VOLTAGE VERSUS FLUX RELATION OF DC SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE USING 3 JOSEPHSON-JUNCTIONS

It has been shown that the voltage versus flux (V-PHI) relation of dc superconducting quantum interference devices (SQUIDs) operating at T=77 K is degraded considerably by thermal noise. Since degradation of modulation voltage in the V-PHI relation becomes more significant when inductance increases, a very restricted range of inductance exists which can be used at T=77 K. In order to reduce the degradation due to thermal noise and to enable the use of large inductances, a new SQUID using three Josephson junctions (3J-SQUID) is proposed. Numerical simulation shows that inductance of 3J-SQUID can be increased to twice as large as that of the conventional case without degrading modulation voltage. It is also shown that modulation voltage is highly improved by damping resistance parallel to inductance. As a result, we can expect to use inductances as large as L<500 pH if 3J-SQUID with damping resistance is employed. Large inductances available in 3J-SQUID will be useful iri realizing efficient coupling of external magnetic field to the SQUID inductance.