Spin-orbital-entangled quantum magnet on a honeycomb lattice

In heavy transition-metal compounds containing 4d or 5d elements, strong spin-orbit coupling often yields spin-orbital-entangled J_eff-states for d-electrons. The magnetic interactions between such states may give rise to an exotic ground state due to their bond-sensitive character. In particular, quantum liquid state of spin-orbital-entangled objects have been expected to emerge in a simple honeycomb lattice.

Honeycomb-based iridates with J_eff = 1/2 pseudospins have been intensively investigated as a potential realization of Kitaev spin liquid. Although the honeycomb iridates such as Na₂IrO₃ and α-Li₂IrO₃ are shown to undergo magnetic ordering likely due to the additional magnetic interactions other than Kitaev-type exchange, a quantum liquid state has been identified in the hydrogen-exchanged material H₃LiIr₂O₆. No magnetic order or spin-glass freezing is not seen in H₃LiIr₂O₆ down to 50 mK. However, clear evidence for Kitaev-type spin liquid is lacking so far, and the prime driving force for the liquid state remains elusive. In order to investigate the critical role of hydrogen, we synthesized an isotopic material D₃LiIr₂O₆. D₃LiIr₂O₆ displays a large isotope effect in structural and magnetic properties; the antiferromagnetic Weiss temperature increases from 100 K for H₃LiIr₂O₆ to 170 K for D₃LiIr₂O₆. Nevertheless, the quantum liquid state was found to be robust. We argue that the disordered OH/OD bonds likely play a role to stabilize the quantum liquid state.
Another exotic honeycomb magnet is expected for systems with d⁴ configuration. In the case, the spin-orbit coupling produces nonmagnetic J_eff = 0 singlet ground state. However, the d⁴ ions magnetically interact with each other via the excited J_eff = 1 levels, and such magnetic interactions are also shown to inherit bond-dependent character. We argue that a honeycomb ruthenate Ag₃LiRu₂O₆ has the spin-orbit induced singlet ground state and discuss the possible pressure-induced electronic phase transitions.