Abstract
One of the most controversial hypotheses for explaining the heterogeneous dynamics of glasses postulates the temporary coexistence of two phases characterized by a high and by a low diffusivity. In this scenario, two phases with different diffusivities coexist for a time of the order of the relaxation time and mix afterwards. Unfortunately, it is difficult to measure the single-particle diffusivities to test this hypothesis. Indeed, although the non-Gaussian shape of the van-Hove distribution suggests the transient existence of a diffusivity distribution, it is not possible to infer from this quantity whether two or more dynamical phases coexist. Here we provide the first direct observation of the dynamical coexistence of two phases with different diffusivities, by showing that in the deeply supercooled regime the distribution of the single-particle diffusivities acquires a transient bimodal shape. We relate this distribution to the heterogeneity of the dynamics and to the breakdown of the Stokes-Einstein relation, and we show that the coexistence of two dynamical phases occurs up to a timescale growing faster than the relaxation time on cooling, for some of the considered models. Our work offers a basis for rationalizing the dynamics of supercooled liquids and for relating their structural and dynamical properties.
Highlights
Deeply supercooled regime, we find this distribution to temporarily acquire a bimodal shape, proving the transient coexistence of two distinct dynamical phases
In order to prove that the continuous time random walk (CTRW) approach provides a quantitative description of the dynamics of the KA mixture, we have performed a careful analysis of the single particle cage–jump intermittent motion, for temperatures slightly above the mode–coupling one[15,16,17], T mct 0.435
As in the case of the diffusivities, small deviations are observed at the lowest temperatures. These results clearly demonstrate that 〈tw〉 and 〈tp〉 respectively correspond to the β and to the α relaxation time scales of structural glasses[28,30,31], and confirm that the breakdown of the Stokes–Einstein (SE) relation, which is the increase of the product τλD on cooling, is mainly due to the increase of the t p / tw ratio, as in lattice model, but it is affected by the temperature dependence of the jump length
Summary
Deeply supercooled regime, we find this distribution to temporarily acquire a bimodal shape, proving the transient coexistence of two distinct dynamical phases. Particles in a glass spend most of their time confined within the cages formed by their neighbors, seldom hopping to different cages This allows to describe the dynamics through the continuous time random walk (CTRW) formalism, reviewed in the Appendix. We discuss in detail the KALJ system to show that the CTRW approach quantitatively describes the relaxation dynamics of atomistic systems, of kinetic lattice models[28,29]. We use this approach to measure the diffusivity distribution and to investigate its time evolution
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