Ultrafast-induced Coherent Acoustic Phonons in the Two-dimensional Magnet CrSBr*

CrSBr is an air-stable, novel two-dimensional (2D) magnetic van der Waals (vdW) single crystal that received intense study as a future spintronics material. The material shows some interesting properties such as a large optical band gap (~ 1.5 eV), strong spin-phonon coupling (SPC) effect, quasi 1-D electronic structure, strain-induced magnetic phase transitions, and enhanced structural deformability.  In our work, mega-electron volt (MeV) ultrafast electron diffraction (UED) was employed to investigate the ultrafast structural dynamic response of this material. We observed femtosecond pump pulses excited small anisotropic lattice deformations that are to be mediated by coherent acoustic phonons (CAPs). The phonons were interpreted as longitudinal breathing modes. The ultrafast oscillatory behavior of Bragg peak intensities was simulated by incorporating an oscillating deviation parameter ansatz into dynamical scattering intensity expressions. The phonon frequencies were extracted by analyzing these oscillations in Bragg peak intensities and they were in nearly 23 GHz range. These results suggest a promising path for tuning the ultrafast photo-induced properties of CrSBr with sample geometry and dimension.