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Spin-orbit Mott insulators composed of $t^4_{2g}$ transition metal ions may host excitonic magnetism due to the condensation of spin-orbital J=1 triplons. Prior experiments suggest that the 4d antiferromagnet Ca$_2$RuO$_4$ 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 K$_2$RuCl$_6$, 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.

Physical review / B 105(5), 054417 (2022). doi:10.1103/PhysRevB.105.054417, Iron(IV) oxides are strongly correlated materials with negative charge-transfer energy (negative $Δ$), and exhibit peculiar electronic and magnetic properties such as topological helical spin structures in the metallic cubic perovskite SrFeO$_3$. Here, the spin structure of the layered negative-$Δ$ insulator Sr$_2$FeO$_4$ was studied by powder neutron diffraction in zero field and magnetic fields up to 6.5 T. Below $T_N$=56K, Sr$_2$FeO$_4$ adopts an elliptical cycloidal spin structure with modulated magnetic moments between 1.9 and 3.5 $μ_B$ and a propagation vector k=($τ,τ$,0) with $τ$=0.137. With increasing magnetic field the spin structure undergoes a spin-flop transition near 5 T. Synchrotron $^{57}$Fe-Mössbauer spectroscopy reveals that the spin spiral transforms to a ferromagnetic structure at pressures between 5 and 8 GPa, just in the pressure range where a Raman-active phonon nonintrinsic to the K$_2$NiF$_4$-type crystal structure vanishes. These results indicate an insulating ground state which is stabilized by a hidden structural distortion and differs from the charge disproportionation in other Fe(IV) oxides., Published by Inst., Woodbury, NY