LUNGHI

- Vespignani

- Caselle

Vecchi e  nuovi risultati sul modello di Ising in tre dimensioni.

Abstract:
Nonostante gli impressionanti sforzi compiuti negli ultimi 50 anni nel
tentativo di trovare una soluzione esatta, il modello di Ising in 3
dimensioni rimane uno dei problemi irrisolti piu' affascinanti e ricchi di
sorpese della  meccanica statistica teorica. In questo seminario dopo una
breve introduzione al modello ed ad alcuni risultati noti da tempo (in
particolare l'esistenza di una trasformazione di dualita' che lega il
modello con la teoria di gauge di gruppo Z2 e la possibilita' di
descrivere il modello in termini di una teoria di campo phi^4), discuteremo
alcuni progressi piu' recenti ed in particolare:
1] I tentativi di descrivere il modello di Ising usando la teoria
   delle stringhe
2] L'esistenza di stati legati nella fase di bassa temperatura del modello

-Pietronero

MEDI

- Gabrielli

Gravitational evolution of a perturbed lattice and its fluid limit
 
We apply a simple linearization, used standardly in solid state
physiscs, to describe the evolution under its self-gravity of an infinite
perfect lattice perturbed from its equilibrium. In the limit that the
initial perturbations are restricted to wavelengths much larger than the
lattice spacing, the evolution corresponds exactly to that usually derived
by cosmologists from an analogous linearization of the Lagrangian
formulation of the dynamics of a pressureless self-gravitating fluid,
including the so called Zeldovich approximation. Our less restricted
approximation, however, allows one to trace the evolution of the fully
discrete distribution until the time when the particles approach one
another, with modifications of the fluid limit explicitely depending on
the lattice spacing.

- Politi

Dinamiche di crescita di fronti unidimensionali

Consideriamo una superficie o interfaccia in una dimensione, u(x,t), il cui profilo piatto è linearmente instabile e la cui evoluzione
temporale è descritta da una equazione differenziale del tipo u_t = F[u]. Ci chiediamo se è possibile stabilire alcune delle sue proprietà dinamiche dalla sola conoscenza delle sue soluzioni
stazionarie, F[u]=0. Per alcune classi di equazioni è possibile dimostrare in maniera rigorosa che questo legame esiste e passa attraverso la dipendenza della lunghezza d'onda di tali soluzioni dalla loro ampiezza: se al crescere della seconda (l'ampiezza) cresce anche la prima (la lunghezza d'onda), e solo in questo caso, abbiamo un processo di coarsening per cui, dinamicamente, il fronte sviluppa una lunghezza caratteristica che cresce nel tempo. La legge con cui cresce è ricavabile analiticamente. 

- Borgonovi

Quantum signature of the Classical Disconnection Border

A quantum  Heisenberg spin chain with anisotropic coupling and all-to-all interspin interaction has been analyzed using Bose
statistics. In (JSP 116, 1435 (2004)) the existence of a disconnection
threshold in the classical version has been demonstrated. The question of finding a quantum signature of the classical disconnection
border has been addressed here. A quantum disconnection border,
motivated by purely quantum considerations is given. The presence of a disconnection border allows the study of
Macroscopic Quantum Phenomena, like Macroscopic Quantum Tunneling and
Macroscopic Quantum Coherence. Indeed, below this border, at variance with the classical case, the total magnetization can flip its sign through
Macroscopic Quantum Tunneling. We also discuss the dynamical relevance of such border with respect to the magnetic reversal times, in the light of the
results found in the classical model.

- Baldassarri

Dinamica di stick-slip nei materiali granulari: Analisi statistica di
un'esperimento

Un materiale granulare sottoposto ad un debole sforzo risponde con una
dinamica intermittente, durante la quale dei periodi di quiete
intervallano rapidi movimenti di assestamento dei grani (dinamica di
stuck-slip). L'estrema irregolarita' della dinamica genera l'impressione 
di una intrinseca irriproducibilita' del fenomeno che sfida la
possibilita' di una sua descrizione quantitativa.

