Peculiarities of rf SQUID response in finite magnetic fields

Abstract

Single-layer washer-type high- T c YBa 2Cu 3O 7− x rf SQUIDs with grain-boundary Josephson junctions, as well as low- T c Nb rf SQUIDs with Nb–Al 2O 3–Nb tunnel junctions, have been investigated in finite magnetic fields. It was shown experimentally that the suppression of the critical current of the Josephson junction due to the magnetic field leads to a modulation of the amplitude of the SQUID output signal. The role of the “unwanted” junction in high- T c rf SQUIDs, which is formed by the grain boundary running through the washer of the SQUIDs on bicrystal substrates, has also been clarified. The drop of the SQUID signal at a finite magnetic field is originated by the penetration of the magnetic field into the unwanted junction. Based on these results, a direct radio-frequency method for the determination of the first critical field H c1 for long Josephson junctions has been developed.