Magnetic field behavior of YBCO step-edge Josephson junctions in rf-washer SQUIDs

The suppression of the critical current in YBCO Josephson junctions by the Earth´s magnetic field strongly affects the operation of SQUIDs outside magnetic shielding. Commonly, one observes a modulation of the SQUID flux-voltage transfer function amplitude, V_sq-pp, with a period of ΔB_0-v leading to an increased white flux noise level or unstable SQUID operation. Here, we report on the investigation of ΔB_0-v of rf-SQUID sensors based on step edge junctions (SEJ) operated in a flip chip configuration with coplanar resonators with integrated flux concentrators. To investigate the origin of the suppression of V_sq-pp, we opened the SQUID loop of some samples and measured the magnetic field dependence of the critical current I_c(B) directly and compared it to V_sq-pp(B). It is shown that a junction width in the submicrometer scale is required for operation of the sensors in the Earth´s magnetic field