Shunted DC SQUIDs with Nb-NbOx-Pb technology

Planar dc SQUIDs have been made and tested. They consist of a 5mm × 5mm square with a 30μm to 100μm hole, the junctions being near the center of the device. The loop inductance is in the range of 100- 200 pH. The technology uses Nb/NbOx/Pb overlapped junctions and Ta shunt resistors. Coupling is discussed ; we show that the input coupling constant k cannot be obtained by direct measurements and an efficiency constant r = I1/nIi, is introduced where n is the number of turns of the input coil, Iithe input current and I1the induced current in the SQUID loop. An experimental efficiency of about 50% has been obtained.