Charge order at the frontier between the molecular and solid states in Ba3NaRu2O9

TL;DR

This study reveals charge ordering in Ba3NaRu2O9 driven by Coulomb interactions, with complex interplay of orbital effects, leading to a unique phase transition at low temperatures that exemplifies phenomena at the molecular-solid state boundary.

Contribution

It demonstrates how long-range Coulomb interactions induce charge order in a quasi-molecular oxide, highlighting the role of orbital and bonding effects in this process.

Findings

Charge crystallizes below 210 K in Ba3NaRu2O9.

Charge order results from long-range Coulomb interactions.

Orbital ordering and metal-metal bonding influence charge separation.

Abstract

We show that the valence electrons of Ba3NaRu2O9, which has a quasi-molecular structure, completely crystallize below 210 K. Using an extended Hubbard model, we show that the charge ordering instability results from long-range Coulomb interactions. However, orbital ordering, metal-metal bonding and formation of a partial spin gap enforce the magnitude of the charge separation. The striped charge order and frustrated hcp lattice of Ru2O9 dimers lead to competition with a quasi-degenerate charge-melted phase under photo-excitation at low temperature. Our results establish a broad class of simple metal oxides as models for emergent phenomena at the border between the molecular and solid states.