Time- and momentum-resolved phonon population dynamics with ultrafast electron diffuse scattering

The coupling within and between lattice and charge degrees of freedom is central to a diverse range of transport and ordering phenomena in materials, from superconductivity to charge density waves. Here, the authors show how ultrafast electron diffuse scattering (UEDS) tracks energy flows -- across the entire Brillouin zone -- between electrons and phonons and within the phonon system itself. The nonequilibrium phonon populations determined by UEDS are used to extract momentum- and mode-dependent electron-phonon and phonon-phonon coupling constants.