Long-Range Order, and a Possible Devil's Staircase in YBa2Cu3oz

It is shown that the oxygen-ordered superstructures observed experimentally in YBa2Cu3Oz can be understood in terms of an Ising Hamiltonian containing screened Coulomb repulsions between any two oxygen sites, augmented with a short-range attractive covalent interaction between oxygen sites adjoined by a copper atom. Spatially modulated commensurate phases separated by smooth soliton-like domain walls are obtained through Monte Carlo simulations. These simulations indicate the existence of a complete devil's staircase of phases. It is predicted that the plateau fine structure of the Tc-z curve and the bond valence sum also exhibit experimentally observable staircase behaviour.

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