arXiv subjects

Marta Sales

Publications and source records attributed to Marta Sales.

9 recordsLinked to original sources

arXiv2022

The effect of global warming on Western Mediterranean seagrasses: towards an agent-based modelling approach

Seagrasses are key structural elements in coastal ecosystems, and studying how temperature affects these species is crucial to anticipate the implications of global warming. In this work, we use an empirically-based numerical model to study the combined dynamics of {\it Posidonia oceanica} and {\it Cymodocea nodosa} and their resilience to sea warming. The model is parametrised using seagrass growth rates measured at the Western Mediterranean Sea. Under favorable growth conditions, our simulations predict the emergence of a coexistence region at the front between mono-specific meadows. This region can be characterised by its width and local shoot densities, which are found to depend on the coupling parameter between {\it Posidonia oceanica} and {\it Cymodocea nodosa} species. Such regions have been empirically observed in Ses Olles de Son Saura (Balearic Islands, Western Mediterranean Sea). A comparison between the field measurements at the study site with the model predictions has been used to fit the value of the coupling parameter. Field data also relates the width of the coexistence region to the average length of {\it Posidonia oceanica} leaves at the front. Remarkably, a linear relationship is found between the coupling parameter and the leaf length. In the presence of sea warming, the model predicts an exponential decay in the population of {\it Posidonia oceanica}, which is highly sensitive to temperature. This behaviour is a direct consequence of the clonal nature of the plant and can be characterised by the model parameters. Considering a scenario of high greenhouse emissions, our model forecasts that {\it Posidonia oceanica} meadows will lose 70$\%$ of their population by the year $2050$. {\it Cymodocea nodosa}, with higher thermal resilience, acts as an opportunistic species conquering the space left by the degraded {\it Posidonia oceanica}.

q-bio.PE↗

arXiv2003

Order-parameter fluctuations in Ising spin glasses at low temperatures

We present a numerical study of the order-parameter fluctuations for Ising spin glasses in three and four dimensions at very low temperatures and without an external field. Accurate measurements of two previously introduced parameters, A and G, show that the order parameter is not self-averaging, consistent with a zero-temperature thermal exponent value θ' \simeq 0, and confirm the validity of the relation G=1/3 in the thermodynamic limit in the whole low-temperature phase, as predicted by stochastic stability arguments.

cond-mat.dis-nn↗

arXiv2002

Temperature shifts in the Sinai model: static and dynamical effects

We study analytically and numerically the role of temperature shifts in the simplest model where the energy landscape is explicitely hierarchical, namely the Sinai model. This model has both attractive features (there are valleys within valleys in a strict self similar sense), but also one important drawback: there is no phase transition so that the model is, in the large size limit, effectively at zero temperature. We compute various static chaos indicators, that are found to be trivial in the large size limit, but exhibit interesting features for finite sizes. Correspondingly, for finite times, some interesting rejuvenation effects, related to the self similar nature of the potential, are observed. Still, the separation of time scales/length scales with temperatures in this model is much weaker that in experimental spin-glasses.

cond-mat.dis-nn↗

arXiv2002

Fragility of the Free-Energy Landscape of a Directed Polymer in Random Media

We examine the sensitiveness of the free-energy landscape of a directed polymer in random media with respect to various kinds of infinitesimally weak perturbation including the intriguing case of temperature-chaos. To this end, we combine the replica Bethe ansatz approach outlined in cond-mat/0112384, the mapping to a modified Sinai model and numerically exact calculations by the transfer-matrix method. Our results imply that for all the perturbations under study there is a slow crossover from a weakly perturbed regime where rare events take place to a strongly perturbed regime at larger length scales beyond the so called overlap length where typical events take place leading to chaos, i.e. a complete reshuffling of the free-energy landscape. Within the replica space, the evidence for chaos is found in the factorization of the replicated partition function induced by infinitesimal perturbations. This is the reflex of explicit replica symmetry breaking.

cond-mat.dis-nn↗

arXiv2001

Fragility of the glassy state of a directed polymer in random media: a replica approach

