TUNNELING EVIDENCE FOR PREDOMINANTLY ELECTRON PHONON COUPLING IN SUPERCONDUCTING BA1-XKXBIO3 AND ND2-XCEXCUO4-Y

AMONG the superconducting oxides, the cubic, copper-free Ba1-xKxBiO3 (BKBO) and the electron-doped compound Nd2-xCexCuO4-y, (NCCO) stand out as being somewhat different from the rest. Nevertheless, an understanding of the pairing mechanism in BKBO (transition temperature, Tc ≈ 30K) and NCCO (Tc ≈ 23 K) may give important insights into the mechanisms of the higher- Tc superconductors. Here we report tunnelling spectroscopy measurements on BKBO and NCCO, using point-contact junctions that exhibit low leakage currents and sharp conductance peaks at the gap voltages V Combining double low line ±Δ/e. Reasonably symmetric and reproducible structures are observed in the high-bias tunnelling conductances which are characteristic of phonon effects as seen in conventional superconductors. We have inverted the tunnelling data and obtained the Eliashberg functions, α2F(ω), where F(ω) is the phonon density of states at energy hω. For BKBO, α2F(ω) bears a close resemblance to the available phonon density of state determined by inelastic neutron scattering, most importantly consistently reproducing the minima. The fact that the α2F(ω) are not identical for different junctions leads to some uncertainty, but the calculated values of Tc are in good agreement with experiment for both BKBO and NCCO. Also, there is a good match between the calculated values of the total electron-phonon coupling constant λ and the measured 2Δ/κTc, consistent with predominantly phonon-mediated pairing mechanisms in these compounds. © 1990 Nature Publishing Group.