High pressure tuning of magnon-polarons in the layered antiferromagnet FePS$_3$

TL;DR

This study demonstrates how applying high pressure to the layered antiferromagnet FePS$_3$ induces magnon-phonon resonance, leading to the formation of magnon-polarons and revealing new coupled excitations in the material.

Contribution

It provides the first experimental observation of pressure-induced magnon-phonon coupling and magnon-polaron formation in FePS$_3$, with detailed characterization of these phenomena.

Findings

Magnon-phonon resonance occurs near 4 GPa

Three coupled modes emerge at resonance

Magnon energy remains nearly pressure independent

Abstract

Magnetic layered materials have emerged recently as promising systems to introduce magnetism in structures based on two-dimensional (2D) materials and to investigate exotic magnetic ground states in the 2D limit. In this work, we apply high hydrostatic pressures up to P = 8.7 GPa to the bulk layered antiferromagnet FePS$_3$ to tune the collective lattice excitations (phonons) in resonance with magnetic excitations (magnons). Close to P = 4 GPa, the magnon-phonon resonance is achieved and the strong coupling between these collective modes leads to the formation of new quasi-particles, the magnon-polarons, evidenced in our low temperature Raman scattering experiments by a particular avoided crossing behavior between the phonon and the doubly degenerate antiferromagnetic magnon. At the pressure-induced magnon-phonon resonance, three distinct coupled modes emerge. As it is mainly defined by intralayer properties, we show that the energy of the magnon is nearly pressure independent. We additionally apply high magnetic fields up to B = 30 T to fully identify and characterize the magnon excitations, and to explore the different magnon-polaron regimes for which the phonon has an energy lower-, equal to-, or higher- than the magnon energy. The description of our experimental data requires introducing a phonon-phonon coupling not taken into account in actual calculations.