Point-contact tunneling spectroscopy in high-temperature superconductors

Tunneling-spectroscopy measurements were made on high-temperature superconductors using the point-contact technique that has been used successfully to reproduce the phonon structure found in thin-film junctions on niobium. For Ba-xKxBiO3, 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 values at high bias, and sharp structures are clearly defined at the sum and difference gaps. These results demonstrates that there is a well-defined energy gap in Ba1-xKxBiO3. Eliashberg functions were obtained for two 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 Tc. Results represent the first demonstration that point-contact tunneling can provide the same degree of sensitivity as thin-film tunneling spectroscopy