Directly observing atomic-scale relaxations of a glass forming liquid using femtosecond X-ray photon correlation spectroscopy
arxiv(2023)
Abstract
Glass forming liquids exhibit structural relaxation behaviors, reflecting
underlying atomic rearrangements on a wide range of timescales. These behaviors
play a crucial role in determining many material properties. However, the
relaxation processes on the atomic scale are not well understood due to the
experimental difficulties in directly characterizing the evolving correlations
of atomic order in disordered systems. Here, taking the model system Ge15Te85,
we demonstrate an experimental approach that probes the relaxation dynamics by
scattering the coherent X-ray pulses with femtosecond duration produced by
X-ray free electron lasers (XFELs). By collecting the summed speckle patterns
from two rapidly successive, nearly identical X-ray pulses generated using a
split-delay system, we can extract the contrast decay of speckle patterns
originating from sample dynamics and observe the full decorrelation of local
order on the sub-picosecond timescale. This provides the direct atomic-level
evidence of fragile liquid behavior of Ge15Te85. Our results demonstrate the
strategy for XFEL-based X-ray photon correlation spectroscopy (XPCS), attaining
femtosecond temporal and atomic-scale spatial resolutions. This twelve orders
of magnitude extension from the millisecond regime of synchrotron-based XPCS
opens a new avenue of experimental studies of relaxation dynamics in liquids,
glasses, and other highly disordered systems.
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