i A comparison of barrier type tunnel junction and point contact tunnel junction formed on the same high Tc material

Abstract. By making a combination of both point contact and barrier type tunnel junctions 11 on a single sample of the high Tc superconductor BSCCO (2212) single crystal, we have jy shown that as the tunneling tip is slowly retracted from the surface a point contact junction f gradually evolves from a N-S short to a high resistance tunnel junction. The scaled dynamic jj conductance (dl/dV) of this point contact tunnel junction becomes almost identical to that | of a conventional barrier type tunnel junction and both show a linear d//dF — 7 curve. The |; observation implies that at high resistance a point contact junction behaves in the same way '$; as a barrier type tunnel junction. We suggested that the almost linear tunneling conductance i! obtained in both the cases most likely arises due to an intrinsic characteristic of the surface 1 of the crystal comprising of a mosaic of superconducting regions of the order of a few I' nanometers. We also conclude that the barrierless (N-S) point contact obtained by piercing || the surface oxide layer of the crystal shows Andreev reflection which we suggest as the origin | of the zero bias anomaly often observed in point contact junctions. 1