  Removing electrons from the CuO2 plane of cuprates alters the electronic
correlations sufficiently to produce high-temperature superconductivity.
Associated with these changes are spectral weight transfers from the high
energy states of the insulator to low energies. In theory, these should be
detectable as an imbalance between the tunneling rate for electron injection
and extraction - a tunneling asymmetry. We introduce atomic-resolution
tunneling-asymmetry imaging, finding virtually identical phenomena in two
lightly hole-doped cuprates: Ca1.88Na0.12CuO2Cl2 and Bi2Sr2Dy0.2Ca0.8Cu2O8+d.
Intense spatial variations in tunneling asymmetry occur primarily at the planar
oxygen sites; their spatial arrangement forms a Cu-O-Cu bond centered
electronic pattern without long range order but with 4a0-wide unidirectional
electronic domains dispersed throughout (a0: the Cu-O-Cu distance). The
emerging picture is then of a partial hole-localization within an intrinsic
electronic glass evolving, at higher hole-densities, into complete
delocalization and highest temperature superconductivity.
