NONLINEAR AC RESPONSE OF YBCO WEAK-LINK BRIDGES AND QUANTUM CIRCUIT-THEORY

We have fabricated microbridge weak links on YBCO films with artificia l single grain boundary. The weak link bridges show typical Shapiro st ep behavior upon application of microwave radiation at 20GHz. However, application of signals at 0.1-4MHz induced nonlinear responses at tem peratures below T-c of the weak link bridges. The ac nonlinearity was independent of external magnetic fields and microwave radiation up to 20GHz. We propose a circuit theory model based upon a quantum macrosco pic Hamiltonian of the Josephson junction to explain the experimental results. The theoretical predictions are in good agreement with the ex perimental results.