Atomic visualization of copper oxide structure in the infinite-layer cuprate SrCuO2

We report the atomic-scale structure of epitaxial films of the parent infinite-layer compound $\mathrm{SrCu}{\mathrm{O}}_{2}$ prepared on ${\mathrm{SrTiO}}_{3}$ by molecular beam epitaxy. In situ scanning tunneling microscopy study reveals a stoichiometric copper oxide (${\mathrm{CuO}}_{2}$) -terminated surface featured by $2\ifmmode\times\else\texttimes\fi{}2$ reconstruction, caused primarily by structural distortions of four adjacent ${\mathrm{CuO}}_{2}$ plaquettes. Furthermore, the subsurface Sr atoms have been rarely discernible, showing intra-unit-cell rotational symmetry breaking. These observations can be reasonably modeled by a periodic up-down buckling of oxygen ions on the ${\mathrm{CuO}}_{2}$ plane. Further postannealing leads to the removal of surface oxygens and an incommensurate stripe phase. Our study provides indispensable structural information to help understand the exotic properties of cuprate superconductors.