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 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 pronounced 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.

Figures (22)

• Figure 1: (a) Crystal and magnetic structure of NiPS3: (a) in the ab plane where Ni atoms (blue spheres) form a honeycomb lattice with zigzag AFM ordering along the a-axis (red and cyan arrows); (b) in the ac plane along the c-axis, showing weak interlayer ferromagnetic coupling. (c) The peak transient reflectivity at the time delay of 10 ps vs temperature (blue line) capturing the AFM-PM transition along with its temperature derivative (red circles). (d) Electronic ground state of NiPS3 ($^3$A$_{2g}$), where Ni$^{2+}$ 3d-orbitals are split into lower-energy t$_{2g}$ and higher-energy e$_g$ levels by the octahedral crystal field from surrounding S$^{2-}$ ligands. (e) Upon 3.1 eV photoexcitation, a LMCT occurs, promoting an electron from the S 3p-orbitals to the Ni e$_g$ manifold. (f) SOC enables a transition within the e$_g$ levels, where the excited electron flips and goes to d$_{x^2 - y^2}$ from d$_{z^2}$, forming a bound SOEE state.
• Figure 2: (a) Temperature-dependent photoinduced differential reflectivity ($\Delta R/R$) traces of NiPS$_3$ (coloured lines) along with fits to Eq. \ref{['eq:1']} (black solid lines). The insets show the same data, normalized to their respective negative peak amplitudes, allowing direct comparison of relaxation dynamics at $5~\text{K} < T < 135~\text{K}$ and $155~\text{K} < T < 294~\text{K}$. (b) 2D false-colour map showing the evolution of transient reflectivity as a function of pump-probe delay (0--500 ps) and temperature. The horizontal green and black dotted lines indicate $T_N$ and $T_{ED}$ respectively. (c) $\Delta R/R$ as a function of temperature for five discrete pump-probe delays to highlight the temperature-dependent evolution of the instantaneous transient reflectivity signal.
• Figure 3: (a) Temperature dependence of the amplitude associated with the coherence of SOEE. (b) The coherence time ($\tau_1$) of SOEE, Solid black line represents the fit (see text). (c) and (d) show the temperature dependence of the amplitude and lifetime of the slower relaxation component ($\tau_2$), attributed to spin-ordering dynamics. The inset in (d) presents a power-law fit (black solid line) to $\tau_{2}$, revealing critical slowing down as the system approaches the Néel temperature. The blue and green vertical dotted line indicates the $T_{ED} \sim 120$ K and $T_N \sim 155$ K respectively. The black dashed line connecting the symbols is drawn as a guide to the eye.
• Figure 4: Parameters extracted as a function of pump excitation fluence. (a) Amplitude and (b) coherence time of the SOEE at different temperatures. (c) and (d) shows the amplitude and spin-ordering dynamics respectively.
• Figure : FIG. S1.2: X-ray diffraction spectrum of NiPS$_3$. Room-temperature XRD pattern of a NiPS$_3$ single crystal showing (001), (002), (003),(004), and (005) reflections, confirming $c$-axis orientation and in good agreement with literature reports XRD1XRD2XRD3.