Al contrario, la descrizione  statistica della dinamica osservata in un
esperimento ha permesso di cogliere comportamenti analoghi a quanto osservato
in un fenomeno estremamente diverso, quale l'effetto Barkhausen
nei materiali magnetici. Questa sorprendente analogia ci ha permesso 
di formulare un modello fenomenologico del sistema che, sebbene si basi su
poche ipotesi motivabili fisicamente, riproduce quasi quantitativamente la
complessa dinamica osservata negli esperimenti.

Lo studio sembra individuare quali siano le caratteristiche fisiche
rilevanti per determinare la dinamica di stick-slip. Se da una parte cio'
giustifica l'universalita' riscontrata con altri fenomeni fisici, quali il
succitato Effetto Barkhausen o l'attrito tra solidi, dall'altra segna la
via per uno studio "microscopico" del mezzo granulare.

- Mossa

link

- Pagnani

Regulatory Control in Boolean Networks

We explore the static behavior of large Boolean networks with methods
recasting Boolean regulation into a constraint satisfaction problem.
Our analysis includes a modified version of the leaf-removal algorithm
as well as belief and survey propagation. This allows to explore the
complex solution-space structure of the problem. We find a phase
transition from simple to complex regulatory control, and identify
relevant regulatory variables which select the fixed points of the
network within the global solution space. Some biological implications
on Biological Gene Regulatory Networks will be also discussed.

- Cencini

Nonlinearly driven synchronization in chaotic systems

Synchronization transitions are investigated in coupled
chaotic systems, in particular chaotic maps.  Depending on the
relative weight of linear versus nonlinear instability mechanisms
associated to the single map two different scenarios for the
transition may occur. When only two maps are considered we always find
that the critical coupling varepsilon_l for chaotic synchronization
can be predicted within a linear analysis by the vanishing of the
transverse Lyapunov exponent lambda_T.  However, major differences
between transitions driven by linear or nonlinear mechanisms are
revealed by the dynamics of the transient toward the synchronized
state.  As a representative example of extended systems a one
dimensional lattice of chaotic maps with power-law coupling is
considered. In this high dimensional model finite amplitude
instabilities may have a dramatic effect on the transition.  For
strong nonlinearities an exponential divergence of the synchronization
times with the chain length can be observed above varepsilon_l,
notwithstanding the transverse dynamics is stable against
infinitesimal perturbations at any instant. Therefore, the transition
takes place at a coupling varepsilon_{nl} definitely larger than
varepsilon_l and its origin is intrinsically nonlinear. The
linearly driven transitions are continuous and can be described in
terms of mean field results for non-equilibrium phase transitions with
long range interactions. While we have some evidences that the
transitions dominated by nonlinear mechanisms is discontinuous.

- Castellano

Avalanche asymmetry in crackling noise and the sign of the effective mass. 

The response of materials and the functioning of devices are
often associated to crackling noise, which encodes fundamental
physical properties of the system. The asymmetry of avalanche shapes,
commonly observed in many diverse noisy phenomena, still needs an
explanation.  We show that such asymmetry is a
direct signature of the presence of inertial effects and, in
particular, of the sign of the effective mass. We present experiments
on the Barkhausen effect (the noise induced in magnets by the jerky
motion of domain walls as they interact with impurities) and
quantitatively explain the results in terms of the propagation of eddy
currents in the material. The leftward asymmetry of avalanche pulses
indicates the presence of a negative effective mass for ferromagnetic
domain walls.  These results provide a method to determine the
underlying effective mass from a generic noisy signal.

