Driving a glass to flow: how X-rays reveal the dynamics of an amorphous material

by Lara Piemontese (UniPD)

Wednesday 14 Oct 2026, 12:00 → 13:00 Europe/Rome

P2D (Dipartimento di Fisica e Astronomia - Edificio Ricci-Curbastro (ex-Paolotti))

Glasses are among the most widespread yet least understood materials in condensed matter physics. Rigid like solids at the atomic scale, yet
structurally disordered like liquids, they exist in a perpetual state of arrested relaxation — frozen far from equilibrium, slowly evolving on
timescales that can exceed the age of the universe.
How can we drive a glass out of this arrested state? And how does it respond when we continuously push it out of equilibrium?
We explore these questions choosing, among the different kinds of external perturbations, a rather unusual
tool: an X-ray beam. It is indeed known that a focused synchrotron X-ray beam can do something unexpected: it
does not merely probe a glass, but it drives it. We show that hard X-ray photons absorbed by GeSe₂ samples
trigger localized bond-breaking events and generate coordination defects. Under sustained irradiation, the
glass undergoes a transition from a solid-like, elastically responding state to a plastically flowing state.
Importantly, in this game the beam is used both as a source of dynamics and as a way of watching that
dynamics in real time. This is made possible by X-ray Photon Correlation Spectroscopy (XPCS) technique.
Its combination with Fast Differential Scanning Calorimetry (FDSC) allows us to study how this beam-induced
motion changes with temperature, from the deep glassy state to and beyond the glass transition. At low
temperatures, the dynamics is mainly driven by the X-ray beam and increases with irradiation rate. At high
temperatures, instead, the material's own thermal relaxation dominates and the beam rate has little influence.
In between, the two processes compete: irradiation tends to rejuvenate the glass, while temperature allows it to
relax and anneal. This competition leads to a surprisingly rich picture of glassy dynamics, where the material
does not simply become "more liquid" under irradiation. Instead, its response depends on temperature,
irradiation history, and the balance between externally induced rearrangements and intrinsic relaxation.