1D cuprates ACuO2 (A = Li, Na, K, Ru, Cs): not a simple atomic insulator

Abstract

Insulators are generally categorized into topologically nontrivial and trivial ones based on the presence and absence of topological invariants. While topologically nontrivial insulators have attracted significant attention, advancements in classifying trivial insulators remain restricted. Recently, a class of unconventional atomic insulators, termed Berry-obstructed atomic insulators (BOAIs), has emerged. The unconventionality is not captured by any symmetry eigenvalue but is identified by the mismatch between Wannier charge centers (WCCs) and atomic positions. In this work, we connected the charge state of insulators with their unconventionality. We presented a tight-binding (TB) model to demonstrate how strong covalency naturally endowed BOAIs. As negative charge-transfer energy insulators, one-dimensional (1D) cuprates ACuO₂ (A = Li, Na, K, Ru, Cs) exhibited strong covalent features. It was demonstrated that ACuO₂ were not simple atomic insulators but qualified as BOAIs. The unconventionality was verified through topological quantum chemistry analysis, first-principles calculations, and simulations of obstructed surface states. Our work not only enriches the family of BOAIs but also bridges the charge states of insulators and unconventional phases, which will arouse broad interest from both material and chemical communities.