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## Research interests | ||||||||

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< < | Glass transition in structural glasses, Fluctuations in granular systems, Non-equilibrium statistical mechanics. | |||||||

> > | SUPERCOOLED LIQUIDS AND THE GLASS TRANSITION | |||||||

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< < | ## Publications | |||||||

> > | In this field my research activity has been devoted to the study of
amorphous order in supercooled liquids, mainly through numerical
simulations (papers [2-6] below) but also with analytical methods (paper [0-1]),
e.g. the study of istantonic solutions of a replica field theory. I am
a proficient programmer in C, the programming language that I used for
writing codes for the simulations of both lattice (spin models) and
off-lattice (liquids) models of glass-formers. My main contributions
to the field are the following. First I want to mention the numerical
measure, presented in [4], of the distribution of the overlap order
parameter, which is associated to the glass-transition, in the deeply
supercooled regime of a glass-forming liquid. The deeply supercooled
regime is when a viscous liquid is cooled far below its melting
temperature without cristallizing: a regime which is particularly
delicate to handle in numerical simulations. Our numerical results,
obtained for a realistic 3D glass-former, suggest the existence of a
transition at low temperatures from a low-overlap (liquid) phase to a
high-overlap (glassy) phase, in agreement with standard mean-field
predictions. The ``overlap'' is the order parameter which in the
field-theoretical formulation characterizes the glass
transition. Another relevant result that I have obtained in this field
was due to a collaboration with G. Biroli: by finding non-perturbative
solutions of the appropriate replica field-theory we clarified which
is the role of geometric confinement in systems which usually become
glassy by lowering the temperature. We obtained the results of [1]
working on a toy model where exact calculations are possible: this
notwidthstanding, we expect them to be particularly relevant for a
better understanding of the physics of glass-forming materials
confined in porous matrices. Working together with people expert in
the field of the glass transition I had indeed the chance to learn
several different methods, not only numerical. In the paper [0], which
is still in preparation, we approached the problem of the glass
transition in a lattice spin model by means of several
techniques. These techniques include the use of algorithms developed
to determine the solvability of constrained satisfaction problems, an
issue that is well know to be related to the problem of the glass
transition; also, we made use of the cavity method and the Belief
Propagation equations, which allows one to exactly calculate all
thermodynamic potentials for systems on random graph geometries.
[0]
[1]
[2] | |||||||

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< < | ANOMALOUS DIFFUSION | |||||||

> > | [3] The ratchet effect in an ageing glass Gradenigo G., Sarracino A., Villamaina D., Grigera T. S. and Puglisi A. J.Stat.Mech. (2010) L12002. (http://iopscience.iop.org/1742-5468/2010/12/L12002?fromSearchPage=true)
[4]
[5] | |||||||

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< < | Einstein relation in superdiffusive systems | |||||||

> > | [6] Numerical determination of the exponents controlling the relationship between time, length, and temperature in glass-forming liquids Cammarota C., Cavagna A., Gradenigo G., Grigera T.S. and Verrocchio P. J.Chem.Phys. 131, 194901 (2009). (http://jcp.aip.org/resource/1/jcpsa6/v131/i19/p194901_s1)
The properties of the erratic motion of a tracer particle, like a
grain of dust within a glass of water, are well known when the
``environment'' is a homogeneous and equilibrium one: in this case one
finds the well-known Brownian motion characterized by a mean squared
displacement of the intruder increasing linearly with the
observational time, < X^2(t)> ~ t. In this situation
it is also well know that when the erratic motion of the tracer
particle is subjected to a small external field $\epsilon$ there is a
finite What is more intriguing, and has been the object of several of my publications [7,8,9,10], is the effect of an external perturbation in the presence of anomalous diffusion: anomalous diffusion is when the mean squared displacement of our tracer particles in the host medium grows like <X^2(t)> ~t^{\nu}, with $\nu\neq 1$. Most often these are situations where the standard tools of equilibrium statistical mechanics cannot be used and one must resort on more general approaches, e.g. the study of the Master Equation governing the process, which are well defined even in absence of an equilibrium environment.
The more interesting result that I have obtained on this subject is
discussed in [7] and [8] and concerns the relation between the shape
of probability distributions of displacements in presence of an
external field and the exponent $\nu$ which characterize the
[7]
[8]
[9] | |||||||

Gradenigo G., Sarracino A., Villamaina D. and Vulpiani A. J.Stat.Mech. (2012) L06001 (http://arxiv.org/abs/1205.6621=true) | ||||||||

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< < | On anomalous diffusion and the out of equilibrium response function in one-dimensional models | |||||||

> > | [10] On anomalous diffusion and the out of equilibrium response function in one-dimensional models | |||||||

Villamaina D., Sarracino A., Gradenigo G., Puglisi A. and Vulpiani A. J.Stat.Mech. (2011) L01002. (http://iopscience.iop.org/1742-5468/2011/01/L01002?fromSearchPage=true) | ||||||||

