Controlling the magnetic state of the proximate quantum spin liquid α-RuCl₃ with an optical cavity

Harnessing the enhanced light-matter coupling arising from mode volume compression in optical cavities is a promising route towards functionalizing quantum materials and realizing exotic states of matter. Here, we extend material engineering via cavity quantum electrodynamics fluctuations to magnetic systems, by demonstrating that a Fabry-Pérot cavity can be used to control the magnetic state of the proximate spin liquid α-RuCl₃. Depending on specific cavity properties such as the frequency, photon occupation, and strength of the light-matter coupling, any of the magnetic phases supported by the extended Kitaev model can be stabilized. In particular, in the THz regime, we show that the cavity vacuum fluctuations alone are sufficient to bring α-RuCl₃ from a zigzag antiferromagnetic to a ferromagnetic state.