Vibronic excitations in resonant inelastic x-ray scattering spectra of K₂RuCl₆

The interplay of the strong spin-orbit coupling and superexchange interaction could make the cubic compounds with nonmagnetic d⁴ ions magnetic [1].

Recently, to search the excitonic magnetism, the local quantum states of a cubic Ru compound, K₂RuCl₆, were experimentally investigated using Ru L₃-edge resonant inelastic scattering (RIXS) [2].

The experimental RIXS spectra, however, show an unexpectedly small spin-orbit gap and broad peak around the spin-orbit levels.

The authors attribute the former to the dynamical Jahn-Teller effect in the excited states.

In this work, we address these two issues with ab initio calculations.

We confirmed the presence of the dynamic Jahn-Teller effect in the excited states by using ab initio calculations as in Ref. [3].

Then, we derived a formula of RIXS spectra of the dynamic Jahn-Teller system, and applied it to K₂RuCl₆ with the ab initio model.

We found that the calculated vibronic levels agree well with the spin-orbit gap, and the electronic peak splits into several vibronic peaks, which makes the experimental peak broad.

[1] G. Khaliullin, Phys. Rev. Lett. 111, 197201 (2013).

[2] H. Takahashi et al., Phys. Rev. Lett. 127, 227201 (2021).

[3] N. Iwahara, V. Vieru, and L. F. Chibotaru, Phys. Rev. B 98, 075138 (2018).