Ergodicity breaking of an inorganic glass aging near Tg probed by elasticity relaxation

We performed a series of aging experiments with an inorganic glass (${\mathrm{As}}_{2}{\mathrm{Se}}_{3}$) at a temperature ${T}_{2}$ near the glass transition point ${T}_{g}$ by first relaxing it at ${T}_{1}$. The relaxations of Young's modulus were monitored, which were (almost if not ideally) exponential with ${T}_{1}$-dependent relaxation time \ensuremath{\tau}, corroborating the Kovacs' paradox in an inorganic glass. Associated with the divergence of \ensuremath{\tau}, the quasiequilibrated Young's modulus ${E}_{\ensuremath{\infty}}$ does not converge either. An elastic model of relaxation time and a Mori-Tanaka analysis of ${E}_{\ensuremath{\infty}}$ lead to a similar estimate of the persistent memory of the history, illuminating ergodicity breaking within the accessible experimental time, as described in the Gardner transition theory. Experiments with different ${T}_{2}$ exhibit a critical temperature ${T}_{p}\ensuremath{\sim}{T}_{g}$, i.e., when ${T}_{2}>{T}_{p}$, both \ensuremath{\tau} and ${E}_{\ensuremath{\infty}}$ converge. The results unveil a long-expected phenomenon that structural glass transition could be a zero-to-nonzero transition, manifested by a nonvanishing structural memory in aging when the temperature is below ${T}_{p}$ in the glass transition range. This demonstrates the existence of the ergodicity breaking deep in the glass state and ${T}_{p}$ could be the Gardner transition point of the structural glass.