PubMed Health⌕ Search

Biomedical subjects

L Angelani

Publications and source records attributed to L Angelani.

11 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.

Journal Article↗
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.

Journal Article↗
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.

Journal Article↗
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.

Journal Article↗
PubMed2001

Topological description of the aging dynamics in simple glasses.

We numerically investigate the aging dynamics of a monatomic Lennard-Jones glass, focusing on the topology of the potential energy landscape which, to this aim, has been partitioned in basins of attraction of stationary points (saddles and minima). The analysis of the stationary points visited during the aging dynamics shows the existence of two distinct regimes: (i) at short times the system visits basins of saddles whose energies and orders decrease with t; (ii) at long times the system mainly lies in basins pertaining to minima of slowly decreasing energy. The long time dynamics can be represented by a simple random walk on a network of minima with a jump probability proportional to the inverse of the waiting time.

Journal Article↗
PubMed2000

Saddles in the energy landscape probed by supercooled liquids.

We numerically investigate the supercooled dynamics of two simple model liquids exploiting the partition of the multidimensional configuration space in basins of attraction of the stationary points (inherent saddles) of the potential energy surface. We find that the inherent saddle order and potential energy are well-defined functions of the temperature T. Moreover, by decreasing T, the saddle order vanishes at the same temperature (T(MCT)) where the inverse diffusivity appears to diverge as a power law. This allows a topological interpretation of T(MCT): it marks the transition from a dynamics between basins of saddles (T > T(MCT)) to a dynamics between basins of minima (T < T(MCT)).

Journal Article↗
PubMed2000

Off-equilibrium effective temperature in monatomic Lennard-Jones glass.

The off-equilibrium dynamics of a monatomic Lennard-Jones glass is numerically investigated after sudden isothermal density jumps (crunch) from well equilibrated liquid configurations towards the glassy state. The generalized fluctuation-dissipation relation has been studied and the temperature dependence of the violation factor m is found in agreement with the one step replica symmetry breaking scenario, i.e., at low temperature m(T) is found proportional to T up to an off-equilibrium effective temperature T(eff), where m(T(eff)) = 1. We report T(eff) as a function of the density and compare it with the glass transition temperature T(g) as determined by equilibrium calculations.

Journal Article↗
PubMed2000

Frustration and sound attenuation in structural glasses

Three classes of harmonic disorder systems (Lennard-Jones-like glasses, percolators above threshold, and spring disordered lattices) have been numerically investigated in order to clarify the effect of different types of disorder on the mechanism of high frequency sound attenuation. We introduce the concept of frustration in structural glasses as a measure of the internal stress, and find a strong correlation between the degree of frustration and the exponent alpha that characterizes the momentum dependence of the sound attenuation gamma(Q) approximately Qalpha. In particular, alpha decreases from approximately d+1 in low-frustration systems (where d is the spectral dimension) to approximately 2 for high-frustration systems such as the realistic glasses examined.

Journal Article↗
PubMed2000

Potential energy landscape and long-time dynamics in a simple model glass

We analyze the properties of a Lennard-Jones system at the level of the potential energy landscape. After an exhaustive investigation of the topological features of the landscape of the systems, obtained by studying small size samples, we describe the dynamics of the systems in multidimensional configurational space by means of a simple model. This considers the configurational space as a connected network of minima where the dynamics proceeds by jumps described by an appropriate master equation. Using this model we are able to reproduce the long-time dynamics and the low temperature regime. We investigate both the equilibrium regime and the off-equilibrium one, finding those typical glassy behaviors usually observed in the experiments such as (i) a stretched exponential relaxation, (ii) a temperature-dependent stretching parameter, (iii) a breakdown of the Stokes-Einstein relation, and (iv) the appearance of a critical temperature below which one observes a deviation from the fluctuation-dissipation relation as a consequence of the lack of equilibrium in the system.

Journal Article↗