If liquids, polymers, bio-materials, metals and molten salts can avoid crystallization during cooling or compression, they freeze into a microscopically disordered solid-like state, a glass1,2. On approaching the glass transition, particles become trapped in transient cages—in which they rattle on picosecond timescales—formed by their nearest neighbours; the particles spend increasing amounts of time in their cages as the average escape time, or structural relaxation time τα, increases from a few picoseconds to thousands of seconds through the transition. Owing to the huge difference between relaxation and vibrational timescales, theoretical3,4,5,6,7,8,9 studies addressing the underlying rattling process have challenged our understanding of the structural relaxation. Numerical10,11,12,13 and experimental studies on liquids14 and glasses8,15,16,17,18,19 support the theories, but not without controversies20 (for a review see ref. 21). Here we show computer simulations that, when compared with experiments, reveal the universal correlation of the structural relaxation time (as well as the viscosity η) and the rattling amplitude from glassy to low-viscosity states. According to the emerging picture the glass softens when the rattling amplitude exceeds a critical value, in agreement with the Lindemann criterion for the melting of crystalline solids22 and the free-volume model23.

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Universal scaling between structural relaxation and vibrational dynamics in glass-forming liquids and polymers

The present synchrotron sources with brilliant x-ray beams, due to high photon fluxes, small source sizes and high collimation, have revolutionized x-ray physics. Enormous progress has been initiated in all established x-ray methods, with the aim of the development of new types of spectroscopy. This is particularly true for the spectroscopy of the dynamics in condensed matter. Meanwhile, there are two powerful x-ray methods with very high-energy resolution available for the study of low energetic excitations like phonons. This review summarizes the developments of these methods focusing on these instrumental developments of the spectrometers using either crystal optics in close-to-backscattering geometry or nuclear resonant techniques. Applications to measurements of phonon dispersion curves and of phonon density of states in ordered and disordered solids and in liquids are presented. It is shown how x-ray results are stimulating improvements in the theoretical approaches to the dynamics. New insights into the dynamics of liquids are discussed. The sensitivities of the spectroscopies allow the study of vibrational behaviour in very small amounts of material even in nanometre-sized thin films or particles. We can already analyse the phonon spectrum of a monolayered nuclear resonant isotope. Prospects of the techniques are also demonstrated.

https://iopscience.iop.org/article/10.1088/0034-4885/63/2/203/pdf

Phonon spectroscopy by inelastic x-ray scattering

An X-ray scattering beamline for studying dynamics

Nuclear resonant scattering techniques with synchrotron radiation (SR) are introduced on a basic level. We focus on the theoretical background and on experimental aspects of two popular methods with a widening range of applications, nuclear resonant inelastic x-ray scattering and synchrotron Mossbauer spectroscopy. The inelastic method provides specific vibrational information, e.g., the phonon density of states. The Mossbauer method permits determination of hyperfine interactions. All nuclear resonance techniques take full advantage of the unique properties of SR: intensity, collimation, time structure, and polarization. As a result both methods discussed here have led to novel applications for materials under extreme conditions, proteins with biological functionality, and magnetic nanostructures.

https://iopscience.iop.org/article/10.1088/0953-8984/16/5/009/pdf

Nuclear resonant spectroscopy

The Paul Scherrer Institute is planning the construction of a hard-x-ray free-electron laser, the SwissFEL, by 2016, which will produce intense, ultrashort pulses of transversely coherent radiation in the wavelength range 0.1?7?nm, with future extensions to cover the range 0.08?30?nm. Special design considerations include (a) a compact construction, compatible with the status of a national facility, (b) a uniform 100?Hz repetition rate, well suited to sample manipulations and detector readout, (c) flexible wavelength tuning by the electron beam energy and undulator gaps, (d) soft x-rays at approximately 1?nm wavelength, with circular polarization and Fourier-transform-limited pulses, (e)?hard x-rays of pulse duration 5?20?fs and (f) an independent source of high-energy, half-cycle terahertz pump pulses. The science case for the Swiss FEL project, which emphasizes the dynamics of condensed matter systems and the damage-free imaging of nanostructures, includes novel considerations that make optimal use of these features.

https://iopscience.iop.org/article/10.1088/1367-2630/12/3/035012/pdf

Coherent science at the SwissFEL x-ray laser

Quasielastic Scattering of Synchrotron Radiation by Time Domain Interferometry

The dynamics of glasses shows some distinct differences with respect to the crystalline state. Both in the short time regime (boson peak) and the microscopically slow regime (alpha- and beta-relaxation) there are glass-typical features. The outstanding sharpness of nuclear transitions offers a new tool to investigate the properties of glasses at long times, i.e., some nanoseconds to microseconds. The article will give a short introduction to glass-dynamic and convenient parameters for the theoretical description and will summarize recent results on model systems.

H. Franz

Papers

Quasielastic Scattering of Synchrotron Radiation by Time Domain Interferometry

Quasielastic scattering: slow dynamics of glasses