Does FeSe crackle?  Direct observation of lattice avalanches in the nematic phase of an Fe-based superconductor using X-ray photon correlation spectroscopy

FeSe is an iron-based unconventional superconductor with a mysterious electronic nematic phase that coincides with a structural transition and seemingly enhances superconductivity. Using X-ray photon correlation spectroscopy, we show that the diffuse scattering from the FeSe lattice exhibits abrupt, spontaneous in-plane distortions within the nematic phase, even in the absence of an external driving force, while the q=0 scattering exhibits slow diffusion dynamics. The spontaneous avalanches seen in the diffuse scattering are reminiscent of crackling noise, which exhibit scaling laws in the vicinity of a critical point. From both forward- and Bragg-scattered coherent X-ray experiments, we investigate the momentum dependence of these avalanches and determine the time- and length-scale of lattice fluctuations in order to understand the role lattice instability plays in the nematic phase.