- Lepri

Mode-coupling theory of anomalous transport in one dimension

In one and two dimensions, transport coefficients may diverge in the
thermodynamic limit due to long--time correlation of the corresponding
currents. Indeed, power--law decay of correlations is expected to be a generic 
feature of one--dimensional systems in presence of conservation laws.
A relevant example is the divergence of the thermal conductivity
coefficient observed numerically in chains of anharmonic oscillators 
and 1D gases. To what extent are those observations universal? To answer 
this question we studied a 1d version of mode-coupling equations, which
describe
the relaxation of fluctuations of the displacement field. We show 
that the latter admit scaling solutions in the small-wavenumber limit 
which, at variance with usual linearized hydrodynamics, 
are not exponential. The theory predicts that the heat current 
autocorrelation should decay as t^{-2/3}, which is in agreement with
recent simulation results on the 1D diatomic gas.


- Verrucchi

Quantum critical enhancement of multipartite entanglement in spin systems

We study the field dependence of the entanglement of formation in one-
and two-dimensional quantum spin systems displaying a T=0
field-driven quantum phase transition. Making use of exact results and
quantum Monte Carlo data for the entanglement of formation, we show
how entanglement can be a fundamental tool to understand the quantum
critical behavior and to find ``special states'' that usual magnetic
observables do not detect.

We find that the ground state of anisotropic two-dimensional S=1/2
antiferromagnets in a uniform field takes the classical-like form of a
product state for a particular value and orientation of the field, at
which the purely quantum correlations due to entanglement
disappear. Moreover, the factorized state both in 1D and 2D systems is
found to precede the quantum phase transition. It could hence
represent a crucial step towards a global rearrangement of the ground
state, in view of the quantum phase transition. Finally, we show
that the field-induced quantum phase transition present in the models
is unambiguously characterized by a cusp minimum in the
pairwise-to-global entanglement ratio R, marking the
quantum-critical enhancement of "multipartite" entanglement.

- Illuminati

(non l'ho trovato)

BREVI

- Baronchelli

EMERGENCE OF COMMUNICATION IN LANGUAGE GAMES

 The self-organization of communication
 conventions in a population of agents is object of growing attention.
 Besides its intrinsic theoretical relevance, in fact, it has recently
 acquired importance for many applications to artificial and distributed
 systems.
 In this talk I will illustrate a very simple model in which agents play
 pairwise games in order to negotiate conventions, i.e. associations between
 forms and meanings. The model describes the emergence of a communication
 system and in addition exhibits a rich phenomenology, while its
 simplicity makes it suitable for analytical approaches.

- Cerruti

Charge fluctuations and electron-phonon interaction in the 
finite-U Hubbard model

In the past months several experiments pointed out an important role of the 
electron-phonon (el-ph) interaction in many physical properties of the 
cuprates. These recent findings have triggered a renewed interest for a 
theoretical understanding of the el-ph properties in strongly correlated 
sistems. In this contribution we employ a gaussian expansion within the 
finite-U slave-bosons formalism to investigate the momentum structure 
of the electron-phonon vertex function in the Hubbard model as function 
of U and n.
The suppression of large momentum scattering and the onset of a 
small-q peak structure, parametrized by a cut-off q_c,
are shown to be essentially ruled by the band narrowing factor 
Z_{\rm MF} due to the electronic correlation. 
A phase diagram of Z_{\rm MF} and q_c in the whole U-n space 
is presented.
Our results are in more than qualitative agreement
with a recent numerical analysis and
permit to understand some anomalous features of the Quantum Monte Carlo data.

- Sbragaglia

Lattice Boltzmann Models in microchannel flows

link

- Ngo

Inferring the diameter of a biopolymer from its stretching response

We investigate the stretching response of a thick polymer model by means of
extensive stochastic simulations. The computational results are synthesized
in an analytic expression that characterizes how the force versus elongation
curve depends on the polymer structural parameters: its thickness 
and granularity (spacing of the monomers). The expression is used to 
analyze experimental data for the stretching of various different types of
biopolymers: polypeptides, polysaccharides and nucleic acids. 
Besides recovering elastic parameters (such as the persistence length) that 
are consistent with those obtained from standard entropic models, the approach 
allows to extract viable estimates for the polymers diameter and granularity. 
This shows that the basic structural polymer features have such
a profound impact on the elastic behaviour that they can be 
recovered with the sole input of stretching measurements.
 

