8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Thu 811:10-14:00P8.41Non-monotonic temperature evolution of dynamiccorrelations in glass-forming liquidsSandalo Roldan-Vargas, 1, 2 Walter Kob, 2 and Ludovic Berthier 21 University of Granada, Faculty of Sciences, Calle Fuentenueva s/n, 18071, Granada,Spain2 Laboratoire Charles Coulomb UMR CNRS, Université Montpellier 2, FranceThe viscosity of glass-forming liquids increases by many orders of magnitude if their temperatureis lowered by a mere factor of 2-3. The microscopic mechanisms governing structural relaxationin viscous liquids are not well understood and therefore there exists no well-accepted theoreticalexplanation for this dramatic evolution that ultimately leads to the glass transition. Recentstudies suggest that this widespread phenomenon is accompanied by spatially heterogeneous dynamicsand a growing dynamic correlation length quantifying the extent of correlated particle motion.However, the microscopic origin, nature and geometry of these collective movements remainlargely unknown. Here we use a novel numerical method to detect and quantify spatial correlationswhich reveals a surprising non-monotonic temperature evolution of spatial dynamical correlations,accompanied by a slower, monotonic growth of a second lengthscale of a very different nature.Our results directly unveil a dramatic qualitative change in atomic motions near the mode-couplingcrossover temperature which involves no fitting or indirect theoretical interpretation. We observea maximum in the size of dynamic correlations that has gone so far undetected, which we interpretas a striking signature of an avoided dynamical critical point. At low temperature, we alsoobserve an accelerated growth of a static correlation length scale, suggesting that the developmentof amorphous order controls structural relaxation. Our results impose severe new constraints ontheoretical descriptions of the glass transition, and open several research perspectives, in particularfor experiments, to confirm and quantify our observations in real materials.41