Thin-film YBa2Cu3O72x direct current SQUIDs with Josephson junctions made by direct electron beam writing

We have investigated dc superconducting quantum interference devices ~SQUIDs! with loop areas 8 and 64 mm made by direct electron beam writing in YBa2Cu3O72x films with thickness 25 and 50 nm. The SQUIDs have maximum peak-to-peak voltage modulation of about 20 mV, which corresponds to the transfer factor dV/dF0;50–60 mV/F0 at 30–40 K. By measuring the mutual inductance of the SQUID signal line as a function of temperature and comparing the data with the numerical calculations and the two-fluid model, we extracted the values of the London penetration depth for YBa2Cu3O72x thin films. © 1996 American Institute of Physics. @S0003-6951~96!03443-2#