Resistive anomalies in a Distributed Point Contact Proximity Effect junction: A study of the high critical temperature superconductor thallium(2) barium(2) calcium(2) copper(3) oxygen(10)

We have developed a new type of junction, the Distributed Point Contact Proximity Effect (DPCPE) junction, in order to investigate the electronic properties of the high T{dollar}sb{lcub}rm c{rcub}{dollar} superconductors. Proximity effect tunneling has been used in the past to study conventional superconductors with very reactive surfaces. By coating the reactive surface with an inert metal, it is possible to tunnel directly into the metal which becomes superconducting by the proximity effect. In this manner it is possible to obtain information about the density of states in the superconductor. The high T{dollar}sb{lcub}rm c{rcub}{dollar} superconductors, with their short coherence lengths and thick layers of insulating oxides are ideally suited for study by proximity effect tunneling. We have fabricated zero barrier Bi/Ag/{dollar}rm Tlsb2Basb2Casb2Cusb3Osb{lcub}10{rcub}{dollar} (Tl-2223) composite junctions and measured the differential resistance vs. voltage of these junctions between 75K and 105K in one degree increments. We find the differential resistance to be independent of voltage above T{dollar}sb{lcub}rm c{rcub}{dollar}, but highly voltage dependent below. The voltage dependence below T{dollar}sb{lcub}rm c{rcub}{dollar} is attributed to Andreev reflection processes at the normal/superconducting interface and is thus related to the amplitude of the superconducting gap function and the superconducting density of states.; This background structure is modulated by periodic resistive peaks corresponding to geometrical resonances in the silver layer. These resistive peaks are at least fifty times sharper than would be expected from a process involving a Fermi distribution of quasiparticles at these temperatures, thus suggesting that another process is operative. We believe that the bismuth layer of the junction is driven superconducting by the Tl-2223 compound through the silver layer, and that the transport of particles in the silver layer is ballistic. We propose that the resistive peaks result from a destructive interference of the supercurrent flowing through the silver layer in the composite junction. We have observed these peaks at voltages as high as 200mV. The phenomenon is intimately related to the superconductivity of the material and suggests that the superconductivity in the Tl-2223 compound can not be exclusively mediated by phonons.