Search for unusual orbital configuration and possible low-energy orbital excitations in CuSb₂O₆ 1D magnet

Low-dimensional cuprates display a variety of exotic physical properties. They tend to exhibit either a quasiplanar CuO₄ coordination or CuO₆ octahedra that are highly distorted due to the Jahn-Teller effect, leading to a large (>1 eV) energy separation between the x²−y² and 3z²−r² -like orbital states for the Cu²⁺ 3d hole. In contrast, the quasi-1D magnet Mott insulator CuSb₂O₆ shows fairly regular CuO6 octahedra that seem to promote a close competition between the 3z²−r² and x²−y² levels. In fact, Ab-initio band structure calculations predict a separation of only ~0.3 eV between these levels [1]. X-ray absorption spectroscopy experiments indicate small anisotropy between different geometries of incident light, which supports the idea of a strong competition between x²−y² and 3z²−r² orbitals in the ground state. Two possibilities are considered: 1) the ground state is a combination of these two electronic states or 2) the orbital ordering is well defined even though the dichroic signal is negligible due to a small energy splitting between the two e_g levels. In addition, the small energy separation between e_g levels may lead to new pathways for electron-phonon interaction in this cuprate. Indeed, a recent report from our group displays Raman spectroscopy data that support this general idea, identifying an electronic excitation that strongly interacts with phonons and may be ascribed to an orbiton [2]. In this work, we will show and discuss resonant inelastic X-ray scattering (RIXS) experiments that shed light on this possible unusual ground state, the presumably small Cu e_g orbital splitting, and the possible existence of orbitons in this unusual 1D magnetic cuprate with trirutile structure.

[1] D. Kasinathan, K. Koepernik, and H. Rosner. Phys. Rev. Lett. 100, 237202 (2008)

[2] D. T. Maimone, A. B. Christian, J. J. Neumeier, and E. Granado, Phys. Rev. B 97 174415 (2018)