Orbital inversion and emergent lattice dynamics in infinite layer CaCoO$_2$

TL;DR

This study investigates CaCoO$_2$, revealing orbital inversion and emergent lattice dynamics linked to its unique herringbone structure, with implications for correlated electronic materials.

Contribution

It uncovers orbital inversion and complex lattice behavior in CaCoO$_2$, a prototype for materials with strong electronic and orbital correlations.

Findings

Inter-plane hybridization causes orbital inversion.

Strong low energy mode observed in RIXS data.

Herringbone structure may enable novel correlated materials.

Abstract

The layered cobaltate CaCoO$_2$ exhibits a unique herringbone-like structure. Serving as a potential prototype for a new class of complex lattice patterns, we study the properties of CaCoO$_2$ using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). Our results reveal a significant inter-plane hybridization between the Ca $4s-$ and Co $3d-$orbitals, leading to an inversion of the textbook orbital occupation of a square planar geometry. Further, our RIXS data reveal a strong low energy mode, with anomalous intensity modulations as a function of momentum transfer close to a quasi-static response suggestive of electronic and/or orbital ordering. These findings indicate that the newly discovered herringbone structure exhibited in CaCoO$_2$ may serve as a promising laboratory for the design of materials having strong electronic, orbital and lattice correlations.