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S C I E N T I F I C H I G H L I G H T S

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The supercooled state of molecular liquids reveals a mosaic structure

To investigate the microscopic origin of glass formation, the molecular motions in a supercooled liquid were probed near its glass-transition temperature. The results provide strong experimental support for theories of an intermittent mosaic structure in the deeply supercooled liquid phase.

A glass can be thought of as a liquid that is no longer able to flow [1]. This definition reflects the typical procedure used to produce glasses, that is by quenching from the melt fast enough to prevent crystallisation. When a liquid is cooled down below its melting temperature, its viscosity becomes larger and larger and its molecular dynamics slows down to the point that when the glass-transition temperature (Tg) is

reached, the liquid is so viscous that it appears frozen on the typical observation timescale: a glass has been obtained. Glasses are used in many technological applications, yet the microscopic origin of the dynamic arrest occurring at the glass transition is still debated and remains one of the most intriguing open questions in the field of condensed matter physics.

A possible route to investigate this phenomenon is to study the molecular motions, called relaxation processes, through which a liquid can restore equilibrium following an external perturbation or a spontaneous fluctuation. Relaxations in a supercooled liquid are complex, heterogeneous processes, spanning many orders of magnitude in time. For instance, in a liquid above its melting temperature, the local structure changes on a timescale of a few picoseconds, while close to the glassy state, where the dynamic arrest

PRINCIPAL PUBLICATION AND AUTHORS

Coherent X-ray-optical control of nuclear excitons, K.P. Heeg (a), A. Kaldun (a), C. Strohm (b), C. Ott (a), R. Subramanian (a), D. Lentrodt (a), J. Haber (b), H.-C. Wille (b), S. Goerttler (a), R. Rüffer (c), C.H. Keitel (a), R. Röhlsberger (b,d,e,f,g), T. Pfeifer (a), J. Evers (a), Nature 590, 401-404 (2021); https:/doi.org/10.1038/s41586-021-03276-x (a) Max-Planck-Institut für Kernphysik, Heidelberg (Germany) (b) Deutsches Elektronen-Synchrotron DESY, Hamburg (Germany) (c) ESRF (d) The Hamburg Centre for Ultrafast Imaging, Hamburg (Germany) (e) Helmholtz-Institut Jena, Jena (Germany) (f) GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt (Germany) (g) Institut für Optik und Quantenelektronik, Friedrich-Schiller-Universität Jena, Jena (Germany)

Fig. 15: Observed X-ray interference structures as a function of time (t) and detuning (δ) of the two samples

against each other. a) Measurement data for the case of enhanced emission, (b) for the case of enhanced

excitation.

for X-rays with laser quality (synchrotron radiation and free-electron lasers) has opened up a new field: nuclear quantum optics. The possibilities demonstrated here open the door to new experimental approaches based on the

control of nuclear dynamics, e.g., by preparing nuclei in particular quantum states allowing for measurements that are more precise.