Tunneling and transport properties of superconductors

We have successfully obtained the electron-phonon spectral function, {dollar}alphasp2F(omega),{dollar} from our tunneling data on Ba{dollar}sb{lcub}1-x{rcub}{dollar}K{dollar}sb{lcub}x{rcub}{dollar}BiO{dollar}sb3{dollar} (BKBO) and Nd{dollar}sb{lcub}2-x{rcub}{dollar}Ce{dollar}sb{lcub}x{rcub}{dollar}CuO{dollar}sb{lcub}4-y{rcub}{dollar} (NCCO). Analysis presented in this thesis is novel in its utilization of tunneling {dollar}alphasp2F(omega){dollar} function to estimate the contribution to resistivity from the electron-phonon scattering. We have successfully tested this methodology for conventional superconductors Nb, V, and VN. Applying this analysis to BKBO data, we make a very strong case for the phonon-mediated coupling in BKBO. We also show that the phonon-mediated coupling plays a major role in the superconductivity of NCCO.; Tunneling data on BKBO exhibit a BCS gap and the ratio {dollar}2Delta/ksb{lcub}B{rcub}Tsb{lcub}c{rcub}{dollar} (of about 3.9, corresponds to a moderately coupled superconductor. The tunneling {dollar}alphasp2F(omega){dollar} compares well with the phonon density of states obtained from the inelastic neutron scattering data; and the T{dollar}sb{lcub}c{rcub}{dollar} calculated from the tunneling {dollar}alphasp2F(omega){dollar} matches well with the measured value. We explain the resistivity of BKBO using the tunneling {dollar}alphasp2F(omega){dollar}. The high-T{dollar}sb{lcub}c{rcub}{dollar} (about 27 K), the low {dollar}lambda{dollar} value (about 1), and especially the fact that optical phonons contribute predominantly to the higher T{dollar}sb{lcub}c{rcub}{dollar} observed suggests that a deeper physical understanding of BKBO is relevant to understanding the high-T{dollar}sb{lcub}c{rcub}{dollar} cuprates. Another similarity that BKBO shares with the cuprates is its proximity to a collective-mode insulator state (charge density waves in BKBO, spin density waves in cuprates).; NCCO is a moderate T{dollar}sb{lcub}c{rcub}{dollar} cuprate superconductor that also shares the presence of spin density wave instability with other cuprates. But unlike other cuprates, NCCO is electron-doped. The gap region tunneling data is distinctly non-BCS-like, but nevertheless we can obtain {dollar}alphasp2F(omega),{dollar} which compares well with the phonon density of states and can explain the measured T{dollar}sb{lcub}c{rcub}{dollar}. From our data, we show that NCCO is also a moderately coupled superconductor with stronger coupling to the optical phonons.; Our analysis allows the identification, more convincingly than heretofore, of a {dollar}Tsp2{dollar} contribution to the resistivity of NCCO and furthermore to establishing a semiquantitative connection between variations in the magnitude of this contribution and the plasma frequency {dollar}omegasb{lcub}p{rcub}.{dollar} This leads us to conclude that the electron-electron scattering is important for understanding the physics of NCCO.