  Nonlinear microwave response of different ceramic MgB2 samples has been
investigated by the technique of second-harmonic emission. The second-harmonic
signal has been investigated as a function of temperature, DC magnetic field
and input microwave power. The attention has mainly been devoted to the
response at low magnetic fields, where nonlinear processes arising from motion
of Abrikosov fluxons are ineffective. The results show that different
mechanisms are responsible for the nonlinear response in the different ranges
of temperature. At low temperatures, the nonlinear response is due to processes
involving weak links. At temperatures close to Tc, a further contribution to
the harmonic emission is present; it can be ascribed to modulation of the order
parameter by the microwave field and gives rise to a peak in the temperature
dependence of the harmonic signal.
