A spin–orbital-entangled quantum liquid on a honeycomb lattice*

In 5d Ir⁴⁺ oxides, the spin-orbit coupling for 5d electrons is as large as ~0.5 eV and not small as compared with on-site Coulomb U. This often gives rise to a spin-orbital Mott state with J_eff=1/2 isospins [1]. In the family of insulating 5d Ir⁴⁺ oxides with a honeycomb-based structure, such as a-, b-, g-Li₂IrO₃ and Na₂IrO₃ (and also their 4d analogue a-RuCl₃), Ir⁴⁺ ions are connected by the three orthogonal Ir-O₂-Ir plane bonds, which gives rise to a bond-dependent Ising interactions among J_eff=1/2 isospins. These compounds were pointed out theoretically to be a materialization of Kiatev model with a topological spin liquid as the ground state [2, 3]. However, a long range magnetic ordering rather than a liquid state was observed in these compounds, likely due to the presence of magnetic interactions other than the Kitaev interactions. We recently visited a new generation of honeycomb iridates H₃LiIr₂O₆, where all the interlayer Li⁺ ions in a-Li₂IrO₃ are replaced with H⁺, and discovered that a quantum spin liquid state is realized in H₃LiIr₂O₆ [4]. H₃LiIr₂O₆ does not show any trace of magnetic ordering down to 0.05 K, despite that an energy scale of magnetic interaction is ~ 100 K. We found at low temperatures below ~5K that the magnetization M, the NMR 1/T₁ and the specific heat C are dominated by the contributions from spin defects and follows a scaling with B/T. After subtracting the scaled contribution from the defects, we find only T³-contribution in C(T) within the given resolution, which can be ascribed to the lattice contribution. This suggests the presence of a gap in the magnetic excitations.

[1] B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008) & Science, 1329 (2009).
[2] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009).
[3] A.Kitaev, Annals of Physics 312 2 (2006).
[4] K. Kitagawa et al., Nature, 554, 341–345 (2018).