Pinch Resonances in a Radio Frequency Driven SQUID Ring-Resonator System
  In this paper we present experimental data on the frequency domain response of a SQUID ring (a Josephson weak link enclosed by a thick superconducting ring) coupled to a radio frequency (rf) tank circuit resonator. We show that with the ring weakly hysteretic the resonance lineshape of this coupled system can display opposed fold bifurcations that appear to touch (pinch off). We demonstrate that for appropriate circuit parameters these pinch off lineshapes exist as solutions of the non-linear equations of motion for the system. 
H. Prance, T. D. Clark, R.Whiteman, R.J.Prance, M. J. Everitt, P.   Stiffell and J. F. Ralph