Biomedical subjects

F Zamponi

Publications and source records attributed to F Zamponi.

7 recordsLinked to original sources

PubMed2006

Glassy behavior of light.

We study the nonlinear dynamics of a multimode random laser using the methods of statistical physics of disordered systems. A replica-symmetry breaking phase transition is predicted as a function of the pump intensity. We thus show that light propagating in a random nonlinear medium displays glassy behavior; i.e., the photon gas has a multitude of metastable states and a nonvanishing complexity, corresponding to mode-locking processes in random lasers. The present work reveals the existence of new physical phenomena, and demonstrates how nonlinear optics and random lasers can be a benchmark for the modern theory of complex systems and glasses.

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PubMed2005

Relationship between phase transitions and topological changes in one-dimensional models.

We address the question of the quantitative relationship between thermodynamic phase transitions and topological changes in the potential energy manifold analyzing two classes of one dimensional models, the Burkhardt solid-on-solid model and the Peyrard-Bishop model for DNA thermal denaturation, both in the confining and nonconfining version. These models, apparently, do not fit [M. Kastner, Phys. Rev. Lett. 93, 150601 (2004)] in the general idea that the phase transition is signaled by a topological discontinuity. We show that in both models the phase transition energy v(c) is actually noncoincident with, and always higher than, the energy v(theta) at which a topological change appears. However, applying a procedure already successfully employed in other cases as the mean field phi4 model, i.e., introducing a map M:v-->v(s) from levels of the energy hypersurface V to the level of the stationary points "visited" at temperature T, we find that M (v(c))=v(theta). This result enhances the relevance of the underlying stationary points in determining the thermodynamics of a system, and extends the validity of the topological approach to the study of phase transition to the elusive one-dimensional systems considered here.

Biophysics↗

PubMed2005

Generalized fluctuation relation and effective temperatures in a driven fluid.

By numerical simulation of a Lennard-Jones-like liquid driven by a velocity gradient gamma we test the fluctuation relation (FR) below the (numerical) glass transition temperature T(g) . We show that, in this region, the FR deserves to be generalized introducing a numerical factor X (T, gamma) <1 that defines an "effective temperature" T(FR) =T/X . On the same system we also measure the effective temperature T(eff) , as defined from the generalized fluctuation-dissipation relation, and find a qualitative agreement between the two different nonequilibrium temperatures.

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PubMed2004

Topological properties of the mean-field phi4 model.

We study the thermodynamics and the properties of the stationary points (saddles and minima) of the potential energy for a phi4 mean-field model. We compare the critical energy vc [i.e., the potential energy vT evaluated at the phase transition temperature Tc ] with the energy vtheta at which the saddle energy distribution show a discontinuity in its derivative. We find that, in this model, vc >> vtheta, at variance to what has been found in different mean-field and short ranged systems, where the thermodynamic phase transitions take place at vc=vtheta [Phys. Rep. 337, 237 (2000)]]. By direct calculation of the energy vs T of the "inherent saddles," i.e., the saddles visited by the equilibrated system at temperature T , we find that vsTc approximately vtheta. Thus, we argue that the thermodynamic phase transition is related to a change in the properties of the inherent saddles rather than to a change of the topology of the potential energy surface at T= Tc. Finally, we discuss the approximation involved in our analysis and the generality of our method.

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PubMed2004

Landscapes and fragilities.

The concept of fragility provides a possibility to rank different supercooled liquids on the basis of the temperature dependence of dynamic and/or thermodynamic quantities. We recall here the definitions of kinetic and thermodynamic fragility proposed in the last years and discuss their interrelations. At the same time we analyze some recently introduced models for the statistical properties of the potential energy landscape. Building on the Adam-Gibbs relation, which connects structural relaxation times to configurational entropy, we analyze the relation between statistical properties of the landscape and fragility. We call attention to the fact that the knowledge of number, energy depth, and shape of the basins of the potential energy landscape may not be sufficient for predicting fragility. Finally, we discuss two different possibilities for generating strong behavior.

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PubMed2004

Fragility in p-spin models.

We investigate the relation between fragility and phase space properties - such as the distribution of states - in the mean-field p -spin model, a solvable model that has been frequently used in studies of the glass transition. By direct computation of all the relevant quantities, we find that (i) the recently observed correlation between fragility and vibrational properties at low temperature is present in this model and (ii) the total number of states is a decreasing function of fragility, at variance with what is currently believed. We explain these findings by taking into account the contribution to fragility coming from the transition paths between different states. Finally, we propose a geometric picture of the phase space that explains the correlation between properties of the transition paths, distribution of states, and their vibrational properties. However, our analysis may not apply to strong systems where inflection points in the configurational entropy as a function of the temperature are found.

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PubMed2002

Crossover between equilibrium and shear-controlled dynamics in sheared liquids.

We present a numerical simulation study of a simple monatomic Lennard-Jones liquid under shear flow, as a function of both temperature T and shear rate .gamma. By investigating different observables we find that (i) there exists a line, T(.gamma), in the (T-(.gamma)) plane that sharply marks the border between an "equilibrium" and a "shear-controlled" region for both the dynamic and the thermodynamic quantities; and (ii) along this line the structural relaxation time, tau(alpha)(T(.gamma)), is proportional to .gamma(-1), i.e., to the typical time scale introduced by the shear flow. Above T(.gamma), the liquid dynamics is unaffected by the shear flow, while below T(.gamma) both T and .gamma control the particle motion.

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