- Anfossi

Entanglement and insulator-superconductor transition in the Hirsch model. 

Authors: A. Anfossi, A. Montorsi Abstract: We use the notion of 
entanglement, and its measures, to get a newer and deeper insight on the 
phenomenon of Quantum Phase Transitions (QPTs), i.e. phase transitions 
occurring in quantum systems at vanishing temperature. In particular, We 
consider the Hubbard model with bond charge interaction x (Hisrch 
Model). As a first result [1], we reproduce the exact phase diagram for 
x=1 --integrable case-- by means of measures of entanglemet (Single 
and Two-site Von Neumann Entropy, Quantum Mutual Information, 
Negativity, Purity). Moreover [2], in the non integrable case x<=1, 
we investigate at half filling the superconductor-insulator QPT. The 
study is performed by means of a composite analysis of the numerical 
results obtained within Density-Matrix Renormalization Group algorithm; 
on the one hand, the charge gap closure is studied by standard finite 
size scaling analysis; on the other hand we look at singularities in the 
derivatives of single-site entanglement. The results of the two 
techniques perfectly agree for x >0.5, showing that the validity of 
the results obtained by bosonization technique ceases in this region: 
the transition takes place at a finite Coulomb interaction u_c. 
Numerical extimations of binding energy, spin gap and pairing 
correlations are then exploited to characterize the superconducting 
nature of the region below u_c.

- Zonta

Dimensione dei nodi in polimeri circolari.

 
Vogliamo studiare, via simulazioni Monte Carlo e analisi di scaling, la
lunghezza media all'equilibrio (<l>) di un nodo in un polimero circolare.
Usando due diverse definizioni statistiche di lunghezza di un nodo troviamo 
che <l>~ N^t, dove N e' il grado di polimerizzazione dell'anello. In
particolare emerge che i nodi primi sono debolemente localizzati (0<t<1)
nel regime di buon solvente, ma si delocalizzano (t=1) nella fase di bassa 
temperatura. La localizzione debole implica l'esistenza di una lunghezza
di scala non banale (la lunghezza del nodo) oltre alla dimensione dell'intero 
anello. Mostriamo che questa nuova lunghezza di scala modifica profondamente
la forma della correzione allo scaling per il raggio di girazione medio.
L'esponente della prima correzione risulta quindi differente da quello
congetturato dalla teoria dei campi, dove si ha una integrazione implicita
su tutte le topologie.

- Imparato

link

- Moretti

Meccanica Statistica della Deformazione Plastica: Rilassamento e Dinamica
Vetrosa.

La deformazione plastica nei metalli e' accompagnata dalla proliferazione di
dislocazioni cristalline ed influenzata dal loro comportamento collettivo.
Studi numerici hanno di recente dimostrato che il rilassamento di sistemi
inizialmente disordinati di dislocazioni e' caratterizzato da una dinamica vetrosa.
In questo contesto, presentiamo i risultati di studi analitici e simulazioni
numeriche di tali sistemi, evidenziando il ruolo delle fluttuazioni di densita'
e della natura delle interazioni tra dislocazioni.
Nello spirito dei modelli chimici per reazioni bimolecolari, mostriamo che la
dinamica vetrosa del rilassamento e' guidata dalle variazioni locali nella
distribuzione di dislocazioni. Questa descrizione resta valida anche in
presenza di sollecitazioni esterne, in virtu' della natura a lungo raggio delle
interazioni in gioco.


- Battaglia

FINDING A NEEDLE IN THE HAYSTACK: EFFICIENT SELECTION OF GLASSY STATES

The onset of complexity in many combinatorial
optimization problems can be connected to the existence of phases of
in which the ground states of the corresponding diluted spin
statistical model form an exponential number of disconnected clusters.
Typically, local search algorithms like Simulated Annealing are unable
to find even a single one optimal configuration, due to the presence of an
exponentially larger number of metastable states and local minima.
In our work we show how the Survey Propagation algorithm can
be generalized to include external forcing messages, and used to address
selectively an exponential number of glassy ground states. These
capabilities are used to provide a direct
experimental evidence of replica symmetry breaking in large-size
problems and to realize a new lossy data compressor, exploiting as a
computational resource the clustered nature of the space
of addressable states.

