Spin-orbital entangled matter can yield a rich variety of phases. In a 5d⁴ (t_2g⁴) Mott insulator, the Hund’s coupling yields local S = 1 and L_eff = 1 moments. Spin-orbit coupling in the large U limit should lead to a nonmagnetic ground state with J_eff = 0, which interestingly is a Mott insulator without a magnetic dipole moment. With increasing electron hopping strength, the excited J_eff = 1 triplet states will interact appreciably, and their dispersion will cross the J_eff = 0 level, where an unconventional magnetic ground state may emerge. This process can be viewed as a condensation of magnetic excitations and therefore may be called excitonic magnetism [1]. The excitonic magnetism becomes even more exotic if it appears in the edge-shared honeycomb network of 5d⁴ transition-metal ions as in 5d⁵ candidates to realization of the Kitaev model. Due to the bond-dependent interactions, a hallmark of spin-orbital entangled systems, an exotic spin-superfluid state and/or nematic dimerized states are theoretically proposed.

Some 5d⁴ Ir⁵⁺ compounds are pointed out to be J_eff = 0 Mott insulators [2]. However, the splitting of the ground J_eff=0 and the first exited J_eff=1 states, proportional to the spin-orbit coupling strength of around half an eV, is too large to induce the excitonic magnetism by increasing the exchange coupling. It might therefore be more realistic to consider 4d⁴ Ru⁴⁺ compounds as candidates for the realization of this exotic magnetism. However, most of insulating Ru⁴⁺ oxides known so far are either magnetic insulators or strong dimer insulators, but not the elusive J_eff = 0 Mott insulators. Recently, 4d⁴ Ca₂RuO₄ has been suggested to be  an excitonic magnet with strong interaction beyond the critical point [3]. Nevertheless, the most important part of the proposed phase diagram, the J_eff = 0 Mott insulator and its associated excitonic critical point remains to be fully explored. With this project, we propose a comprehensive search for the J_eff = 0 Mott insulators through material synthesis of honeycomb Ru⁴⁺ oxides, as well as RIXS, NMR, Raman and thermodynamic measurements to exploring the ground state and the properties of the phase diagram.

 

References

[1] Y. -Z. You et al., Phys. Rev. B 80, 085145 (2012).
[2] B. Yuan et al., Phys. Rev. B 95, 235114 (2017).
[3] A. Jain et al., Nat. Phys. 13, 633 (2017).

Principal Investigators

H. Takagi
K. Kitagawa