Decoupling limit of diffusion and structural relaxation predicted by a fragility-tunable glassy model

A full picture of dynamic properties through diverse glasses remains a great challenge in glassy physics. The kinetic fragility is introduced to classify glass-forming liquids and its relevance to glassy properties is expected to outline the family characteristics of glasses. In this paper we propose a distinguishable-particle glassy model with simple pair interactions. This model sensitively tunes the kinetic fragility in an ultrawide range covering real glassy materials. Using the model, we study the decoupling of self-diffusion and structural relaxation time close to the glass transition, and present the fragility dependence of the fractional Stokes-Einstein relation. The results support the existence of a decoupling limit, which corresponds to a possible lower bound of the fractional Stokes-Einstein exponent in very fragile glass-forming liquids. The microscopic mechanism of the fractional Stokes-Einstein relation is verified by using the hopping-dynamics approach associated with single particles.