# Cu-Sb dumbbell arrangement in the spin-orbital liquid candidate Ba$_3$CuSb$_2$O$_9$

## TL;DR

This study investigates the microscopic lattice structure of Ba₃CuSb₂O₉, revealing a stripe order of Cu ions and significant interlayer interactions, which are crucial for understanding its quantum spin-orbital liquid behavior.

## Contribution

The paper provides a detailed density functional theory analysis showing stripe Cu arrangements and interlayer interactions, clarifying the lattice structure of Ba₃CuSb₂O₉.

## Key findings

- Stripe order of Cu ions is energetically favored.
- Interlayer interactions influence the out-of-plane structure.
- Insights into Jahn-Teller distortions and electronic properties.

## Abstract

The absence of both spin freezing and of a static Jahn-Teller effect have lead to the proposition that Ba$_3$CuSb$_2$O$_9$ is a quantum spin-orbital liquid. However, theoretical understanding of the microscopic origin of this behavior has been hampered by a lack of consensus on the lattice structure. Cu ions have been proposed to realize either a triangular lattice, a short-range ordered honeycomb lattice or a disordered lattice with stripelike correlations. Here we analyze the stability of idealized versions of these arrangements using density functional theory. We find stripe order of Cu ions to be energetically favored, hinting towards stripelike local Cu-Cu arrangements, while long-range order is presumably hindered due to disorder effects. Furthermore, we find evidence of significant interlayer interactions between Cu-Sb dumbbells, which affects the out-of-plane arrangement. Analysis of the relaxed crystal structures, electronic properties and tight-binding parameters provides clues as to the nature of the Jahn-Teller distortions.