A DFT Study of Tetragonal Rocksalt Copper Monoxide: A Proxy Structure for Understanding HTSC in the Copper Oxide Perovskites

Copper monoxide does not naturally crystallize in face-centered cubic rocksalt symmetry. It has been shown such may result from Jahn-Teller degeneracies which are relieved by distortion into monoclinic tenorite. It has been found possible to ``force epitaxially'' grow 4-5 layers of fcc tetragonal CuO on suitable perovskite substrates A ``c/a-axis'' elongation of roughly 1.3 -- 1.4 stabilizes the ground state. What is paramount is that fcc tetragonal CuO contains the essential feature of all high temperature superconducting copper oxide perovskites without the complicating inclusion of ``3$^{\mathrm{rd}}$ elements,'' and thus provides a simple ``proxy'' to understand the origin of high-Tc. We report using density-functional-theory towards this end. We add charge to the relevant Cu-O bond states, compensated by background ``jellium'' to simulate doping. We introduce an empirical screening of coulomb repulsion and present our results, suggesting electron-phonon coupling underlies HTSC, in the phase diagram context of the ``Great Quantum Conundrum (Nature, 2011).''