Spin exchange dominated by charge fluctuations of the Wigner lattice in the chain cuprate Na5Cu3O6
Naveed Zafar Ali, Jesko Sirker, Jürgen Nuss, Peter Horsch, Martin Jansen
DOI 10.1103/PhysRevB.84.035113 · Physical Review B
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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 ∞ 1CuO2n− 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 the explicit assignment of the valence states of either +2 or +3 to each copper atom, resulting in a repetition according to Cu2+-Cu3+-Cu2+-Cu2+-Cu3+-Cu2+. 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 quantitatively satisfying theoretical description of the magnetic susceptibility data.
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