Fig. 2: Superconducting region on Cu–O terminated surface.

a Constant-current STM image of the Cu–O terminated YBCO surface measured at a sample bias of −50 meV and a current of 200 pA. The STM image exhibits a clear layered structure perpendicular to [001] direction, corresponding to tunneling into states associated with the CuO-chain and CuO₂-plane layers. b Fourier transform of the STM image in (a). The peaks labeled Q_layer arise from the imaged atomic planes. c Differential conductance spectra obtained from three consecutive atomic layers corresponding to CuO-chain and CuO₂-plane layers. The blue arrow marks the superconductive (SC) gap as a weak bulge on the flanks of the pseudogap. The inset displays the negative second derivative of the differential conductance spectra, −d³I/dV³, measured from the three consecutive layers, revealing the coherence peaks of the SC gap and providing the signature to identify the SC phase. A representative error bar is given for each spectrum.