Direct Observation of Phonon Squeezing in Bismuth by Mega-Electron-volt Ultrafast Electron Diffraction

In this paper, we report the structural dynamics of polycrystalline bismuth (Bi) thin films in response to photoexcitation, visualized by mega-electron-volt ultrafast electron diffraction. The data reveal that the carrier–phonon scattering process involves phonon squeezing within sub-picoseconds (ps) and lattice thermalization within a few ps. Through the time-resolved pair distribution function analysis, we directly observe the changes in the interatomic distance of adjacent Bi atoms in real space, which can be explained by phonon softening and subsequent phonon squeezing.