- Massel

 Dynamical algebra for ultracold fermions in 1D optical lattices

- Puppin

Temperature, energy equipartition and complexity: an experimental view

One of the key issues of modern days statistical physics is the investigation of complex systems, with a particular emphasis on the occurrence of power-law probability distributions for certain observable quantities x: P(x) = x^(-a). 
A major goal of these investigations is the possibility to predict, on the basis of the microscopic properties of the system, the value of the so-called critical exponent a. Surprisingly enough, none of the available paradigms explicitly takes into account one of the central concepts of classical statistical physics, i.e., temperature.

In our experiment we studied the role of temperature in determining the statistical properties of Barkhausen noise in thin Fe films. In the temperature range between 10 K and 300 K the probability distribution for the amplitude of the magnetization avalanches is always a power-law. The critical exponent a, however, undergoes a strong variation since its value changes from a = 1 at 300 K to a = 1.8 at 10 K. At our knowledge this is the first experimental evidence of the role of temperature in a complex system.

In the present paper we suggest the possibility that for complex systems such as magnetic materials, a generalized version of the energy equipartition principle might hold. Within this ansatz the energy released during the dynamical evolution of the system is “shared” between all the available avalanches depending on their size: the amount of energy released through avalanches of each possible size is the same. In our experiment this behaviour is actually observed at room temperature. At low temperature a “freezing” of the system prevents the occurrence of large size events and therefore the critical exponent has a larger value. A similar effects, due to quantum effects, also takes place in classical statistical physics: the well known freezing of the rotational and vibrational degrees of freedom of a gas molucule at low temperature.


- Pigolotti
Aspects of pseudo-chaos: random number generators

I will discuss two properties making a deterministic
algorithm suitable to generate a pseudo random sequence of numbers: high
value of Kolmogorov-Sinai entropy and high-dimensionality. The multi
dimensional Arnold's cat map is proposed as a pseudo-random number
generator using both these mechanisms. Testing and comparisons with other
generators will be presented

- Fanelli

Large scale dynamics of antibodies from tomographic data

The issue of protein dynamics and its implications in the biological function of
proteins are arousing greater and greater interest in different fields of
molecular biology. In cryo-electron tomography experiments one may take several
snapshots of a given biological macromolecule. In principle, a large enough
collection of snapshots of the molecule may then be used to calculate its
equilibrium configuration in terms of the experimentally accessible degrees of
freedom and, hence, to estimate its potential energy. In this talk, we analyze the
results of cryo-electron tomography experiments performed on monoclonal murine
IgG2a antibodies. We measure the equilibrium distribution of the molecule in terms
of the relevant angular coordinates and build a mechanical model of the antibody
dynamics. This approach enables us to derive an explicit expression of the IgG
potential energy. Brownian dynamics simulations are performed and the role of
flexibility in the formation of the comp
lex antigen-antibody investigated.

- Jepps

Transport Complexity in Simple Porous Media

We examine the transport behaviour of non-interacting particles in a
simple channel billiard, at equilibrium and in the presence of an external
field. The channel walls are constructed from straight line-segments. We
observe a sensitive dependence on the model parameters of the transport
properties, which range from sub-diffusive to super-diffusive regimes. In
non-equilibrium, we find a transition in the transport behaviour between
seemingly-chaotic and (quasi-) periodic behaviour. Our results support the
view that normal transport laws do not need chaos, or quenched disorder,
to be realized. Furthermore, they motivate some new definitions of
complexity, which are relevant for transport phenomena.

- Musacchio

High-friction asymptotics for 2D turbulence.

