Nonmagnetic J=0 State and Spin-Orbit Excitations in K2RuCl6

Spin-orbit Mott insulators composed of t(2g)(4) transition metal ions may host excitonic magnetism due to the condensation of spin-orbital J =1 triplons. Prior experiments suggest that the 4d antiferromagnet Ca2RuO4 embodies this notion, but a J = 0 nonmagnetic state as a basis of the excitonic picture remains to be confirmed. We use Ru L-3-edge resonant inelastic x-ray scattering to reveal archetypal J multiplets with a J = 0 ground state in the cubic compound K2RuCl6, which are well described within the LS-coupling scheme. This result highlights the critical role of unquenched orbital moments in 4d-electron compounds and calls for investigations of quantum criticality and excitonic magnetism on various crystal lattices.