Coexisting Z-type charge and bond order in metallic NaRu$_2$O$_4$

TL;DR

This study reveals a novel coexistence of charge order and dimerization in metallic NaRu$_2$O$_4$, driven by a new Z-type bonding, highlighting complex interactions among orbital, charge, and lattice degrees of freedom.

Contribution

It introduces Z-type ordering as a new bonding mechanism explaining the coexistence of charge order and dimerization in a metallic quantum material.

Findings

Discovery of Z-type ordering in NaRu$_2$O$_4$

Observation of coexisting charge order and dimerization in a metal

Identification of complex orbital-charge-lattice interplay

Abstract

How particular bonds form in quantum materials has been a long-standing puzzle. Two key concepts dealing with charge degrees of freedom are dimerization (forming metal-metal bonds) and charge ordering (CO). Since the 1930s, these two concepts have been frequently invoked to explain numerous exciting quantum materials, typically insulators. Here we report dimerization and CO within the dimers coexisting in metallic NaRu$_2$O$_4$. By combining high-resolution x-ray diffraction studies and theoretical calculations, we demonstrate that this unique phenomenon occurs through a new type of bonding, which we call Z-type ordering. The low-temperature superstructure has strong dimerization in legs of zigzag ladders, with short dimers in legs connected by short zigzag bonds, forming Z-shape clusters: simultaneously, site-centered charge ordering also appears. Our results demonstrate the yet unknown flexibility of quantum materials with the intricate interplay among orbital, charge, and lattice degrees of freedom.