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< < | GRANULAR FLUIDS | |||||||

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< < | Entropy production in non-equilibrium fluctuating hydrodynamics | |||||||

> > | FLUCTUATION THEOREM
The most universal property that has been discovered in driven stationary system is the Fluctuation Relation: this is a symmetry of the probability distribution of the entropy produced per unit time, $\sigma$, which reads as $\log[P(\sigma)/P(-\sigma)]=\sigma$. In papers [7] and [9] of the list below I realized, in collaboration with people in Rome, a detailed study of a toy model of a gas such that exact results are available on $P(\sigma)$ and therefore the interplay between the Fluctuation Relation and other phyisical properties of the system can be discussed with remarkable precision. The model of [7] and [9] is a coarse-grained toy model of a gas where deterministic and stochastic ingredients are coexisting. The model is characterized by inelastic collisions among particles alternated by a deterministic motion which, in presence of an external field, is ballistic: this is the stochastic Lorentz gas. Due to the inelastic collisions and the external field the system is ``out-of-equilibrium'': nevertheless, due to the simplicity of the ingredients concourring in the description, the related Boltzmann equation can be solved and the properties of the system determined with accuracy. The Boltzmann equation is a very general one for the probability of the microstates: it requires the only assuption of ``molecular chaos'' and holds also for out-of-equilibrium systems. The unknown of the Boltzmann equation is indeed the probability distribution of microstates: the equation is nonlinear and in general is not possible to solve it exactly. Nevertheless, there are systems simple enough that the equation becomes linear and can also be solved, like the one we studied in [7],[9]. One of the properties which are more often studied in systems like the stochastic Lorentz gas is the probability of very unlikely events. While in an equilibrium system the average entropy produced per unit time is zero, within an out-of-equilibrium system this entropy is positive. Indeed one can assume as an equivalent definition of being ``out-of-equilibrium'' that of having a positive entropy production. We must then say that a positive entropy production is an \emph{average} property of \emph{macroscopic} systems. When a system is small, there can be rare fluctuations leading to a \emph{negative} entropy production. There is a branch of the theory of probability that is called Large Deviation theory and which is dealth with the study of these rare events. In [7] and [9] we used the stochastic Lorentz gas as a benchmark to compare the predictions of the Large Deviation theory on $P(\sigma)$ with the symmetry properties of $P(\sigma)$ which correspond to the Fluctuation Relation.
[11]
[12]
Granular fluids are a paradigmatic example of systems which cannot be described with the standard tools of stastistical mechanics, namely with the Boltzmann distribution of microstates. A fluid, which can be more or less dense, is said ``granular'' when its elementary components, which are usually mesoscopic beads, have inelastic collisions: this means that some energy is lost within every collision. Clearly, in order to prevent all the energy present in the system to be sucked away by the collisions, leaving the system in a quiet death, the characterization of a granular gas is usually provided also specifying the mechanism by which energy is supplied to the systems. Such a mechanism could be a homogeneous driving of all the particles, like the one discussed in several of my papers on that subject [14,17-19], but may also act just across the boundaries of the system. The continuous flux of energy across the system allows the granular gas to approach a non-equilibrium stationary state (NESS) where the distribution of microstates becomes time-independent and is of course different from the Boltzmann one. Granular fluids display several interesting phenomena that are totally absent in equilibrium fluids and that I studied during my three year post-doc in the TNT group. Among these it is worthwile to mention the behaviour of correlations, i.e. of cooperativity, between the elementary degree of the system, which differ remarkably with respect to equilibrium fluids. In equilibrium systems it happens that scale-free correlations, namely the simultaneous cooperation of all the degrees of freedom of the system, can be found only in the vicinity of a phase transition. On the contrary in granular gases and in other out-of-equilibrium systems this high degree of cooperativity can be found even far from critical points, and is just due to the ``out-of-equilibrium'' nature of the system. The intriguing point abount correlations in granular fluids to which I dedicated most of my work on granular system is the precise characterization of how the range of correlations depend on the energy injection mechanism exploited to keep the system in a stationary state. This dependence of correlations on the energy injection mechanism is the main point that I underlined in the study of the correlations among the velocities of granular beads presented in papers [14] and [17-19]. The study of correlations in granular fluids, which has been carried on making use of the linearized hydrodynamic equations with noise, yielded results in fair agreement with the experimental results obtained within the same group, paper [17]. Such a collaboration was for me very stimulating, because I had the opportunity to approach a physical problem from all the point of view: analytical, numerical and experimental. Working on granular fluids I achieved familiarity with one of analytical approaches which are usually exploited to study out-of-equilibrium systems: the study of hydrodynamic equations. Hydrodynamics equations are coupled partial differential equations which can be obtained from the Boltzmann equation postulating that only few coarse-grained field like local density $\rho(x)$, local velocity $v(x)$ and local temperature $T(x)$ are relevant. In a granular fluids the hydrodynamics equations, which represent a corase-grained description fo the model, can be linearized around the stationary solution, obtaining more simple and tractable expressions [18], and provide a very useful tool to study the extent of correlations and fluctuations within the system.
[13]
[14] | |||||||

