Abstract:
Among the frontier condensed matter studies, van der Waals (vdW) materials represent a highly celebrated class that features intertwined electronic order and collective phenomena. At this front, elucidating the elementary excitations and dynamics out of the fundamental electronic degrees of freedom, namely spin, charge, orbital, lattice, is of paramount importance. Here I present a recent study using resonant inelastic X-ray scattering (RIXS) to elaborate the optically-forbidden spin-orbital excitations of a vdW antiferromagnet FePS3 and their role for magnetism. We observed the spectral enhancement of spin-orbital multiplet excitations at about ~100 and ~220 meV, as well as the quasi-elastic response, when entering the antiferromagnetic zig-zag phase with an order-parameter-like evolution in temperature. By comparing with model calculations, we discovered the trigonal lattice distortion, spin-orbit interaction and metal-ligand charge-transfer to be essential for these emergent excitations. We further reveal their spectral robustness down to the few atomic-layer limit by mechanical exfoliation, in accordance with the persistent antiferromagnetism reported previously. Our study highlights the crucial role of lattice and orbital anisotropy for stabilizing the quasi-two-dimensional magnetism and tailoring vdW magnets.