Excitonic magnetism and valence bond solid in spin-orbit coupled d⁴ ruthenates

In 4d and 5d transition-metal oxides, spin-orbit coupling produces exotic phases beyond those based on isotropic spin by entangling the spin and angular momenta. In ruthenates with d⁴ configuration, spin-orbit coupling stabilises a nominally nonmagnetic J_eff = 0 singlet ground state. However, when intersite magnetic exchange is comparable to the spin-orbit gap, the upper-lying J_eff = 1 triplets acquire dispersion and may undergo a quantum phase transition into a triplon condensate known as an excitonic magnet. Adjusting the balance between the spin-orbit gap and magnetic interaction therefore provides a route toward approaching the quantum critical regime of spin-orbit condensate.

A useful way to tune this competition is through lattice geometry, which controls the exchange pathways via octahedral connectivity. In this talk, I present a materials-based exploration of this tuning across ruthenium oxides, including double perovskite, layered perovskite, honeycomb and pyrochlore lattices that span regimes from Van Vleck paramagnetism to excitonic magnetism. Specifically, pyrochlore In₂Ru₂O₇ synthesised under high pressure exhibits a series of structural transitions from J_eff =  0 Mott insulating state to a nonmagnetic phase characterised by Ru₂O molecular units. These findings show that competing valence bond instabilities can emerge and dominate the ground state in the vicinity of the predicted quantum critical regime of excitonic magnetism.