Tunneling spectroscopy measurements on low-leakage junctions of Nd 1.85Ce0.15CuO4-y and Ba1-xK xBiO3

Tunneling measurements are reported for ND_1.85Ce_0.15CuO_4-y (NCCO) and Ba_1-xK_xBiO₃ (BKBO) using the point-contact technique with an Au or Nb tip. The junctions display very low zero-bias conductance values of <1% for BKBO and ~10% for NCCO and show sharp conductance peaks at eV=±Δ. In the case of BKBO, the normalized conductance can be fit with a BCS (Bardeen-Cooper-Schrieffer) density of states with thermal smearing only. The energy gaps are in good agreement with optical measurements and lead to values of 2Δ/kT_c in the range of 3.5-4.0 for both compounds, implying moderate coupling strength. The tunneling conductance data at high bias voltages display reproducible structures which are characteristic of phonon effects as found in conventional superconductors. The Eliashberg functions were obtained, and the resulting values of λ determined from this analysis are consistent with the measured values of the strong-coupling ratio, 2Δ/kT_c. It is concluded that the pairing mechanism in both materials is principally phonon mediated