Detailed measurements of the response of an rf SQUID in the regime LIc< Φ0/2π

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}< \Phi_{0}/2\pi
. 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.