Two-dimensional turbulent flows can be strongly affected
by the interaction with the three-dimensional environment
in which they are embedded. This interaction is often frictional,
and can be modeled by means of a linear damping term
in the equation of motion. In both experiments and numerical simulations
it has been observed that, due to the presence of friction,
the kinetic energy spectrum deviates from the classical
Kraichnan prediction k^(-3).
We discuss the high-friction asymptotics, deriving
explicit expressions for the spectral slope and for the
scaling exponents of vorticity structure functions.


- Jug

TEORIA DELLA RISPOSTA MAGNETODIELETTRICA ANOMALA NEI VETRI SILICATI
MULTICOMPOSTI A BASSA TEMPERATURA: SOLUZIONE DI UN MISTERO?


Recenti esperimenti condotti dai gruppi di Hunklinger/Strehlow a Heidelberg/Berlin
e di Ladieu-LeCochec-Pari a Saclay hanno indicato un'incontrovertibile risposta
anomala della permittivita' dielettrica nei vetri silicati multicomposti a bassa
temperatura (T<100 mK) e in presenza di deboli campi magnetici (B<<1 T). Lo studio
sistematico di questi materiali ha rivelato sia un'esaltazione della permittivita'
a bassissimi campi (B~0-0.03 T) che una decisa riduzione della stessa a campi piu'
elevati (B>0.1 T); un'andamento oscillatorio sembra instaurarsi a campi ancora piu'
alti. Questo comportamento e' inspiegabile in termini termodinamici e non ha ancora 
trovato una spiegazione teorica convincente, nonostante alcune proposte di una certa 
risonanza (nuovo stato di tunneling coerente della materia/Fulde/, accoppiamento
mediato da nuclei a quadrupolo elettrico/Enss-Wuerger/Fulde/). 
Nella mia presentazione mostrero' come il comportamento magnetodielettrico anomalo
vada messo in relazione a quello magnetotermico negli stessi materiali, altrettanto 
anomalo, che ho gia' provveduto a spiegare per mezzo di un'estensione del modello
'standard' del tunneling tra due stati di un potenziale locale a doppia buca. Il mio
modello prende le mosse da studi di simulazione e di diffusione neutronica/Kob/ in 
questi vetri silicati, che indicano come si debba tenere conto di almeno due specie
dinamiche presenti nella loro composizione chimico-fisica. Il modello delle due 
specie di centri di tunneling/Jug/ spiega moltissima della fenomenologia osservata in 
questi vetri negli ultimi decenni e l'interpretazione delle anomalie magnetotermiche
verra' brevemente ripresentata. La seconda parte della presentazione vertera' sulla
teoria sviluppata di recente per l'interpretazione delle anomalie magnetodielettriche.
Non solo l'esaltazione della permettivita' a deboli campi verra' spiegata, ma anche
la successiva depressione per campi crescenti che nessun gruppo teorico ha sinora
saputo affrontare. Si mostrera' come l'accordo tra teoria e esperimenti dimostri 
in modo inequivocabile la presenza di potenziali di tunneling a piu' di due minimi
in questi vetri e l'importanza di una separazione microfasica nel materiale, con
formazione di nanocristalli responsabili della sensibilita' a debolissimi campi.
La teoria rappresenta la base per una completa comprensione in termini di meccanica
statistica quantistica delle straordinarie proprieta' fisiche di questi materiali che
potrebbero rivelarsi utili sensori sia di bassissime temperature che di debolissimi 
campi magnetici e rappresentare un nuovo stimolante campo di studi.

- Mognetti

Transizioni di fase a temperatura finita: Modello di Heisenberg
ed altri esempi.

Sono stati studiati una classe di modelli statistici su
reticolo con simmetria O(N) e generica interazione a
primi vicini ferromagnetica.
Simulazioni M.C. e stime esatte mettono in evidenza la
possibilita' di transizioni di fase continue a temperatura
finita il cui parametro critico e' l'energia. Argomentazioni
di simmetria ed evidenze numeriche prevedono poi un
comportamento critico di Ising. D'altra parte uno studio
analitico a N=infinito evidenzia una criticita' del 
campo medio.
Spiegheremo l'apparente paradosso includendo ordini in
1/N,  mostrando come la regione critica di Ising scali
come 1/N nella direzione termica e come 1/N^{3/2} in 
quella magnetica. Questo spiega il motivo per cui a
N=infinito soltanto il campo medio e' osservato.
Mostreremo poi come la tecnica sviluppata sia poi
estendibile ad altri sistemi statistici, in
particolare sono stati considerati modelli Multicritici,
ed un modello di Yukawa.

