How close can vapor-deposited glasses approach the ideal glass
In plain words
Glasses grown slowly from vapor onto a cooled surface can be as stable as ordinary glasses aged for thousands of years or more. How far this route can go toward the hypothetical ideal glass, and what limits it, is unknown.
Precise statement
Physical vapor deposition at substrate temperature near $0.85 T_g$ produces glasses with fictive temperature $T_f$ (the temperature at which the equilibrium liquid has the same structure and enthalpy) far below that of liquid-cooled glasses, attributed to enhanced mobility in a surface layer. Determine the lowest attainable $T_f$ as a function of substrate temperature and deposition rate, the mechanism that sets this limit, and whether $T_f$ can approach extrapolated Kauzmann temperatures $T_K$. An answer is measured $T_f(T_{\mathrm{sub}}, \mathrm{rate})$ for several molecules explained by a quantitative surface-mobility theory.
What would settle it
Calorimetric $T_{f}$ of vapor-deposited glasses over a wide range of deposition rates and substrate temperatures, with a theory of surface-layer equilibration that predicts the observed limit.
Status in the literature
Unverified note
Ultrastable vapor-deposited organic glasses were reported in 2007 (Swallen et al., Science, https://doi.org/10.1126/science.1135795); the lower bound on $T_f$ is not established (2026).