Spin-Coupled Coherent Acoustic Phonons in van der Waals Antiferromagnet NiPS3.

Controlling magnetic order with coherent lattice motion is a key frontier for ultrafast spintronic and quantum technologies, yet direct access to propagating spin-coupled acoustic modes in 2D antiferromagnets has been limited. Here, the propagating longitudinal and transverse acoustic phonons in the van der Waals antiferromagnet NiPS3 are optically generated and detected by using ultrafast spectroscopy. Both the longitudinal and transverse acoustic branches exhibit a pronounced magnetic-state dependence: the shear-like transverse acoustic mode is amplified by nearly a factor of 3 below the Néel temperature, possibly resulting from the ultrafast demagnetization-driven shear stress. The propagating coherent acoustic phonons induce a distinct optical polarization-rotation response that disappears above magnetic ordering, providing a fully all optical readout of dynamic spin-lattice coupling in a zero-net-moment antiferromagnet. These findings reveal a particularly strong interplay among strain, optical anisotropy, and antiferromagnetic order in a layered van der Waals antiferromagnet.