Anomalous Anisotropic Quasiparticle Decay Dynamics in NiPS₃ Induced by Electric Dipole and Spin Ordering

Two-dimensional van der Waals antiferromagnetic (AFM) material NiPS3 has attracted extensive attention in the exploration of strongly correlated many-body phenomena due to its unique spin ordering and electric dipole. The spin ordering in NiPS3 has been reported as the origin of its strong optical anisotropy, while the electric dipole, excited with the assistance of AFM spin ordering, has been reported to yield extraordinarily sharp photoluminescence. Here, we investigate the anisotropic quasiparticle decay dynamics of NiPS3 single crystals using polarization-resolved femtosecond transient optical spectroscopy. We observe quasiparticle decay via spin–phonon coupling, electron–phonon coupling, and thermal diffusion processes, all showing strong anisotropy in the AFM phase of NiPS3. Such anisotropy disappears in the paramagnetic phase of NiPS3 due to spin disorder. The electron–phonon coupling decay process is significantly slowed down in the AFM phase of NiPS3, which is a result of the formation of an electric dipole. The thermal diffusion process in NiPS3 is susceptible to spin fluctuations, which are significantly slowed near the Néel temperature.