Ultrafast optical excitation of magnetic dynamics in van der Waals magnets: Coherent magnons and BKT dynamics in NiPS${}_3$

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 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 NiPS${}_3$, we find quantitative agreement with recent experiments by Afanasiev et al. [Science Advances 7, abf3096 (2021)]. We further propose pump-probe experiments for monolayer NiPS${}_3$ and detail how anomalous relaxational behaviour may reveal excitations of a (proximate) BKT phase in a proposed effective XY model.

15+6 pages, 5 figures; v2 (added Fig. 1, clarifying comments in Sec. II, and additional minor modifications)

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• Magnetic 2D materials and heterostructures
• Emergence of the persistent spin helix in semiconductor quantum wells
• Nonthermal pathways to ultrafast control in quantum materials
• Suppression of magnetic ordering in XXZ-type antiferromagnetic monolayer NiPS3
• Controlling the anisotropy of a van der Waals antiferromagnet with light
• Quadrupolar correlations and spin freezing in S = 1 triangular lattice antiferromagnets
• Exciton-driven antiferromagnetic metal in a correlated van der Waals insulator
• Non-diffusive spin dynamics in a two-dimensional electron gas
• Controlling ligand-mediated exchange interactions in periodically driven magnetic materials

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