Soft spin-amplitude fluctuations in a Mott-insulating ruthenate

Abstract

Magnetism in transition-metal compounds (TMCs) has traditionally been associated with spin degrees of freedom, because the orbital magnetic moments are typically largely quenched. On the other hand, magnetic order in 4f- and 5d-electron systems arises from spin and orbital moments that are rigidly tied together by the large intra-atomic spin-orbit coupling (SOC). Using inelastic neutron scattering on the archetypal 4d-electron Mott insulator Ca${}_2$RuO${}_4$, we report a novel form of excitonic magnetism in the intermediate-strength regime of the SOC. The magnetic order is characterized by ``soft'' magnetic moments with large amplitude fluctuations manifested by an intense, low-energy excitonic mode analogous to the Higgs mode in particle physics. This mode heralds a proximate quantum critical point separating the soft magnetic order driven by the superexchange interaction from a quantum-paramagnetic state driven by the SOC. We further show that this quantum critical point can be tuned by lattice distortions, and hence may be accessible in epitaxial thin-film structures. The unconventional spin-orbital-lattice dynamics in Ca${}_2$RuO${}_4$ identifies the SOC as a novel source of quantum criticality in TMCs.

Cited by in corpus (8)

• Hallmarks of Hund's coupling in the Mott insulator Ca${}_2$RuO${}_4$
• On the possibility of excitonic magnetism in Ir double perovskites
• Doping-Induced Ferromagnetism and Possible Triplet Pairing in d4 Mott Insulators
• Magnon dispersion in Ca2RuO4: impact of spin-orbit coupling and oxygen moments
• LDA+DMFT approach to ordering phenomena and the structural stability of correlated materials
• Excitonic magnetism in $d^6$ perovskites
• Spontaneous Spin Textures in Multiorbital Mott Systems
• Magnetic field dependence of excitations near spin-orbital quantum criticality