Detailed measurements of the response of an rf SQUID in the regime LI<inf>c</inf>< Φ<inf>0</inf>/2π

Abstract

Using a superconducting ring containing a Nb point contact inductively coupled to a resonant circuit fed with an rf current source, (typical rf SQUID configuration) we have measured the tank voltage as a function of frequency in the regime LI_{c} . For LI_{c} \ll \Phi_{0}/2\pi , the curves of tank voltage versus drive current for various values of external flux \Phi_{x} through the ring agree with the calculations of Hansma. For higher critical currents, there is an asymmetry in these curves for frequencies above and below ω 0 , the resonant frequency of the circuit. Calculations using an expanded theory including the effects of a conductance G_{o}(1+\alpha \cos \phi) and of screening flux give good agreement with the data. In principle α can be determined from these measurements, however the \cos \phi term and the screening flux produce qualitatively similar effects. A quantitative determination of α requires a precise knowledge of G o /I c . Using values of G o and I c from independent impedance measurements, the data yield a negative value of α in agreement with another experiment.