Ultrafast optical excitation of magnetic dynamics in van der Waals magnets: Coherent magnons and BKT dynamics in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>NiPS</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math>
Urban F. P. Seifert, Mengxing Ye, Leon Balents
Abstract
Optical pump-probe experiments carried out in the time domain reveal both the intrinsic low-energy dynamics and its connections to higher-energy excitations in correlated electron systems. In this work, we propose two microscopic mechanisms for the optical generation of coherent magnetic modes 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. The subsequent long-time relaxational dynamics can then be probed using, e.g., the magneto-optical Kerr effect or transient grating spectroscopy. As an example, we apply our framework to ${\mathrm{NiPS}}_{3}$, which is magnetically ordered in the bulk, and is conjectured to realize the XY model in the monolayer limit. Our theory makes explicit how the material's low-energy response depends sensitively on the microscopic details of the light-spin coupling as well as pump fluence, frequency, and polarization. For the case of bulk ${\mathrm{NiPS}}_{3}$, we find quantitative agreement with recent experiments [D. Afanasiev et al., Sci. Adv. 7, eabf3096 (2021)]. We further propose pump-probe experiments for monolayer ${\mathrm{NiPS}}_{3}$ and detail how anomalous relaxational behavior may reveal excitations of a (proximate) Berezinskii-Kosterlitz-Thouless phase in a proposed effective XY model.