Engineered Tunnel Barriers for a YBCO Single Electron Transistor

Abstract

Despite the great scienti c effort, the microscopic origin of superconductivity in the high-temperature superconductor Y Ba2Cu3O7-8 is still not understood. In recent work, Gustafsson, D. et al. discovered that the superconducting order parameter in a Y Ba2Cu3O7-8 mesoscopic island did not have a purely d-wave symmetry, as it is commonly assumed. By using a Single-Electron Transistor (SET) made of a Y Ba2Cu3O7-8 source, mesoscopic island, drain and gate, they discovered that the Y Ba2Cu3O7-8 island had a fully gapped superconductivity. This indicates the existance of a subdominant imaginary order parameter in Y Ba2Cu3O7-8 that gaps the nodes of the d-wave order parameter. In order to investigate the doping dependence of this subdominant order parameter, one should use a new design for the SET, with normal metal source, drain and gate. To have a working SET, the tunnel resistance between the electrodes and the islands needs to be higher than the quantum resistance. The goal of this work is to engineer the Y Ba2Cu3O7-8-Au tunnel junction with a su - ciently high tunnel resistance. For that, tunnel barriers between Y Ba2Cu3O7-8 and Au were created by an Ar ion milling procedure of the surface using different etching times and di erent voltages. The current-voltage characteristics of the Au-Y Ba2Cu3O7-8 junctions were investigated and the contact resistivity between Y Ba2Cu3O7-8 and Au was measured. The surface etching seems to create low transparency interfaces. In addition, a normal Y Ba2Cu3O7-8 layer is formed at the surface, which can be turned superconducting by ozone treatment. Suitable tunnel barriers for a Y Ba2Cu3O7-8 SET can be created using mild surface etching and ozonation.