the system deep inside the metastable phase, by looking for a point where the
system shows critical behavior. We show that spinodal points obtained by this
method agree both with the thermodynamical spinodal point in mean field systems
and with the pseudo-spinodal point obtained by extrapolation of
meta-equilibrium behavior in short range systems. With this definition, a
practical determination can be achieved without regard for equilibration
issues.Comment: 10 pages, 12 figure
Observation of Fluctuation-Dissipation-Theorem Violations in a Structural Glass
The fluctuation-dissipation theorem (FDT), connecting dielectric
susceptibility and polarization noise was studied in glycerol below its glass
transition temperature Tg. Weak FDT violations were observed after a quench
from just above to just below Tg, for frequencies above the alpha peak.
Violations persisted up to 10^5 times the thermal equilibration time of the
configurational degrees of freedom under study, but comparable to the average
relaxation time of the material. These results suggest that excess energy flows
from slower to faster relaxing modes.Comment: Improved discussion; final version to appear in Phys. Rev. Lett. 4
pages, 5 PS figures, RevTe