All hard X-ray transient grating spectroscopy

A microscopic understanding of matter requires the study of excitations across an extended spatial, time and frequency range. The tens of nanometer length-scale is crucial for transport phenomena in liquids and crystals. It holds the key to understanding phenomena such as charge, spin and heat diffusion at the frontier of diffusive and ballistic regimes. At the nanometer scale, disorder, diffusion, and relevant physical phenomena deviate from classical descriptions. It also covers the dynamics of charge/spin ordering and of the lattice. Optical-domain transient gratings (TG) spectroscopy demonstrated access to all relevant degrees of freedom (e.g. charge, lattice, orbital and spin) of matter, but are limited to typically micron-size grating periods. Extreme-Ultraviolet TG spectroscopy has represented a major leap forward as it allowed access to mesoscopic scales. [1–4] Using hard X-rays for excitation and probing can extend TG spectroscopy beyond the aforementioned length scales. While hard X-ray generated TGs and their probing by optical pulses were recently reported, [5,6] here we present an X-ray TG (XTG) study, in which few-femtosecond hard X-ray pulses are used both for excitation and probing. Our study establishes key ingredients that allow previously inaccessible length (and therefore, momentum) regions. Ultrafast all X-ray TG thus emerges as a complementary approach to methods, such as inelastic neutron [7] and X-ray [8] scattering techniques, that are commonly used in the study of microscopic transport phenomena.