STEP-EDGE JUNCTION YBA2CU3OX DC SQUID AT 77-K

We fabricated low-noise YBa2Cu3Ox dc SQUIDs using a step edge junction -technology. The epitaxial YBa2Cu3Ox-films were prepared by laser depo sition, covering Ar+ ion milled step edge SrTiO3- or LaAlO3-substrates . For device patterning, a SiO inhibit technique was used. The Josephs on junctions are of grain boundary type, with linewidths of 5 mum. The SQUID hole was varied between 5 mum 10 mum and 50 mum * 50 mum, rep resenting SQUID inductances between 20 PH and 140 pH. The current-volt age characteristic fits approximately the RSJ model. The flux to volta ge modulation was very regular and nonhysteretic over more than 100 fl ux quanta (PHIo); the optimum transfer function dV/dPHI at the appropr iate flux bias is 100 muV/PHIo at 77K for samples with 20 pH. The best value of the energy resolution for samples with 20 pH is epsilon(n,w) (f>30 Hz)=610(-31) J/Hz in the white noise region and dc bias modula tion. The energy sensitivity was measured as a function of the SQUID i nductance. The coloured noise could be reduced by one order of magnitu de applying an ac-bias to the SQUIDs obtaining an energy resolution fo r samples with 20 pH of 310(-30) J/Hz at 1 Hz. Thin-film input coils have been developed, in which a persistent current of about 1 mA could be observed for more than two hours.