Scanning tunneling spectroscopy of a-axis YBa₂Cu₃O_7-δ films: k-selectivity and the shape of the superconductor gap

Tunneling spectroscopy of epitaxial (100) oriented YBa₂Cu₃O_7-δ films was performed using an STM at 4.2 K. On atomically smooth areas, tunneling spectra revealing clear U-shaped gaps with relatively low zero bias conductance were measured. These spectra can be well fitted to the tunneling theory into a d-wave superconductor only when introducinga strong dependence of the tunneling probability on the wave vector k. Possible origins for this k-selectivity in STM measurements will be discussed. On other areas, V-shaped gaps as well as zero bias conductance peaks are observed, indicating relaxation of k-selectivity and the effect of nanofaceting, respectively.