Universal bifurcation in glass relaxation dynamics revealed by ultra-low-frequency spectroscopy

Probing ultra-slow relaxation dynamics far below the glass transition temperature (T_g) remains a significant challenge. Here, we introduce static square-wave mechanical spectroscopy (SSWMS), a technique that accesses the microhertz regime with high precision. Applying this method to six glasses, we resolve two fundamental issues. First, we map the complete kinetic evolution, revealing a transition from Vogel-Fulcher-Tammann dynamics to a metastable state characterized by an apparent exponential temperature dependence, which is fundamentally distinct from the non-equilibrium evolving pathway of quenched glasses. Second, we verify the intrinsic β-α correlation, demonstrating that the coupling model quantitatively predicts cooperative α-relaxation based on local β-motions across the entire range from the supercooled liquid to the deep glassy state. This work establishes a unified physical framework for glass kinetics and validates SSWMS as a powerful tool for interrogating the long-term stability of disordered matter.