Lattice Dynamics Study in Noncentrosymmetric Antiferromagnet*

Lattice dynamics, probed through phonon measurements, can offer critical insights into electron-electron and electron-lattice correlations in quantum materials. Here, I will present our phonon measurements of the intercalated transition metal dichalcogenide (TMD)  Fe_0.35NbS₂,  a noncentrosymmetric structure that exhibits charge order that is strongly coupled to a zigzag antiferromagnetic order below T_c ~ 40 K [1]. To advance our understanding of the mechanism of charge ordering and its interaction with the lattice, we investigated phonon dispersions across multiple temperatures by high-resolution inelastic X-ray (IXS). Surprisingly, no phonon softening was observed at the critical charge ordering wave-vector upon T_c. Instead, we identified a non-reciprocal phonon effect, which evolves with temperature. To quantify the characteristics of acoustic phonon response, I performed line fittings using a pseudo-voigt function and the Python Lmfit library. From this analysis, I extracted phonon dispersions and phonon widths as a function of temperature, revealing a connection with the charge order temperature in this system. These results report a possible new way of interplay between the lattice dynamics and electronic order in this noncentrosymmetric antiferromagnet Fe_0.35NbS₂, distinct from the conventional phonon softening mechanism.

Wu, S., Basak, R., Birgeneau, R. J., et. al.  Discovery of Charge Order in the Transition Metal Dichalcogenide Fe𝑥⁢NbS2. Phys. Rev. Lett. 131, 186701 (2023).