Unraveling Momentum-Dependent Electron-Phonon Coupling and its Role in the Origin of Charge Density Wave Phases*

The nature of the couplings within and between lattice and charge degrees of freedom is central to condensed matter and materials physics. These interactions are essential to phenomena as diverse as superconductivity, charge density waves and carrier mobility in semiconductors and metals. Despite their fundamental role, detailed momentum-dependent information on the strength of electron-phonon coupling (EPC) and phonon-phonon coupling (PPC) across the entire Brillouin zone has proved elusive. This talk will describe a new technique, ultrafast electron diffuse scattering (UEDS), which provides such information. Specific applications of UEDS to 2D materials including graphite, TiSe2 and TaSe2 will be presented. These data demonstrate that UEDS patterns can separate the influence of the electronic susceptibility from the inelastic exchange of energy between the electron and phonon systems through the technique’ profound sensitivity to photoinduced changes to the phonon system. In TiSe2, this confirms key role of excitonic correlations to the phonon softening and CDW mechanism.