honeycomb system

4d and 5d transition-metal compounds hosting spin-orbit-entangled states have emerged as a rich platform of unconventional electronic phases. The interactions between spin-orbit-entangled pseudospins in these compounds are distinct from those of spin-only magnets, which may give rise to exotic magnetic ground states. The examples include Kitaev magnetism in d⁵ J_eff = 1/2 systems and multipolar physics in d¹ (J_eff = 3/2) and d² (J_eff = 2) compounds.

 

In this talk, I will focus on the d⁴ compounds with strong spin-orbit coupling. While spin-orbit coupling yields nonmagnetic J_eff = 0 singlet state for an isolated d⁴ ion, it has been proposed that their interactions via excited states lead to a rare example of exotic magnet called excitonic magnet. Excitonic magnets are expected to display a variety of intriguing phenomena including magnetic exciton condensation, quantum criticality and Higgs mode excitation. Particularly, d⁴ honeycomb compounds are proposed to realize frustrated exitonic magnetism where unconventional magnetic ground states such as a spin-nematic state and bosonic Kitaev liquid may emerge.

 

Despite these tantalizing perspectives, honeycomb-based excitonic magnets have remained elusive. We found that the honeycomb ruthenate Ag₃LiRu₂O₆ realizes a honeycomb lattice of spin-orbit-entangled singlet states serves as a promising starting point for frustrated excitonic magnetism. Under pressure, Ag₃LiRu₂O₆ undergoes successive phase transitions to other spin-orbital singlet phases instead of developing excitonic magnetism. While the high-pressure phase is characterized by  molecular-orbital formation as found in other honeycomb-based compounds, the intermediate phase represents a spin-orbit-entangled weak dimer state which we argue is induced by a pseudo-Jahn-Teller effect and is unique to d⁴ systems. The identification of competing spin-orbital phases on a honeycomb lattice should give a clue to the realization of unconventional magnetic ground states.