Spin exchange dominated by charge fluctuations of the Wigner lattice in the newly synthesized chain cuprate Na5Cu3O6

TL;DR

This study investigates the magnetic properties of Na5Cu3O6, a newly synthesized chain cuprate with a Wigner lattice, revealing that charge fluctuations significantly influence spin exchange interactions beyond traditional superexchange mechanisms.

Contribution

The paper demonstrates that charge fluctuations across the Wigner charge gap dominate spin exchange in Na5Cu3O6, providing a new understanding of magnetic interactions in charge-ordered cuprates.

Findings

Charge order forms a Wigner lattice with specific Cu valence states.

Magnetic susceptibility is affected by excitations across the Wigner charge gap.

Including charge fluctuation effects yields a good match with experimental data.

Abstract

Na5Cu3O6, a new member of one dimensional charge ordered chain cuprates, was synthesized via the azide/nitrate route by reacting NaN3, NaNO3 and CuO. According to single crystal X-ray analysis, one dimensional CuO2 chains built up from planar, edge-sharing CuO4 squares are a dominant feature of the crystal structure. From the analysis of the Cu-O bond lengths we find that the system forms a Wigner lattice. The commensurate charge order allows to explicitly assign the valence states of either +2 or +3 to each copper atom resulting in a repetition according to Cu(2+)-Cu(3+)-Cu(2+)-Cu(2+)-Cu(3+)-Cu(2+). Following the theoretical analysis of the previously synthesized compounds Na3Cu2O4 and Na8Cu5O10, the magnetic susceptibility was expected to show a large dimer gap. Surprisingly, this is not the case. To resolve this puzzle, we show that the magnetic couplings in this compound are strongly affected by excitations across the Wigner charge gap. By including these contributions, which are distinct from conventional superexchange in Mott-insulators, we obtain a quantitative satisfying theoretical description of the magnetic susceptibility data.