DC-MAGNETIC-FIELD DEPENDENCE OF THE SURFACE IMPEDANCE OF A MULTILAYERSUPERCONDUCTOR SYSTEM IN THE MIXED-STATE

We study the electrodynamics of a superlattice that consists of two ki nds of layers: the layers of a type-II superconductor and the layers o f a normal metal in a perpendicular magnetic field. The problem is tre ated in the framework of London electrodynamics taking into account si multaneously the nonlocality of the intervortex interaction and the Ab rikosov vortex elasticity. The dependence of the surface impedance Z o n the dc-magnetic-field induction B much less than H(c2) (H(c2) is the upper critical field) is analzyed for different values of the paramet ers of the superlattice. It is shown that, in the low-frequency limit, three types of Z (B) dependence are possible: (i) Z is-proportional-t o B for small fields and Z is-proportional-to B1/2 for large fields, w ith a crossover at B almost-equal-to H(c1) (H(c1) is the lower critica l field); (ii) Z is-proportional-to B1/2 for any fields; or (iii) Z = AB1/2 for small fields and Z = CB1/2 for large fields, the coefficient s A and C being nonequal, with the crossover AB1/2-->CB1/2 at B almost -equal-to H(c1). The type-(i) dependence characteristic of a uniform s uperconductor converts into a type-(ii) dependence as one diminishes t he thickness of the superconductor layers. The physical origin of this conversion is explained. It is suggested that observed dependence in multilayer superconductor systems Z is-proportional-to B1/2 is due to the effect discussed in the paper.