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Effect of Glass Stability on the Low Frequency Vibrations of Vapor Deposited Glasses

I. Festi1,*, E. Alfinelli1, D. Bessas2, F. Caporaletti3, A. I. Chumakov2, M. Moratalla4, M. A. Ramos4, M. Rodríguez-López5,6, C. Rodríguez-Tinoco5,6 et al.

J. Rodríguez-Viejo5,6 and G. Baldi1,†

  • *Contact author: irene.festi@unitn.it
  • †Contact author: giacomo.baldi@unitn.it
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Phys. Rev. X 16, 021021 – Published 28 April, 2026

DOI: https://doi.org/10.1103/311v-1ftn

Abstract

Ultrastable 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 quasilocalized 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 ultrastable and conventional glasses down to an exceptionally low frequency of approximately 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 quasilocalized 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.

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Popular Summary

A glass may seem familiar and common, but deep down it vibrates in surprisingly complex ways. A long-standing debate exists about whether low-frequency vibrations are controlled by tiny defectlike excitations inside the material. We tested this idea by measuring the vibrational spectrum of ultrastable glasses prepared by vapor deposition and comparing them with ordinary glasses. We used a high-resolution x-ray technique that reaches extremely low and previously inaccessible frequencies. We found that making a glass more stable almost suppresses defect-related excitations and weakens the so-called boson peak, a universal vibrational feature of disordered solids. Yet the very lowest-frequency harmonic vibrations stay essentially the same. This unexpected result shows that the lowest frequency part of the spectrum does not depend directly on defects but is largely governed by how sound waves are attenuated by disorder in the material. The combination of method and results provides a powerful new route to uncover how disorder governs vibrational motion in amorphous materials.

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Phys. Rev. X 16, 021021– Published 28 April, 2026
  • Received 19 May 2025
  • Accepted 23 February 2026
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