HARMONIC EMISSION AT MICROWAVE-FREQUENCIES IN YBA2CU3O7 SINGLE-CRYSTALS NEAR T-C

Nonlinear microwave emission by YBa2Cu3O7 single crystals is investiga ted at temperatures close to T-c. Second- and third-harmonic signals a re studied as a function of the temperature and input power, for diffe rent values of the external magnetic field. The results are discussed in the framework of a phenomenological theory based on the two-fluid m odel which assumes that both the microwave and static fields perturb t he partial concentrations of the normal and condensate fluids. It is s uggested that, at temperatures very close to T-c, this mechanism is th e only one responsible for the harmonic emission by YBa2Cu3O7 single c rystals in the Meissner state. It is also effective in the mixed state where the nonlinearity mechanism suggested by Coffey may play a role. (C) 1998 Elsevier Science B.V. All rights reserved.