Van Vleck excitons in Ca2Ru O4

A framework is presented for modeling and understanding magnetic excitations in localized, intermediate coupling magnets where the interplay between spin-orbit coupling, magnetic exchange, and crystal-field effects are known to create a complex landscape of unconventional magnetic behaviors and ground states. A spin-orbit exciton approach for modeling these excitations is developed based upon a Hamiltonian which explicitly incorporates single-ion crystalline electric field and spin exchange terms. This framework is then leveraged to understand a canonical Van Vleck jeff=0 singlet ground state whose excitations are coupled spin and crystalline electric-field levels. Specifically, the anomalous Higgs mode [Jain, Nat. Phys. 13, 633 (2017)10.1038/nphys4077], spin-waves [Kunkemöller, Phys. Rev. Lett. 115, 247201 (2015)10.1103/PhysRevLett.115.247201], and orbital excitations [Das, Phys. Rev. X 8, 011048 (2018)10.1103/PhysRevX.8.011048] in the multiorbital Mott insulator Ca2RuO4 are captured and good agreement is found with previous neutron and inelastic x-ray spectroscopic measurements. Furthermore, our results illustrate how a crystalline electric-field-induced singlet ground state can support coherent longitudinal, or amplitude excitations, and transverse wavelike dynamics. We use this description to discuss mechanisms for accessing a nearby critical point.