Low-energy orbital texture of Ca₂RuO₄ and Ca₃Ru₂O₇

Transition metal (TM) oxides with 4d valence electrons exhibit unconventional magnetic and electronic properties, which put them into the spotlight of many investigations. The origin of these intriguing properties lays in the competition of comparable energy scales set by local interactions, including the Hund’s rule and crystal field terms, together with intrinsic spin-orbit coupling (SOC) of the TM ions. By entangling the electron spin to the shape of the electronic cloud, SOC makes the electronic spin-orbital states highly sensitive to the intersite connectivity and effective dimensionality of the underlying lattice.

On this background, a wide scientific interest has been put towards the Ruddlesden-Popper type ruthenates (Sr, Ca)_n+1Ru_nO_3n+1, which show a very rich variety of exotic ground states including superconductivity and Mott physics [1-3]. This talk presents combined oxygen K-edge x-ray absorption and resonant inelastic x-ray scattering studies of the single layer Ca₂RuO₄ and the bilayered Ca₃Ru₂O₇ [4-5]. Excitations of ruthenium d orbitals were accessed indirectly through their hybridization with oxygen p orbitals. With this method, it is possible to investigate the low-lying electronic structure site and orbital specific for these closely related compounds. Whereas Ca₂RuO₄ is an antiferromagnetic band-Mott insulator at low temperature, the ground state of Ca₃Ru₂O₇ exhibits in-plane ferromagnetic order and a metallic behaviour. This direct comparison of the low-lying spin-orbital excitations demonstrates remarkable differences. Furthermore, the experimentally observed dispersion of excitations allows their interpretation as spin-orbital quasiparticle branches.

[1] Fatuzzo et al., PRB 91, 155104 (2015)

[2] Das, L. et al., PRX 8, 011048 (2018)

[3] Gretarsson et al., PRB 100, 045123 (2019)

[4] von Arx et al., PRB 102, 235194 (2020)

[5] von Arx et al., in preparation