Competing spin-orbital singlet states in d<sup>4</sup> honeycomb system
ORAL · Invited
Abstract
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 Jeff = 1/2 systems and multipolar physics in d1 (Jeff = 3/2) and d2 (Jeff = 2) compounds.
In this talk, I will focus on the d4 compounds with strong spin-orbit coupling. While spin-orbit coupling yields nonmagnetic Jeff = 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.
In this talk, I will focus on the d4 compounds with strong spin-orbit coupling. While spin-orbit coupling yields nonmagnetic Jeff = 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.
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Publication: arXiv:2205.12123
Presenters
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Tomohiro Takayama
Max Planck Institute for Solid State Research
Authors
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Tomohiro Takayama
Max Planck Institute for Solid State Research