  We report on atomic-scale visualization of the structure of infinite-layer
cuprate SrCuO2 thin films grown on Nb-doped SrTiO3 substrates by molecular beam
epitaxy. In-situ scanning tunneling microscopy study reveals stoichiometric
copper oxide (CuO2) plane with a 2 x 2 surface reconstruction, prompted by
preferential clustering of four adjacent CuO2 plaquettes. By imaging the
subsurface Sr atoms, intra-unit-cell rotational symmetry breaking is observed,
which, together with the adjacent CuO2 clustering, can be well accounted for by
a periodic up-down buckling of oxygen ions on the CuO2 plane. Further
post-annealing leads to an incommensurate stripe structure of the surface
layer. Our findings provide important structural information for deeply
understanding the electronic structure of superconducting CuO2 plane as well as
high temperature superconductivity in cuprates.
