Tunnelling evidence for predominantly electron–phonon coupling in superconducting Ba_1−xK_xBiO₃ and Nd_2−xCe_xCuO_4−y

Abstract

AMONG the superconducting oxides, the cubic, copper-free Ba_1−xK_xBiO₃ (BKBO) and the electron-doped compound Nd_2−xCe_xCuO_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, 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 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/κT_c, consistent with predominantly phonon-mediated pairing mechanisms in these compounds.