Ultrafast Electron Diffraction of Monoclinic GaTe and the Dynamic Breaking of Friedel's Law*

GaTe is a low-symmetry layered materials with strong anisotropy, which has been widely studied in recent years. Various applications, such as photodetectors, nonlinear optics and thermoelectric devices, have been demonstrated based on GaTe. Here the electron-phonon coupling and the lattice dynamics of GaTe are studied by ultrafast electron diffraction (UED). Besides the Debye-Waller effect caused by the incoherent electron-phonon coupling, we observed a coherent breathing phonon mode in GaTe. Different from the LA breathing modes of high-symmetry 2D materials, the displacement direction of the LA breathing phonon modes in GaTe has deviated incommensurately from the stacking lattices. In-phase and out-of-phase oscillations of diffraction intensities are observed for each Friedel pairs, which in fact break the Friedel's law dynamically. This dynamic breaking of Friedel's law can be explained by the wobbling of the crystal lattice and the Ewald sphere, and the related study can be used as a reference for UED characterization of other low-symmetry thin films.