Phonon and magnon dynamics across antiferromagnetic transition in 2D layered van der Waals material CrSBr

We report temperature-dependent reflectivity spectra of the layered van der Waals magnet CrSBr in the far-infrared region. Polarization-dependent measurements resolve the vibrational modes along the E$\mathrm{\parallel }a$- and $b$-axes and reveal the clear structural anisotropy. While the $a$-axis phonons notably harden on cooling, the $b$-axis phonon frequencies are almost temperature-independent. A phonon splitting due to the antiferromagnetic phase transition is observed for the 180~cm${}^{-1}$ $a$-axis vibrational mode, accompanied by a phonon softening below $T_N$. Furthermore, an additional mode with strong magnetic characteristics at $\sim$360~cm${}^{-1}$ is identified and attributed to the magnon excitation of CrSBr.