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The macroscopic characteristics of a material are determined by its elementary excitations, which dictate the response of the system to external stimuli. The spectrum of excitations is related to fluctuations in the density-density correlations and is typically measured through frequency-domain neutron 1 or X-ray [2][3][4] scattering. Time-domain measurements of these correlations could yield a more direct way to investigate the excitations of solids and their couplings both near to and far from equilibrium. Here we show that we can access large portions of the phonon dispersion of germanium by measuring the diffuse scattering from femtosecond X-ray freeelectron laser pulses. A femtosecond optical laser pulse slightly quenches the vibrational frequencies, producing pairs of highwavevector phonons with opposite momenta. These phonons manifest themselves as time-dependent coherences in the displacement correlations 5 probed by the X-ray scattering. As the coherences are preferentially created in regions of strong electron-phonon coupling, the time-resolved approach is a natural spectroscopic tool for probing low-energy collective excitations in solids, and their microscopic interactions.Density fluctuations in nominally periodic media reduce the intensity of the Bragg diffraction peaks and consequently increase the weak diffuse scattering between these peaks, the details of which reflect the amplitudes and spatial frequencies of the fluctuations 6 . The scattered intensity is determined by the dynamic structure factor S(Q, ω) at momentum Q and frequency ω, which is proportional to the Fourier transform of the correlation function of the density-density fluctuations. For phonons, these correlations are u q (0)u −q (t ) , where u q is the phonon amplitude at reduced wavevector q = Q − K Q and K Q is the closest reciprocal lattice vector to Q, and in this context the expectation value is a thermal average 7 . In typical X-ray or neutron scattering experiments the measured diffuse scattering is proportional to the equal-time correlations u q (0)u −q (0) (refs 3,7,8) of the inelastically scattered photons from a highly monochromatic beam. As we demonstrate here in a single crystal of the prototypical semiconductor germanium, a femtosecond laser pulse generates temporal coherences in the equal-time correlation functions g (τ ) = u q u −q parameterized by the pump-probe delay τ between the optical pulse and the X-ray probe. As the X-ray pulse from the free-electron laser (FEL) is short compared with the vibrational motion, we assume that the scattering is effectively instantaneous. Under this approximation we measure g (τ ) stroboscopically, which unlike in the thermal case has an oscillatory contribution from a two-phonon squeezed state generated by the laser pulse, as well as a contribution from incoherent changes in populations 9 . In this paper we focus on the oscillatory component, which yields large portions of the phonon dispersion directly from the measurement without any particular model of the interatomic force...

In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials Ag4In3Sb67Te26 and Ge15Sb85 at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics.

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Nanoscale dimensions in materials lead to unique electronic and structural properties with applications ranging from site-specific drug delivery to anodes for lithium-ion batteries. These functional properties often involve large-amplitude strains and structural modifications, and thus require an understanding of the dynamics of these processes. Here we use femtosecond X-ray scattering techniques to visualize, in real time and with atomic-scale resolution, light-induced anisotropic strains in nanocrystal spheres and rods. Strains at the percent level are observed in CdS and CdSe samples, associated with a rapid expansion followed by contraction along the nanosphere or nanorod radial direction driven by a transient carrierinduced stress. These morphological changes occur simultaneously with the first steps in the melting transition on hundreds of femtosecond timescales. This work represents the first direct real-time probe of the dynamics of these large-amplitude strains and shape changes in few-nanometre-scale particles.