Magnetically brightened dark electron-phonon bound states in a van der Waals antiferromagnet

TL;DR

This study uncovers magnetically tunable bound states of d-orbitals and phonons in a 2D van der Waals antiferromagnet, revealing strong electron-phonon coupling and potential for controlling magnetic properties.

Contribution

It demonstrates the existence of optically dark electron-phonon bound states in NiPS3 that become accessible with magnetic order, and quantifies their strong coupling strength.

Findings

Bound states manifest as equally spaced phonon replicas.

States are optically dark above Néel temperature and become accessible with magnetic order.

Electron-phonon coupling exceeds known values in 2D systems.

Abstract

In van der Waals (vdW) materials, strong coupling between different degrees of freedom can hybridize elementary excitations into bound states with mixed character. Correctly identifying the nature and composition of these bound states is key to understanding their ground state properties and excitation spectra. Here, we use ultrafast spectroscopy to reveal bound states of d-orbitals and phonons in 2D vdW antiferromagnet NiPS3. These bound states manifest themselves through equally spaced phonon replicas in frequency domain. These states are optically dark above the N\'eel temperature and become accessible with magnetic order. By launching this phonon and spectrally tracking its amplitude, we establish the electronic origin of bound states as localized d-d excitations. Our data directly yield electron-phonon coupling strength which exceeds the highest known value in 2D systems. These results demonstrate NiPS3 as a platform to study strong interactions between spins, orbitals and lattice, and open pathways to coherent control of 2D magnets.