Ultrafast Electron Diffuse Scattering as a Tool for Studying Phonon Transport: Phonon Hydrodynamics and Second Sound Oscillations

Hydrodynamic phonon transport phenomena, like second sound, have been observed in liquid Helium temperatures more than 50 years ago. More recently second sound has been observed in graphite at over 200\,K using transient thermal grating techniques. In this work we explore the signatures of second sound in ultrafast electron diffuse scattering (UEDS) patterns. We use density functional theory and solve the Boltzmann transport equation to determine time-resolved non-equilibrium phonon populations and subsequently calculate one-phonon structure factors and diffuse scattering patterns to simulate experimental data covering the regimes of ballistic, diffusive, and hydrodynamic phonon transport. For systems like graphite, UEDS is capable of extracting time-dependent phonon occupancies across the entire Brillouin zone and ultimately lead to a more fundamental understanding of the hydrodynamic phonon transport regime.Comment: 7 pages, 4 figure

Bismuth Nanofilms as Efficient Broadband THz Antireflection Coating

Stroh A, Zhang W, Fabretti S, et al. Bismuth Nanofilms as Efficient Broadband THz Antireflection Coating. In: 2020 45th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz). IEEE; 2020: 1-2.We demonstrate the broadband THz antireflection coating based on bismuth nanofilms. The antireflection effect stems from the complex-valued conductivity of bismuth, providing excellent impedance matching with high-index dielectrics such as silicon, in the range of at least 0-4 THz. The details of THz conductivity of bismuth will also be presented