Removing electrons from the CuO 2 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: Ca 1.88 Na 0.12 CuO 2 Cl 2 and Bi 2 Sr 2 Dy 0.2 Ca 0.8 Cu 2 O 8+δ . 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 4 a 0 -wide unidirectional electronic domains dispersed throughout ( a 0 : 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.

An Intrinsic Bond-Centered Electronic Glass with Unidirectional Domains in Underdoped Cuprates