Low noise operation of a DC squid in a large β regime and its application to the design of a high TC squid

Noise characteristics of a large-β dc SQUID have been studied experimentally in the case when a SQUID inductance is resistively shunted. Here, ß=2LIo/¢o is a modulation parameter, L is a SQUID inductance, Io is a critical current and ¢o is the flux quantum. Energy resolutions of the SQUIDs, ε, have been measured for various β values up to β=19. It is shown experimentally that energy resolutions of the present large-β SQUIDs are as small as that of the conventional SQUID with β=1. For example, energy resolution ε=25h was obtained even for β=12(L=340pH and Io=37µA), where his the Planck constant. The obtained experimental results agree quantitatively with theoretical ones. It is also shown that the present resistively-shunted-inductance scheme is very useful in developing SQUIDs operating at T=77k with high-Tc superconductors, where the value of β will become necessarily large in order to avoid noise effects. Numerical simulations on the performance of the high-Tc SQUID are presented.