Tunnelling evidence for predominantly electron–phonon coupling in superconducting Ba1−xKxBiO3 and Nd2−xCexCuO4−y

AMONG the superconducting oxides, the cubic, copper-free Ba 1− x K x BiO 3 (BKBO) and the electron-doped compound Nd 2− x Ce x CuO 4− y , (NCCO) stand out as being somewhat different from the rest. Nevertheless, an understanding of the pairing mechanism in BKBO (transition temperature, T c ≈ 30K) and NCCO ( T c ≈ 23 K) may give important insights into the mechanisms of the higher- T c 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 = ±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, α 2 F ( ω ), where F ( ω ) is the phonon density of states at energy hω . For BKBO, α 2 F ( 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 α 2 F ( ω ) are not identical for different junctions leads to some uncertainty, but the calculated values of T c 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/ κ T c , consistent with predominantly phonon-mediated pairing mechanisms in these compounds.