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Scientific Report 2007-2009
Condensed matter physics and biophysics
Kosterlitz-Thouless (BKT) character, focusing on systems that are effectively low dimensional.
To this category belong layered materials with weakly-coupled planes, like the high-temperature
cuprate superconductors, as well as confined 2D structures, like thin films or conducting layers at
the interface of artificial hetero-structures (C1). Under investigation is also the connection with
”strong correlation” (C3). Indeed, superconductivity appears with highest critical temperatures
in strongly correlated materials, and in particular by doping a Mott insulator, a state in which the
carriers are localized by the mutual repulsive interactions. Such approach predicts a first-order
transition between a superconductor and a anti-ferromagnet state as a function of pressure, and a
bell-shaped superconducting region which reminds of the doping dependence of T c in the copper
oxides. A distinct, theoretical scheme is based on the idea that correlated materials are easily
prone to charge instabilities. In this case the anomalous normal and superconducting properties
naturally emerge from the abundance of soft charge fluctuations occurring when the system is
close to the charge instability. Finally, another approach focuses on the common element of all
exotic superconductors, the small value of the Fermi energy (or the Fermi velocities). From the
point of view of the many body theory of superconductivity this situation requires a generalization
which includes novel pairing channels beyond Migdal theorem.
Theoretical approaches are also applied to the study of quantum degeneracy in Fermi-Bose
atomic mixtures, in the attempt to reach temperatures significantly smaller than the Fermi temperature
to be able to observe the expected unconventional pairing mechanisms (C7).
A significant attention is devoted to the glass transition phenomenon. In the last years, interest
has shifted from spin-glasses (after Parisi developed his replica-symmetry-breaking scheme for the
infinite-range Ising spin glass) to structural and colloidal glasses, where disorder is self-generated
by the system. Here the goal is to understand if there is an unconventional thermodynamic
transition associated to the vanishing of the molecular mobility and how the temperature and
pressure dependence of the dynamics can be properly described. Beside theoretical and numerical
investigation of glass systems, interest is devoted to understanding peculiar phenomena taking
place in disordered systems. One of these is the theoretical and experimental investigation of the
nature of collective excitations in disordered solids, a topic reinvigorated by the discovery that
disordered materials, such as glasses and liquids, support the propagation of sound waves in the
Terahertz frequency region, made possible recently thanks to the development of the Inelastic X-
ray Scattering (IXS) technique (C8) and the availability of specific beam lines at ESRF (Grenoble).
Significant efforts are also made in the direction of understanding dynamic arrest with mechanism
different from packing and the differences between gels and glasses (C9) and anomalous systems
where dynamic arrest or crystallization take place on heating and/or decreasing packing (C10).
Statistical physics has proven to be a very fruitful framework to describe phenomena outside
the realm of traditional physics. The last years have witnessed the attempt by physicists to
study collective phenomena emerging from the interactions of individuals as elementary units in
social structures. Our department is particularly active on a wide list of topics ranging from
opinion, cultural and language dynamics (C11) to the dynamics of online social communities.
In all these activities a crucial element is the information shared in specific groups and one is
interested in understanding how this information emerges, spreads and gets shared, is organized
and eventually retrieved. A similar knowledge transfer is taking place from Physics to Finance
and Economics, a topic which, after the sub-prime crisis in the financial world, has attracted
renewed interest. Indeed, standard risk analysis usually neglects concepts like collective behavior,
contagion, network domino effect, coherent portfolios, lack of trust, liquidity crisis, and, in general
psychological components in the traders behavior (C12). The research we develop is based on the
introduction of suitable models with heterogeneous agents and a different perspective in which
the interaction between agents (direct or indirect) is explicitly considered together with the idea
that the system may become globally unstable in the sense of self-organized criticality.
Collective behavior is commonly found also in biology, occurring at several scales and levels
of complexity. Animal groups - like insect swarms and bird flocks - are paradigmatic cases of
Sapienza Università di Roma 48 Dipartimento di Fisica