A phenomenological model is proposed which describes the effect of both dc and rf magnetic fields H on the microwave surface impedance, Zₛ=Rₛ+jXₛ, of a superconductor containing weak links. Two types of the weak links are considered; a weak link between two grains, shunted by another grain, and a nonshunted weak link. In both cases, the dependences of Rₛ and Xₛ on applied H were found to be anomalous. Under certain conditions, the values of Zₛ(H) can fall below the zero-field values. Comparison with experiment is performed, and very good qualitative and (in some cases) quantitative agreement is found.
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Velichko et al. (2001) studied this question.
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