The two-level systems in cryogenic solids, or how to avoid stressful memories

TL;DR

This paper investigates the presence and reduction of two-level systems in various stable glasses, highlighting the role of configurational entropy and bonding in their stability.

Contribution

It clarifies the conditions under which two-level systems are depleted in ultrastable glasses and discusses the mechanisms behind enthalpic stabilization.

Findings

Ultrastable vapor-deposited glasses have fewer TLSs and are more enthalpically stable.

Stable glasses of model liquids show reduced TLSs, unlike geological amber.

Depletion of TLSs correlates with lower configurational entropy, not aging alone.

Abstract

Structural glasses prepared by bulk quenching a liquid melt universally exhibit puzzling low-energy excitations commonly known as the ``two-level systems'' (TLSs). Recent studies indicate that ultrastable glassy films made by vapor deposition exhibit substantially fewer TLSs and, at the same time, are more stable enthalpically than conventional glasses made by quenching a melt. A similar phenomenon is observed in very stable glasses of model liquid mixtures prepared using swap Monte Carlo sampling. However, in a separate set of enthalpically stable solids, exemplified by amber matured over geological times, the two-level systems persist. In addressing this seeming conflict, we emphasize that a depletion of the TLSs, if any, means the configurational entropy of the material is lower than that of conventional glasses made by bulk-quenching a melt. Ageing does induce reduction in configurational entropy, but amber, we speculate, achieves enthalpic stabilization through increased bonding, not ageing. We separately comment on the discrepancy among existing predictions for the extent of cooperativity of the two-level systems. Several experiments are suggested to test the present picture.