Origins of Charge-Density Waves in Niobium Diselenide and Tantalum Disulfide using Ultrafast Electron Diffuse Scattering*

Investigations of structural dynamics on the sub-picosecond timescale can provide a detailed fundamental understanding of the non-equilibrium properties of materials. While structural transformations are related to intensity changes in Bragg peaks, thermal diffuse scattering can reveal complete information on the underlying phonon modes. Because thermal diffuse scattering intensities are many order of magnitude lower that Bragg intensities, very sensitive measurements are required.
In this talk, I will present ultrafast electron diffuse scattering to directly observe electron-phonon interactions in charge-density wave (CDW) materials with a time resolution of ~100 fs. It was found that CDW materials and high-temperature superconductors (HTS) have a similar origin and hence, gaining deep insights in CDW materials is expected to enhance our understanding of HTS. Using a combination of density functional theory and experimental studies, the rich electron-phonon interactions of two CDW materials (i.e., niobium diselenide and tantalum disulfide) were investigated. Measurements suggest photoexcitation significantly stiffens/softens acoustic modes only at certain wave-vectors, a phenomenon we have also investigated computationally.