Abstract
Nonlinear microwave emission by YBa 2Cu 3O 7 single crystals is investigated at temperatures close to T c. Second- and third-harmonic signals are studied as a function of the temperature and input power, for different values of the external magnetic field. The results are discussed in the framework of a phenomenological theory based on the two-fluid model which assumes that both the microwave and static fields perturb the partial concentrations of the normal and condensate fluids. It is suggested that, at temperatures very close to T c, this mechanism is the only one responsible for the harmonic emission by YBa 2Cu 3O 7 single crystals in the Meissner state. It is also effective in the mixed state where the nonlinearity mechanism suggested by Coffey may play a role.
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