Mean-field analysis of collective-mode excitations in excitonic magnet Ca₂RuO₄

In 4d⁴ electron systems with t_2g orbitals, where the orbital angular momentum is not fully quenched, nonmagnetic singlet state becomes the ground state in the atomic limit due to spin-orbit coupling, while in crystals, intersite superexchange interactions could lead to the excitonic magnetic ordering due to hybridization between low-spin singlet and high-spin triplet states. Ca₂RuO₄ is a candidate material for such excitonic magnets.
In the present study, we applied the mean-field and random phase approximations to analyze the collective-mode excitations in the two-dimensional three-orbital Hubbard model for Ca₂RuO₄ derived from ab-initio calculations. We then found that when the system is in the excitonic magnetic phase, the calculated magnetic excitation spectra are in good agreement with experimental ones.
In this talk, we will present collective-mode spectra in the ordered phase of the above model and discuss their parameter dependence such as spin-orbit coupling and crystal field in detail. We will also discuss the role of excitonic magnetic ordering in inelastic neutron scattering spectra of Ca₂RuO₄. If time permits, results for resonant inelastic X-ray scattering spectra will also be presented.