Characterization of YBa₂Cu₃O₇step-edge Josephson junctions

Abstract

The authors have fabricated and characterized thin film YBa2Cu3O7 step-edge microbridges for application in SQUIDS. Epitaxial YBCO films were pulsed-laser-deposited on SrTiO3 and had Tc's of 88-90 K. Sharp steps in SrTiO3 were obtained by photolithographic techniques and Ar ion milling with step heights (h) between 100 and 250 mm. They investigated step-edge junctions (SEJ) with different film thicknesses (d) from 100 to 250 mm but with a constant ratio d/h=1. The width of the microbridges was 2-2.5 mu m. The Tc's of the bridges were 65-85 K, depending on film thickness and the duty cycle during ion milling. The I-V curves were RSJ-like and clear Shapiro steps were observed. The junction normal resistance was independent of temperature. With applied magnetic field the critical current Ic showed a T-dependent modulation, indicating that there was self-shielding and Ic non-uniformity in the junctions. Low-frequency noise measured in RF SQUIDS was relatively low. They showed that it was generated predominantly in the junctions.