Ultrafast optical excitation of magnetic dynamics in van der Waals magnets: Coherent magnons and BKT dynamics in NiPS₃

Optical pumping of collective excitations in correlated electron systems and probing the subsequent response has received increased attention in past years for its ability to reveal intrinsic low energy dynamics of the system of interest.

In this talk, we propose two microscopic mechanisms for the optical generation of coherent magnetic excitations in van der Waals magnets, and derive the corresponding effective light-spin interactions: Either through pumping atomic orbital excitations resonantly or via a light-induced Floquet spin Hamiltonian, the ground state of the system is driven out of equilibrium. Subsequent spin dynamics can then be probed using established magneto optical techniques.

As an example, we apply our framework to NiPS₃, which is magnetically ordered in the bulk, and is conjectured to realize the XY model in the monolayer limit. We find quantitative agreement with recent experiments by Afanasiev et al. in Sci. Adv. 7 eabf3096 (2021).

We further propose pump-probe experiments for monolayer NiPS₃ and detail how anomalous relaxational behaviour may reveal excitations of a (proximate) BKT phase in a proposed effective XY model.