Gradenigo G., Puglisi A., Sarracino A.J.Chem.Phys. 137, 014509 (2012) (http://jcp.aip.org/resource/1/jcpsa6/v137/i1/p014509_s1=true) | ||||||||

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< < | Non-equilibrium fluctuations in a driven stochastic Lorentz gas G. Gradenigo, U. Marini Bettolo Marconi, A. Puglisi, A. Sarracino Phys.Rev.E 85, 031112 (2012) (http://pre.aps.org/abstract/PRE/v85/i3/e031112=true)
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> > | [15] Out-of-equilibrium generalized fluctuation-dissipation relations | |||||||

Gradenigo G., Puglisi A., Sarracino A., Villamaina D. and Vulpiani A. Chapter in the book: R.Klages, W.Just, C.Jarzynski (Eds.), Nonequilibrium Statistical Physics of Small Systems: Fluctuation Relations and Beyond (Wiley-VCH, Weinheim, 2012; ISBN 978-3-527-41094-1) (http://www.maths.qmul.ac.uk/~klages/smallsys/smallsys_rk.html=true) | ||||||||

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< < | Dynamics of a massive intruder in a homogeneously driven granular fluid | |||||||

> > | [16] Dynamics of a massive intruder in a homogeneously driven granular fluid | |||||||

A. Puglisi, A. Sarracino, G. Gradenigo, D. Villamaina Comments: 6 pages, 2 figures, to be published on "Granular Matter" in a special issue in honor of the memory of Prof. Isaac Goldhirsch | ||||||||

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< < | Structure factors in granular experiments with homogeneous fluidization | |||||||

> > | [17] Structure factors in granular experiments with homogeneous fluidization | |||||||

A. Puglisi, A. Gnoli, G. Gradenigo, A. Sarracino, D. Villamaina J.Chem.Phys. 136, 014704 (2012) (http://jcp.aip.org/resource/1/jcpsa6/v136/i1/p014704_s1=true) | ||||||||

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< < | Fluctuating hydrodynamics and correlation lengths in a driven granular fluid | |||||||

> > | [18] Fluctuating hydrodynamics and correlation lengths in a driven granular fluid | |||||||

Gradenigo G., Sarracino A., Villamaina D. and Puglisi A.
J.Stat.Mech. (2011) P08017 (http://iopscience.iop.org/1742-5468/2011/08/P08017?fromSearchPage=true) | ||||||||

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< < | Growing non-equilibrium length in granular fluids: from experiment to fluctuating hydrodynamics | |||||||

> > | [19] Growing non-equilibrium length in granular fluids: from experiment to fluctuating hydrodynamics | |||||||

Gradenigo G., Sarracino A., Villamaina D. and Puglisi A.
EPL 96 14004 (2011) (http://iopscience.iop.org/0295-5075/96/1/14004?fromSearchPage=true) | ||||||||

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< < | Irreversible dynamics of a massive intruder in dense granular fluids | |||||||

> > | [20] Irreversible dynamics of a massive intruder in dense granular fluids | |||||||

Sarracino A., Villamaina D., Gradenigo G. and Puglisi A.
EPL 92, 34001 (2010). (http://iopscience.iop.org/0295-5075/92/3/34001?fromSearchPage=true) | ||||||||

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< < | STRUCTURAL GLASSES
| |||||||

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< < | Evidence for a spinodal limit of amorphous excitations in glassy systems Cammarota C., Cavagna A., Gradenigo G., Grigera T.S. and Verrocchio P. J.Stat.Mech. (2009) L12002. (http://iopscience.iop.org/1742-5468/2009/12/L12002?fromSearchPage=true) | |||||||

> > | DYNAMICAL SYSTEMS | |||||||

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< < | Numerical determination of the exponents controlling the relationship between time, length, and temperature in glass-forming liquids Cammarota C., Cavagna A., Gradenigo G., Grigera T.S. and Verrocchio P. J.Chem.Phys. 131, 194901 (2009). (http://jcp.aip.org/resource/1/jcpsa6/v131/i19/p194901_s1) | |||||||

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< < | DYNAMICAL SYSTEMS | |||||||

> > | [21] Fluctuations in partitioning systems with few degrees of freedom L. Cerino, G. Gradenigo, A. Sarracino, D. Villamaina, A. Vulpiani, accepted on Phys. Rev. E (2014). | |||||||

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< < | A study of the Fermi-Pasta-Ulam problem in dimension two | |||||||

> > | [22] A study of the Fermi-Pasta-Ulam problem in dimension two | |||||||

Benettin G. and Gradenigo G., Chaos, 18, 013112 (Mar 2008). (http://chaos.aip.org/resource/1/chaoeh/v18/i1/p013112_s1) |

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