Anisotropic plasmonic properties at the graphene/CrSBr interface

CrSBr is a stable 2D magnetic material with strong anisotropy in its electron effective mass in the two in-plane directions. It has been observed to have surface plasmon polariton (SPP) modes with strong anisotropy when a graphene layer is placed on top, as measured by scattering-type scanning near-field optical microscopy (s-SNOM). Using first-principles calculations, we estimated the charge transfer between the graphene and the CrSBr layer which matches well with the measurement. We studied the plasmonic dispersions and lifetimes within the framework of random phase approximation. We explained the observed anisotropic SPP lifetimes and dispersions from our dielectric function calculations based on random phase approximation and plasmonic damping joint density of states analysis. The calculated SPP lifetime in one direction being 10 times larger than the SPP lifetime in the other in-plane direction matches well with the experiments [1]. We show that the charge transfer between the CrSBr layer induces the strong anisotropy in the plasmon propagations which otherwise could not happen in isolated CrSBr. Our result is important for realizing uniaxial plasmon propagation in 2D materials.