Nonlinear microwave response of YBaCuO (review article)
The review of modern situation concerning the studies of the generation of microwave harmonics in YBaCuO is given. Far from Tc the nonlinear response of ceramics YBaCuO has Josephson nature in a weak magnetic fields or is described in terms of the critical state model in a strong external magnetic field H. Near Tc, the signals of the second P2ω(T) and the third P3ω(T) harmonics have the form of spikes of generation. Yet the signal P2ω(H) shows a hysteresis and is proportional to the square of incident power P2ω∝P2ω while the signal P3ω∝P3ω and doesn't depend on the field H. Such behavior of the nonlinear response near Tc is typical of both ceramics and single crystals of YBaCuO. We use the Portis model to describe the signal P2ω and the model based on Eliashberg theory to describe the signal P3ω. The question about the model which can simultaneously explain the behavior of the second and the third harmonics remains open.