Halogen-mediated exchange in the coupled-tetrahedra quantum spin systems Cu₂Te₂O₅X₂(X=Br,Cl)

Abstract

Motivated by recent discussion on possible quantum critical behavior in the coupled Cu-tetrahedra system Cu₂Te₂O₅Br₂, we present a comparative ab initio study of the electronic properties of Cu₂Te₂O₅Br₂ and the isostructural Cu₂Te₂O₅Cl₂. A detailed investigation of the copper-copper interaction paths reveals that the halogen ions play an important role in the inter-tetrahedral couplings via X₄ rings (X=Br,Cl). We find that, contrary to initial indications, both systems show a similar electronic behavior with long range exchange paths mediated by the X₄ rings.