Fig. 1: Phonon replicas observed in the transient absorption spectrum of NiPS₃.

a Schematic of the broadband transient absorption spectroscopy setup. A pulse with energy (1.88 eV) larger than the bandgap excites the system, and a subsequent probe pulse with white light continuum measures the transient reflectivity in a broad spectral region, with 1.5 meV spectral resolution. b Spectral dependence of transient reflectivity measured at 10K, 2 ps after the pump. Previously reported coherent many-body spin–orbit-entangled excitons (orange region) and a broad d–d transition (blue region) can clearly be seen. Two sharp features are labeled as Peak I and Peak II which have been previously identified as a spin–orbit-entangled exciton and a magnon sideband¹¹. The d–d peak is dressed by phonons, with at least 9 replicas visible, indicating a strong coupling between them. The inset shows the Fourier transform of the extracted spectral oscillations, with a mode peaking at 28.5 meV, which is close to the 7.5 THz A_1g phonon energy. c Crystal structure of NiPS₃. Octahedron as shown in the solid square, formed by a nickel (red) and six sulfur (yellow) atoms. Phosphorus atoms are shown in dark blue. Arrows indicate the A_1g phononic displacements for the 7.5 THz A_1g mode.