We study the stability of the glassy equilibrium state of a directed polymer in random media against weak perturbations including the intriguing problem of temperature-chaos. The statistical correlation of two real replica systems under perturbations, such as small difference of temperature, is studied using the mapping to a quantum problem of interacting bosons. Explicit replica symmetry breaking takes place being induced by the infinitesimally weak perturbations in the $n \to 0$ limit followed by the thermodynamic limit which leads to complete decorrelation of the two real replicas. The overlap lengths anticipated by phenomenological scaling arguments are retrieved as a correlation length associated with this explicit replica symmetry breaking.

cond-mat.dis-nn↗

arXiv2001

Rejuvenation in the Random Energy Model

We show that the Random Energy Model has interesting rejuvenation properties in its frozen phase. Different `susceptibilities' to temperature changes, for the free-energy and for other (`magnetic') observables, can be computed exactly. These susceptibilities diverge at the transition temperature, as (1-T/T_c)^-3 for the free-energy.

cond-mat↗

arXiv2001

Scaling approach to order-parameter fluctuations in disordered frustrated systems

We present a constructive approach to obtain information about the compactness and shape of large-scale lowest excitations in disordered systems by studying order-parameter fluctuations (OPF) at low temperatures. We show that the parameter $G$ which measures OPF is 1/3 at T=0 provided the ground state is unique and the probability distribution for the lowest excitations is gapless and with finite weight at zero-excitation energy. We then apply zero-temperature scaling to describe the energy and volume spectra of the lowest large-scale excitations which scale with the system size and have a weight at ze ro energy $\hat{P}_v(0)\sim l^{-θ'}$ with $v=l^d$. A low-temperature expansion reveals that, OPF vanish like $L^{-θ}$, if $θ> 0$ and remain finite for space filling lowest excitations with $θ=0$. The method can be extended to extract information about the shape and fractal surface of the large-scale lowest excitations.

cond-mat.stat-mech↗

arXiv2000

Order-parameter fluctuations (OPF) in spin glasses: Monte Carlo simulations and exact results for small sizes

The use of parameters measuring order-parameter fluctuations (OPF) has been encouraged by the recent results reported in \cite{RS} which show that two of these parameters, $G$ and $G_c$, take universal values in the $\lim_{T\to 0}$. In this paper we present a detailed study of parameters measuring OPF for two mean-field models with and without time-reversal symmetry which exhibit different patterns of replica symmetry breaking below the transition: the Sherrington-Kirkpatrick model with and without a field and the Ising p-spin glass (p=3). We give numerical results and analyze the consequences which replica equivalence imposes on these models in the infinite volume. We give evidence for the transition in each system and discuss the character of finite-size effects. Furthermore, a comparative study between this new family of parameters and the usual Binder cumulant analysis shows what kind of new information can be extracted from the finite $T$ behavior of these quantities. The two main outcomes of this work are: 1) Parameters measuring OPF give better estimates than the Binder cumulant for $T_c$ and even for very small systems they give evidence for the transition. 2) For systems with no time-reversal symmetry, parameters defined in terms of connected quantities are the proper ones to look at.

cond-mat.dis-nn↗

arXiv2000

A conjectured scenario for order-parameter fluctuations in spin glasses

We study order-parameter fluctuations (OPF) in disordered systems by considering the behavior of some recently introduced paramaters $G,G_c$ which have proven very useful to locate phase transitions. We prove that both parameters G (for disconnected overlap disorder averages) and $G_c$ (for connected disorder averages) take the respective universal values 1/3 and 13/31 in the $T\to 0$ limit for any {\em finite} volume provided the ground state is {\em unique} and there is no gap in the ground state local-field distributions, conditions which are met in generic spin-glass models with continuous couplings and no gap at zero coupling. This makes $G,G_c$ ideal parameters to locate phase transitions in disordered systems much alike the Binder cumulant is for ordered systems. We check our results by exactly computing OPF in a simple example of uncoupled spins in the presence of random fields and the one-dimensional Ising spin glass. At finite temperatures, we discuss in which conditions the value 1/3 for G may be recovered by conjecturing different scenarios depending on whether OPF are finite or vanish in the infinite-volume limit. In particular, we discuss replica equivalence and its natural consequence $\lim_{V\to\infty}G(V,T)=1/3$ when OPF are finite. As an example of a model where OPF vanish and replica equivalence does not give information about G we study the Sherrington-Kirkpatrick spherical spin-glass model by doing numerical simulations for small sizes. Again we find results compatible with G=1/3 in the spin-glass phase.

cond-mat.dis-nn↗