| Tunneling spectroscopy measurements have been made on high temperature superconductors {dollar}rm Basb{lcub}1-x{rcub}Ksb{lcub}x{rcub}BiOsb3{dollar}, {dollar}rm Ndsb{lcub}2-x{rcub}Cesb{lcub}x{rcub}CuOsb{lcub}4-y{rcub}{dollar}, {dollar}rm Bisb{lcub}1.7{rcub}Pbsb{lcub}0.3{rcub}Srsb2CaCusb2Osb{lcub}x{rcub}{dollar} and {dollar}rm Tlsb2Basb2CaCusb2Osb{lcub}x{rcub}{dollar} using the point contact technique that has been used successfully to reproduce the phonon structure found in thin film junctions on niobium. For {dollar}rm Basb{lcub}1-x{rcub}Ksb{lcub}x{rcub}BiOsb3{dollar}, ideal tunneling characteristics have been observed. Using a Au tip, the normalized conductance can be fit perfectly with a thermal smeared BCS density of states. Using a superconducting Nb tip, the zero bias conductance is approximately 0.2% of its value at high bias, and sharp structures are clearly defined at the sum and difference gaps. These results demonstrate that there is a well-defined energy gap in {dollar}rm Basb{lcub}1-x{rcub}Ksb{lcub}x{rcub}BiOsb3{dollar}. The tunneling characteristics of the {dollar}rm Ndsb{lcub}2-x{rcub}Cesb{lcub}x{rcub}CuOsb{lcub}4-y{rcub}{dollar}, {dollar}rm Bisb{lcub}1.7{rcub}Pbsb{lcub}0.3{rcub}Srsb2CaCusb2Osb{lcub}x{rcub}{dollar} and {dollar}rm Tlsb2Basb2CaCusb2Osb{lcub}x{rcub}{dollar} are quite symmetrical, displaying BCS-like gaps and low zero-bias normalized conductance values. The ratio of 2{dollar}Delta{dollar}/{dollar}rm ksb{lcub}B{rcub}Tsb{lcub}c{rcub}{dollar} for {dollar}rm Basb{lcub}1-x{rcub}Ksb{lcub}x{rcub}BiOsb3{dollar} and {dollar}rm Ndsb{lcub}2-x{rcub}Cesb{lcub}x{rcub}CuOsb{lcub}4-y{rcub}{dollar} is approximately 3.9, which indicates moderate coupling strength for both materials. For {dollar}rm Bisb{lcub}1.7{rcub}Pbsb{lcub}0.3{rcub}Srsb2CaCusb2Osb{lcub}x{rcub}{dollar} and {dollar}rm Tlsb2Basb2CaCuOsb{lcub}x{rcub}{dollar}, the ratio is about 6, implying stronger coupling of electrons to the excitation. For {dollar}rm Basb{lcub}1-x{rcub}Ksb{lcub}x{rcub}BiOsb3{dollar} and {dollar}rm Ndsb{lcub}2-x{rcub}Cesb{lcub}x{rcub}CuOsb{lcub}4-y{rcub}{dollar}, the tunneling data at high bias display reasonably symmetric and reproducible phonon structures. The Eliashberg functions, {dollar}alphasp2{dollar}F({dollar}omega{dollar}), have been obtained for both materials by the modified McMillan-Rowell analysis. They bear a striking resemblance to the phonon density of states generated from the neutron scattering. For both materials, there is a good agreement between the calculated and the experimental values of T{dollar}sb{lcub}rm c{rcub}{dollar} and a good match between the calculated values of the total electron-phonon coupling parameter and measured 2{dollar}Delta{dollar}/{dollar}rm ksb{lcub}B{rcub}Tsb{lcub}c{rcub}{dollar}, consistent with predominantly phonon-mediated pairing mechanisms in these compounds. Also, the above results represent the first demonstration that point contact tunneling can provide the same degree of sensitivity as thin film tunneling spectroscopy. It is a useful alternative method for tunneling into materials which are particularly sensitive to surface contaminations. |
