Decoupling of Diffusion from Structural Relaxation and Spatial Heterogeneity in a Supercooled Simple Liquid (original) (raw)

We report a molecular dynamics simulation of a supercooled simple monatomic glass-forming liquid. It is found that the onset of the supercooled regime results in formation of distinct domains of slow diffusion which are confined to the long-lived icosahedrally structured clusters associated with deeper minima in the energy landscape. As these domains, possessing a low-dimensional geometry, grow with cooling and percolate below T c , the critical temperature of the mode coupling theory, a sharp slowing down of the structural relaxation relative to diffusion is observed. It is concluded that this latter anomaly cannot be accounted for by the spatial variation in atomic mobility; instead, we explain it as a direct result of the configuration-space constraints imposed by the transient structural correlations. We also conjecture that the observed tendency for lowdimensional clustering may be regarded as a possible mechanism of fragility. PACS numbers: 64.70.Pf 1 Fragile liquids[1], having been cooled below a characteristic temperature, T A , which is typically close to the melting point, undergo a transition to the supercooled dynamics regime with super-Arrhenius slowing down and stretched exponential relaxation. Mode-coupling theory [2] appears only to be successful in interpreting early stages of supercooled dynamics. Further cooling results in a fundamental transformation of the liquid state that has not yet been comprehended in terms of theoretical models [3]. This transformation is manifested by three principal phenomena observed in the vicinity of the glass transition point T g : (i) the liquid undergoes a structural transformation shifting to the area of its energy landscape with deeper minima [4] (ii) a long-range slowly relaxing spatial heterogeneity arises[5] that is observed as formation of structurally[6] and dynamically[7] distinct long-lived domains (iii) a new type of liquid dynamics develops where the structural relaxation becomes retarded relative to the translational diffusion, thus breaking the Stokes-Einstein relation [8]. It appears sensible to ask whether these three observations represent different aspects of the same phenomenon, and, if so, what is its primary mechanism.