Nonlinear AC response of YBCO weak link bridges and quantum circuit theory

Microbridge weak links fabricated on YBCO films with artificial single grain boundary are discussed. The weak link bridges show typical Shapiro step behaviour upon application of microwave radiation at 20 GHz. However, application of signals at 0.1-4 MHz induced nonlinear responses at temperatures below T/sub c/ of the weak link bridges. The AC nonlinearity was independent of external magnetic fields and microwave radiation up to 20 GHz. The authors propose a circuit theory model based upon a quantum macroscopic Hamiltonian of the Josephson junction to explain the experimental results. The theoretical predictions are in good agreement with the experimental results. >