Exploring the spin-orbital ground state of Ba3CuSb2O9

TL;DR

This paper investigates the complex ground state of Ba3CuSb2O9 by analyzing a spin-orbital model, revealing phases like a spin-orbital liquid and singlet cluster formations, with theoretical results aligning qualitatively with experiments.

Contribution

It introduces a comprehensive spin-orbital model on honeycomb lattices, uncovering new phases and providing insights into the material's experimental behavior.

Findings

Identification of an SU(4) spin-orbital liquid phase.

Discovery of cluster singlet phases on decorated honeycomb lattice.

Qualitative agreement with experimental observations despite structural disorder.

Abstract

Motivated by the absence of both spin freezing and a cooperative Jahn-Teller effect at the lowest measured temperatures, we study the ground state of Ba3CuSb2O9. We solve a general spin-orbital model on both the honeycomb and the decorated honeycomb lattice, revealing rich phase diagrams. The spin-orbital model on the honeycomb lattice contains an SU(4) point, where previous studies have shown the existence of a spin-orbital liquid with algebraically decaying correlations. For realistic parameters on the decorated honeycomb lattice, we find a phase that consists of clusters of nearest-neighbour spin singlets, which can be understood in terms of dimer coverings of an emergent square lattice. While the experimental situation is complicated by structural disorder, we show qualitative agreement between our theory and a range of experiments.