YBCO/PBCO/YBCO Edge Junctions and DC-SQUIDs

Abstract

One of the first demonstrated low power applications of high-Tc superconductivity was the DC-SQUID making use of natural grain boundaries as Josephson junctions^1/3. That was (and still is) a difficulty in high current applications turned out to be a benefit in the early stages of the research in high-T_c superconducting electronics: grain boundaries in polycrystalline thin films of YBa₂Cu₃0_7−x (in short YBC0) work as Josephson junctions with superconductor/normal metal/superconductor (SNS) like behavior⁴. Based on these early investigations two main directions are currently followed in the Josephson junction and SQUID research, (i) the preparation of artificial grain boundaries on bicrystals⁵ and on edges in the substrate⁶, and (ii) the development of SNS-like junctions with thin film sandwiches of YBCO/normal metal⁷ or semiconductor/YBCO^8/10. Up to now the best performance at 77 K has been demonstrated for a DC-SQUID with artificial grain boundary junctions on SrTiO₃ bicrystals¹¹. It offers relative low noise and a periodic voltage modulation by an external magnetic field at temperatures up to 87 K. However, bicrystal fabrication, control of the critical current and the normal resistance, and reproducibility pose a serious problem.