Regular-Triangle Trimer and Charge Order Preserving the Anderson Condition in the Pyrochlore Structure of CsW$_2$O$_6$

TL;DR

This study reports a novel phase transition in CsW$_2$O$_6$ where W 5d electrons form regular-triangle trimers, preserving cubic symmetry and exhibiting charge, orbital, and spin orderings driven by electronic instabilities.

Contribution

It reveals a unique self-organization of 5d electrons into regular-triangle trimers that satisfy the Anderson condition, a phenomenon not previously observed in pyrochlore oxides.

Findings

Formation of W3 trimers preserving cubic symmetry.

Charge order satisfying the Anderson condition.

Orbital order and spin-singlet pairing in trimers.

Abstract

Since the discovery of the Verwey transition in magnetite, transition metal compounds with pyrochlore structures have been intensively studied as a platform for realizing remarkable electronic phase transitions. We report the discovery of a unique phase transition that preserves the cubic symmetry of the beta-pyrochlore oxide CsW$_2$O$_6$, where each of W 5d electrons are confined in regular-triangle W3 trimers. This trimer formation is an unprecedented self-organization of d electrons, which can be resolved into a charge order satisfying the Anderson condition in a nontrivial way, orbital order caused by the distortion of WO6 octahedra, and the formation of a spin-singlet pair in a regular-triangle trimer. Electronic instability due to the unusual three-dimensional nesting of Fermi surfaces and the localized nature of the 5d electrons characteristic of the pyrochlore oxides were found to play important roles in this unique charge-orbital-spin coupled phenomenon.