We use Ru $L_3$-edge resonant inelastic x-ray scattering to study the full range of excitations in ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ from meV-scale magnetic dynamics through to the eV-scale interband transitions. This bilayer $4d$-electron correlated metal expresses a rich phase diagram, displaying long-range magnetic order below 56 K followed by a concomitant structural, magnetic, and electronic transition at 48 K. In the low-temperature phase, we observe a magnetic excitation with a bandwidth of $\sim 30$ meV and a gap of $\sim 8$ meV at the zone center, in excellent agreement with inelastic neutron scattering data. The dispersion can be modeled using a Heisenberg Hamiltonian for a bilayer $S=1$ system with single-ion anisotropy terms. At a higher energy loss, $dd$-type excitations show heavy damping in the presence of itinerant electrons, giving rise to a fluorescencelike signal appearing between the $t_{2g}$ and $e_g$ bands. At the same time, we observe a resonance originating from localized $t_{2g}$ excitations, in analogy to the structurally related Mott insulator ${\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_4$. But whereas ${\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_4$ shows sharp separate spin-orbit excitations and Hund's-rule driven spin-state transitions, here we identify only a single broad asymmetric feature. These results indicate that local intraionic interactions underlie the correlated physics in ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, even as the excitations become strongly mixed in the presence of itinerant electrons.

• Received 4 December 2020
• Accepted 25 January 2021

DOI:https://doi.org/10.1103/PhysRevB.103.085108

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Published by the American Physical Society