Correlation of tunneling spectra with surface nanomorphology and doping in thin YBa2Cu3O7 − δ films

Tunneling spectra measured on thin epitaxial YBa2Cu3O7 − δ films are found to exhibit strong spatial variations, showing U- and V-shaped gaps as well as zero-bias conductance peaks typical of a d-wave superconductor. A full correspondence is found between the tunneling spectra and the surface morphology down to a level of a unit-cell step. Splitting of the zero-bias conductance peak is seen in optimally doped and overdoped films, but not in the underdoped ones, suggesting that there is no transition to a state of broken time-reversal symmetry in the underdoped regime.