  Asymmetric scanning tunneling microscopy (STM) of the CuO2 plane of
Ca2-xNaxCuO2Cl2, x = 0.125, shows a square domain structure with edge length
four times the compound's lattice constant a0 (Cu-O-Cu distance). The domain
structure is a direct consequence of the 4a0 by 4a0 superlattice formed by
vertical Na+ pairs (oriented parallel to the crystal's c axis) that substitute
Ca2+ ions. The surrounding O2- ions are displaced away from, and the Cu2+ ions
toward the Na+ pairs. Contrary to the fourfold symmetry of the CuO2 plane, the
stable displacement configuration has a twofold symmetry, dominated by large
and, respectively, small displacement of opposite O2- ions being nearest
neighbors to each vertical Na+ pair. The ion displacements give rise to
sufficient squeeze of certain O2- ions that, by the Coulomb-oscillator model of
superconductivity, prevents lateral overswing of their excited 3s electrons.
The axial 3s oscillations are predominantly oriented in the directions of O2-
ion displacements. The observed ladder pattern in the domains provides a direct
imaging of the 3s Coulomb oscillators. The 'sidepieces' of the ladders
correspond to long unidirectional pathways for 3s electrons in the CuO2 plane.
They account for superconductivity. The findings lend support to the validity
of the Coulomb-oscillator model of superconductivity.
