Physical properties of glassy materials.    The second great challenge for these glassy materi- als, i.e. near or below Tg is to understand their peculiar physical properties. Obviously, the partic- ular liquid molecular arrangements, including the number of molecules dynamically correlated in the supercooled liquid, has a determining influence on the properties of the solid. Recently, Novikov and Sokolov have found such inheritance in the properties of glasses, and showed that the viscous character of a liquid just above the glass transition temperature (the supercooled liquid) can deter-
mine    the    Poisson    ratio    of Other authors suggested that the differences observed in the vibrational properties, har- monic and anharmonic contributions of glasses, could originate from the different T-variations of the viscosity of the supercooled liquids from which the glasses were formed. Recently Shi and Falk also showed that the mechanical re- sponse of a metallic glass well below Tg was shown to depend on the quench history of the liquid and supercooled liquid. In fact one can say that the complex and history dependent state of matter is inherently associated to the rugous energy landscape attached to the glass. In this energy landscape picture the dynamics of ageing or externally driven glasses is quite intuitive (figure 2). The glass described by an energy of ≃ dN coordinates (or distances), where d stands for dimension and N for the number of particles in the sample, is in a metastable state obtained from a quenching procedure. This energy can evolve by activated processes in the case of thermal glass relaxation at T ≃ Tg (ageing) or by an external drive in athermal quasistatic simulations. In both situations local rearrangements have been observed in simulations or more recently and less commonly in experiments. These rear- rangements, that correspond to the realisation in coordinate space of an often irreversible change of local minima in the energy landscape of the glass, have been shown to be usualy very localized involving the cooperative displacement of a few tens or hundreds of elementary particles or grains, therefore happening on a length scale of only a few interparticle diameters. In ageing glasses they were shown to take the form of spring like cooperative chains, in sheared materials they have usu- aly been described in terms of T1 events (foams), or shear transformation zones (metallic glasses) or quadrupolar events (Lennard-Jones glasses, see figure 5). These localized rearragements have been recently postulated to be good microscopical canditates to be the elementary building blocks of the deformation mechanisms involved in disordered glassy materials. In fact to make a parallel with cristals these local events are now thought to be the equivalent in glassy materials of the dislocations and grain boundaries present in lattice structures of crystals and one of the great challenges is to characterize the motion and interactions of these elementary units, in the spirit of what was initiated by Peierls, Nabarro, Friedel in the 50’s and now well understood for defects and dislocations in cristals.