Soft Report - Dipartimento di Fisica - Sapienza

Scientific Report – Non Equilibrium Dynamics and ComplexityVibrational Dynamics and Viscous Flowin Glass Forming LiquidsThe connection between slow and fast degrees offreedom in glass forming materials has been in focusof several investigations for long times. On thetheoretical side, landscape based approachessuggest a correlation between the curvature of thepotential energy minima and the barrier heights. Onthe experimental side, a number of correlationsbetween the temperature dependence of theviscosity (fragility) and other quantities related tofast degrees of freedom gradually emerged. A newimpulse in this context has been stimulated by arecently reported result connecting fragility with thelow temperature behaviour of the non ergodicityfactor.When reported on an Arrhenius plot, i.e. logarithm ofviscosity versus inverse temperature, practically allglass forming liquids stand in such a way that theycan be ordered according to their steepness at theglass transition temperature, T g. On a morequantitative groung, one defines the kinetic fragilitym as:m =limT →Tgd logηd(T T )Recent extensive inelastic x-ray scattering (IXS)measurements of the dynamic structure factor haveallowed to constitute a sizeable library of highfrequency (THz) dynamical properties of glasses. Inparticular, the IXS measurements allow for thedetermination of the non-ergodicity factor, f(Q,T),i.e. the long time limit of the normalized densitydensitycorrelation function. This quantity representsthe amount of decorrelation introduced by thevibrational dynamics, and it depends on both the (Tdependent)amplitude of the vibrations and thedegree of disorder of the glassy structure.We show that the low temperature dependence ofthe non ergodicity factor for several glasses stands ina fashion similar to the one exhibited by a T g scaledArrhenius plot (best known as Angell plot). It isindeed possible to define a glass fragility as thederivative of f(Q,T) in the T=0 limit (there is almostno Q dependence in the small Q region of interesthere) .mαdf ( Q,T )= limT → 0 d(T T )This conceptually suprising link between vibrationalmotion in glasses and diffusive processes insupercooled liquids represents a further aspect thatrequires to be clarified and, at the same time,suggests a new direction of investigation for the theultimate understanding of the glass transitionphenomenology.ggfragility m12010080604020BeF 2silicaglycerolPB 1.4mTCP00.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8SeoTPmtolsalolnBBFig. 1. Upper panel: Angell plot for a strong,intermediate and fragile system. Middle panel: Glassfragility for the same three system. Lower panel:Correlation between liquid and glass fragilityReferences[1] C.A. Angell, Science, 267, 1924 (1995)[2] T. Scopigno, G. Ruocco, F. Sette and G. Monaco,Science 302, 850 (2003).[3] V.N. Novikov. A.P. Sokolov, Nature 431, 961 (2004)[4] J. Dyre, Nature Materials 3, 749, (2004)AuthorsG. Monaco (a), G. Ruocco (b,c), T. Scopigno (c), F.Sette (a).(a) ESRF, Grenoble, France.(b) Dip. Di Fisica, Univ. Di Roma, Roma, Italy.(c) CRS SOFT-INFM-CNR, Roma, Italy.αSOFT Scientific Report 2004-0674