paper

Spin and charge excitations in the correlated multiband metal Ca3Ru2O7

arXiv:2012.10180 · doi:10.1103/PhysRevB.103.085108

Abstract

We use Ru $L_{3}$ -edge resonant inelastic x-ray scattering (RIXS) to study the full range of excitations in Ca $_{3}$ Ru $_{2}$ O $_{7}$ from meV-scale magnetic dynamics through to the eV-scale interband transitions. This bilayer $4 d$ -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 fluorescence-like signal appearing between the $t_{2 g}$ and $e_{g}$ bands. At the same time, we observe a resonance originating from localized $t_{2 g}$ excitations, in analogy to the structurally related Mott-insulator Ca $_{2}$ RuO $_{4}$ . But whereas Ca $_{2}$ RuO $_{4}$ 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 intra-ionic interactions underlie the correlated physics in Ca $_{3}$ Ru $_{2}$ O $_{7}$ , even as the excitations become strongly mixed in the presence of itinerant electrons.

8 pages, 4 figures