Spin and charge excitations in the correlated multiband metal Ca3Ru2 O7

Abstract

We use Ru L3-edge resonant inelastic x-ray scattering to study the full range of excitations in Ca3Ru2O7 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 ∼30 meV and a gap of ∼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 t2g and eg bands. At the same time, we observe a resonance originating from localized t2g excitations, in analogy to the structurally related Mott insulator Ca2RuO4. But whereas Ca2RuO4 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 Ca3Ru2O7, even as the excitations become strongly mixed in the presence of itinerant electrons.