Soft Report - Dipartimento di Fisica - Sapienza

Investigation of the Relation Between Local InherentStructures Properties and the Diffusivity: the Role ofFluctuations in Supercooled Liquids DynamicsSupercooled liquids are characterized by dynamicswhich take place on at least two well-separated timescales: a fast microscopic dynamics (associated toatomic or molecular motions at fixed liquid structure)and a slow structural relaxation time (which requiresstructural changes and diffusion processes). In thelatest years many efforts have been made toelucidate the physics of supercooled liquids. Thetime separation between the two mechanismscontrolling the physics of supercooled liquids allowsthe development of a thermodynamic approachbased on the analysis of the potential energy surface(PES) geometry. At sufficiently low temperatures,the motion of the system can be separated intomotion confined in one PES basin (on the short timescale), and interbasin motion (on the long timescale). The study of the PES of simple models, aidedby the increased computational resources, hasprovided important insights into the mechanismscontrolling the slowing down of dynamics on coolingand the PES approach has become a powerfulformalism for the analysis of the low temperaturephysics, strongly supporting the reasonable hopethat in supercooled states structural properties areconnected to dynamics.In our work we go one step further performing adetailed study of thousands independent equilibriumtrajectories for two models for supercooled liquids:a system of 216 molecules of water interacting via atwo body potential (SPC/E) and a Kob-AndersenLennard Jones system of 155 atoms. Due to the highnumber of analyzed trajectories and to the smallsystems size we could perform a careful study offluctuating quantities. We focus on the relationbetween supercooled liquids dynamics and PESproperties and we separate in a model-free approachthe role of temperature and the role of the exploredpotential energy landscape basin depth (eIS)I inthe particle dynamics, as shown in the fig. 1. Weidentify the connection between diffusioncoefficients on the depth of the local potentialenergy minima explored. We express the averagediffusion coefficient for the system as a sum overcontributions of the sampled basins, establishing amodel unbiased connection betweenthermodynamics and dynamics in the potentialenergy to landscape framework. Moreover, ourstudy allows to clarify the role played by fluctuationsof static and dynamic properties in the slowing downof the dynamics of liquids, providing aninterpretation within the energy landscape picturefor the presence of dynamical heterogeneities andconnecting the peculiar dynamical behavior observedin supercooled states to the observed non-linearityin the relation between local diffusion and basindepth.Refences[1] E. La Nave and F. Sciortino, J. Chem. Phys. B108, 19663 (2004).[2] E. La Nave, S. Sastry and F. Sciortino, submittedto PRL (2006) and cond-mat/0512729 (2005).Fig. 1. Top Panel: Diffusivity D as a function of eISfor a Lennard-Jones system. Each point representan independent trajectory. The diffusivity is afunction of both T and eIS. Lower Panel: Same datawhere the role of temperature is sorted outrescaling D by the correct function.AuthorsE. La Nave (1), S. Sastry(2) and F. Sciortino (1)(1) Dipartimento di Fisica and INFM-CNR Udr andCRS-SOFT: Complex Dynamics in StructuredSystems, Universita' di Roma ``La Sapienza'',Piazzale Aldo Moro 2, I-00185, Roma, Italy.(2) Jawaharlal Nehru Centre for Advanced ScientificResearch, Jakkur campus, Bangalore - 500064,India.93SOFT Scientific Report 2004-06