- Lanotte

Proprieta' statistiche del moto di particelle inerziali in un fluido turnbolento


Verranno presentati alcuni risultati preliminari di simulazioni numeriche ad alta risoluzione 
di traccianti passivi (privi di massa) e particelle inerziali disperse un fluido turbolento.
Mentre la distribuzione stazionaria dei traccianti passivi e' omogenea, le particelle inerziali
tendono a distribuirsi in modo disomogeneo formando cluster. In particolare verra' esporato il 
duplice ruolo associato all'inerzia: di tipo dinamico, in base al quale le particelle tendono 
a scappare dalle regioni ellittiche del fluido, i.e. a forte vorticita'; di filtro temporale, 
poiche' le particelle inerziali a differenza dei traccianti passivi, hanno un tempo di 
risposta finito.

- vaia

Reentrant behavior and dissipative transition in 2D Josephson junction arrays

The effects of dissipation are studied in the proximity of a quantum phase 
transition. The prototypical system is a resistively shunted 
two-dimensional Josephson junction array, simulated by means of an advanced 
Fourier path-integral Monte Carlo algorithm. A reentrant 
superconducting-to-normal phase transition is the most dramatic signature 
of the strong nonlinear fluctuations characterizing the quantum critical 
region at finite temperature.
It is shown that a region of quantum criticality exists at finite 
dissipation and the zero-T phase diagram is constructed. The latter is of 
crucial importance for the fabrication of samples with interaction 
parameters suited to match the quantum critical region. It is shown how 
quantum fluctuations are quenched by dissipation, leading to the removal of 
the reentrance. As dissipation can drive the system in and out of the 
critical region, it reveals as a fundamental aspect for both theoretical 
and experimental investigations.


- De Stefanis

Simple lattice model for the solvation of an apolar molecule in water

We investigate a lattice-fluid model defined on a two-dimensional
triangular lattice, with the aim of reproducing qualitatively some
anomalous properties of water as a solvent for nonpolar (inert) molecules.
Water molecules are of the ``Mercedes Benz'' type, i.e., they possess
an equilateral triangle symmetry, with three bonding arms.
Bond formation depends both on orientation and local density.
The insertion of nonpolar molecules (no bonding arms) displays the
qualitative features that are typical signatures of the hydrophobic
hydration: a large negative transfer entropy; a large
positive transfer free energy; a steep temperature dependence of
the transfer entalpy and entropy, i.e, a large positive transfer
heat capacity. In order to understand the microscopic bases of the
hydrophobic effect we include the analysis of the hydrogen-bond coordination
of a water molecule in the bulk and in the first hydration shell.
The results are compared with experiments where possible.
The finite temperature analysis is carried out by a generalized first order
approximation on a triangle cluster.
In the very last part of the talk we briefly introduce a
work-in-progress application
of the water model to the study of the hydration of a polymer (SAW
on a triangular lattice) by means of Dynamic Monte Carlo methods.

- Cicuta

Matrici aleatorie e conteggio di cammini

Un metodo sistematico permette di valutare l'aspettazione  <Tr  M^p> per
matrici  M  di arbitrario ordine n x n, con entrate M_{ij} variabili
aleatorie indipendenti identicamente distribuite.
Il metodo e' facilmente utilizzato su computer e porta ad una valutazione
esatta di una parte dello sviluppo di Taylor del  risolvente G(z).
Queste informazioni sono utili per lo studio dei riscalamenti necessari
nel limite n \to \infty, per la determinazione dello spettro delle matrici
laplaciane, per discutere la validita' ed i limiti del teorema di
addizione delle matrici aleatorie.