(1)
We demonstrate a recent technique, ultrafast electron diffuse scattering, which provides detailed momentum-dependent information on electron-phonon and phonon-phonon coupling. We will explore how scattering from different phonon branches can be distinguished by substituting time-resolution for energy-resolution, and how crystal symmetry considerations help reduce the number of possible relaxation pathways. Using graphite as a model two-dimensional system, we show that ultrafast electron diffuse scattering maps energy flow from the electron system to optical modes, all the way to relaxation into acoustic modes. The applicability of this technique to other two-dimensional systems will also be discussed
(2)
We combine ultrafast electron diffraction and time-resolved terahertz spectroscopy measurements to unravel the connection between structure and electronic transport properties during the photoinduced insulator-metal transitions in vanadium dioxide. We determine the structure of the metastable monoclinic metal phase, which exhibits anti-ferroelectric charge order arising from a thermally activated, orbital-selective phase transition in the electron system. The relative contribution of this photoinduced monoclinic metal (which has no equilibrium analog) and the photoinduced rutile metal (known from the equilibrium phase diagram) to the time and pump-fluence dependent multi-phase character of the film is established, as is the respective impact of these two distinct phase transitions on the observed changes in terahertz conductivity. Our results represent an important new example of how light can control the properties of strongly correlated materials and elucidate that multi-modal experiments are essential when seeking a detailed connection between ultrafast changes in optical-electronic properties and lattice structure in complex materials..