Effects of Weak Links in the Nonlinear Microwave Response of MgB2 Superconductor

We report experimental results of second-harmonic (SH) response at microwave frequencyin several ceramic MgB2 samples, prepared by different methods. The SH signal has beeninvestigated as a function of the temperature and DC magnetic field. The investigation hasbeen carried out at low magnetic fields, where nonlinear processes arising from motion ofAbrikosov fluxons are ineffective. We show that the low-temperature SH emission is ascribableto processes involving weak links. Comparison among the peculiarities of the SH signalradiated by the different samples shows that the presence of weak links strongly depends onthe sample preparation method, as well as on the purity and morphology of the componentsused for the preparation.