Fig. 1. Crystal structure and optical properties of NiPS₃.

a Crystal and magnetic structure of NiPS₃. The Ni atoms (blue spheres) are arranged in a two-dimensional honeycomb lattice in the ab-plane. The magnetic moments (red and blue arrows), which point mostly along the a-direction, form a zigzag antiferromagnetic pattern with weak ferromagnetic coupling in the c-direction. b Optical absorption (α) of NiPS₃ below the charge-transfer gap at 20 K. The features around 1.5 eV are the spin–orbit-entangled exciton transitions reported in ref. ⁵. The broad structures around 1.1 and 1.7 eV are on-site d-d transitions. The data is adapted from ref. ⁵. c Absorption spectrum in the THz range at various temperatures. The traces are offset vertically by 10 cm⁻¹ for clarity. The low-temperature curves show an absorption peak corresponding to a magnon resonance. The magnon energy softens with increasing temperature and the mode disappears above T_N = 157 K.