Topological Defects Control Dynamic Crossover in Glass-Forming Liquids

Understanding the dramatic slowdown of dynamics in glass-forming liquids remains one of the central challenges in glass physics. Using extensive molecular dynamics simulations, we uncover a topological origin of this phenomenon. Vortices in the hopping displacement vector fields are identified as the fundamental carriers of structural relaxation, whose density exhibits a pronounced maximum near the mode-coupling critical temperature. This nonmonotonic behavior signals a crossover of relaxation mechanisms from locally cooperative relaxation mediated by compact vortex structures to long-range facilitation governed by extended vortex lines. The peaking and subsequent decline of vortex density are accompanied by a sequence of topological phase transitions, which play a central role in the super-Arrhenius slowdown upon approaching the glass transition. Our results reveal a topological mechanism underlying dynamic slowdown and establish a direct link between topological defects and glassy dynamics.