Tunneling and transport properties of superconductors

We obtained the electron-phonon spectral function, α2F(ω), from our tunneling data on Ba1-xKxBiO3 (BKBO) and Nd2-xCexCuO4-y (NCCO). The analysis is novel in its utilization of tunneling α2F(ω) function to estimate the contribution to resistivity from the electron-phonon scattering. We tested this methodology for conventional superconductors Nb, V, and VN. We make a strong case for the phonon-mediated coupling in BKBO. We show that phono-mediated coupling plays a major role in the superconductivity of NCCO. Tunneling data on BKBO exhibit a BCS gap and this ratio 2Δ/kB Tc (of about 3.9) corresponds to a moderately coupled superconductor. The tunneling α2F(ω) compares well with the phonon density of states obtained from the inelastic neutron scattering data; and the Tc calculated from the tunneling α2F(ω) matches well with the measured value. We explain the resistivity of BKBO using the tunneling α2F(ω). The high-Tc (about 27 K), the low λ value (about 1), and the fact that optical phonons contribute heavily to the higher Tc suggests that a deeper physical understanding of BKBO is needed to understand the high-Tc cuprates. NCCO is a moderate Tc cuprate superconductor that also shares the presence of spin density wave instability with other cuprates. But NCCO is electron-doped. The gap region tunneling data is distinctly non-BCS-like, but nevertheless we can obtain α2F(ω), which compares well with the phonon density of states and can explain the measured Tc. We show that NCCO is also moderately coupled with stronger coupling to the optical phonons. Our analysis allows the identification of a T2 contribution to the resistivity of NCCO and establishes a connection between variations in the magnitude of this contribution and the plasma frequency ωp