Magnetic Field Behavior of YBCO step-edge Josephson junctions in rf-washer SQUIDs

he 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 fluxvoltage transfer function amplitude, Vsq.pp, with a period of AB,, leading to an increased white flux noise level or unstable SQUID operation. Here, we report on the investigation of AB,., 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 Vsq-pp, we opened the SQUID loop of some samples and measured the magnetic field dependence of the critical current I,(B) directly and compared it to VSqJB). It is shown that a junction width in the submicrometer scale is required for operation of the sensors in the Earth's magnetic field.