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

TL;DR

This paper develops microscopic models for optically inducing and probing magnetic dynamics in van der Waals magnets, specifically NiPS$_3$, revealing how light interacts with magnetic excitations and proposing experiments to detect Berezinskii-Kosterlitz-Thouless (BKT) physics.

Contribution

It introduces two mechanisms for optical generation of magnetic modes in van der Waals magnets and applies these to NiPS$_3$, connecting microscopic light-spin interactions with experimental observations.

Findings

Quantitative agreement with recent experiments on bulk NiPS$_3$

Proposals for pump-probe experiments in monolayer NiPS$_3$

Identification of BKT phase excitations via relaxational behavior

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.