Séminaire Optique/Spectro

Mardi 9 Décembre 2014 à 14h00.

Atomic motion in disordered systems studied with coherent X-rays

''Many different disordered materials undergo structural arrest and physical aging, ranging from structural glasses to soft matter and biological systems [1-3]. Albeit the apparent differences, metallic glasses, fluid lubricants, pharmaceutical compounds, window glasses and emulsions do share common features. In fact, following different experimental routes, like decreasing the temperature in a molecular glass former, increasing the packing fraction in a colloidal suspension, or applying an external stress in a granular media, these materials can be driven in an out-of-equilibrium configuration, where they cease to flow under conditions that still remain unknown [4]. Within the different disordered systems, structural glasses play a key role, since glasses are often considered as archetypes for systems far from thermodynamical equilibrium. However, and despite the strong efforts done in the field, a microscopic description of the glassy state is still missing. The main difficulty arises from the difficulty of achieving information on the dynamics at the atomic level with both experiments and numerical simulations. In the last years X-ray Photon Correlation Spectroscopy (XPCS) has emerged as a very powerful technique able to follow the atomic motion in glasses, revealing a very complex scenario [5-7]. Specifically, while metallic glasses and jammed soft materials (i.e. concentrated colloidal suspensions, gels...) display a similar microscopic dynamics characterized by a hierarchy of aging regimes [1,5,6], a different picture comes out when more conventional network glasses are investigated [7]. Collectively, these findings modify the conception of the glassy state and ask for a new microscopic theory which is still missing. Here the main results will be discussed together with a brief overview of the unique possibilities offered today by XPCS in this field of investigation. References [1] - L. Cipelletti et al. Faraday Discuss., 123, 237 (2003). [2] - A. J. Liu and S. R. Nagel, Nature, 396, 21 (1998). [3] - C. A. Angell et al. J. Appl. Phys., 88, 3113 (2000). [4] - L. Berthier and G. Biroli, Rev. Mod. Phys. 83, 587 (2011). [5] - B. Ruta et al. Phys. Rev. Lett. 109, 165701 (2012). [6] - B. Ruta et al. J. Chem. Phys. 138, 054508 (2013). [7] - B. Ruta et al. Nature Commun. 5, 3939 (2014).''