Ultrafast Dynamics of Spin-Orbit Entangled Excitons Coupled to Magnetic Ordering in van der Waals Antiferromagnet NiPS3

Abstract: Spin-orbit entangled excitons (SOEE) in two-dimensional (2D) antiferromagnets provide a direct access to explore unconventional many body interactions in correlated electron systems. In this work, we carry out a detailed investigation using non-degenerate isotropic and anisotropic pump-probe reflection spectroscopy to probe the ultrafast dynamics of SOEE and their coupling to spin fluctuations in NiPS3. Transient reflectivity data reveals acoustic phonon oscillations at ~ 27 GHz, along with two distinct relaxation timescales: fast (1-9 ps) and slower components (1-4 ns) associated with SOEE coherence and spin reordering, respectively. Both timescales exhibit pronouns temperature dependence near the exciton dissociation (TED = 120 K) and Neel TN = 155 K) temperatures. The SOEE coherence shortens from ~ 8-9 ps at T < TED to ~ 3 ps at T > TED with a finite tail persisting beyond TN. The spin reordering time grows near 120 K, and shows critical slowing down around TN. Pump fluence studies further corroborate their spin origin. Our findings uncover the direct interplay between the excitonic and spin degrees of freedom across ultrafast and longer timescales, offering new opportunities to probe and engineer emergent many-body interactions in 2D antiferromagnets.