Effect of glass stability on the low frequency vibrations of vapor deposited glasses

TL;DR

This study investigates how the stability of vapor-deposited glasses influences their low frequency vibrational modes, revealing that stability affects higher frequency modes but not the lowest frequency harmonic vibrations.

Contribution

The paper provides experimental evidence that glass stability impacts vibrational modes at the boson peak but not at the lowest frequencies, challenging previous assumptions about two-level states.

Findings

Low frequency vibrational modes are unaffected by glass stability.

Higher frequency modes near the boson peak are highly sensitive to stability.

Presence of quasi-localized modes is not necessary to explain low frequency vibrations.

Abstract

Ultra-stable glasses prepared from the physical vapor deposition of organic molecules present a very low density of two-level states, the kind of glass defects that determine their peculiar low temperature thermal properties. Numerical simulations suggest that quasi-localized harmonic vibrational modes emerge in the soft regions associated with two-level states. However, the connection between the low frequency vibrational modes and the local structural instabilities of glasses remains unexplained. Here we exploit a recently developed spectrograph for nuclear resonant analysis of inelastic X-ray scattering to probe the density of vibrational states of amorphous thin films of ultra-stable and conventional glasses down to an exceptionally low frequency of $\sim 70$ GHz. We show that the glass stability does not affect the harmonic vibrational modes at the lowest frequencies, despite a reduction of almost an order of magnitude in the density of two-level states. At the same time, the vibrational modes at higher frequencies, around the boson peak maximum, are extremely sensitive to the glass stability. Although we cannot exclude the possible existence of quasi-localized modes in glasses, we show that their presence is not strictly necessary to describe the measured density of low frequency vibrations. The experimental developments here presented pave the way to the solution to the long-standing debate on the low frequency vibrations in glasses.