Nonmagnetic <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>J</mml:mi><mml:mo>=</mml:mo><mml:mn>0</mml:mn></mml:mrow></mml:math> State and Spin-Orbit Excitations in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">K</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>RuCl</mml:mi></mml:mrow><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:mrow></mml:math>
H. Takahashi, H. Suzuki, J. Bertinshaw, S. Bette, C. Mühle, J. Nuss, R. Dinnebier, A. Yaresko, G. Khaliullin, H. Gretarsson, T. Takayama, H. Takagi, B. Keimer
Abstract
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 ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$ 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 ${\mathrm{K}}_{2}{\mathrm{RuCl}}_{6}$, 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.