MAGNETIC-FIELD DEPENDENCE OF THE SURFACE IMPEDANCE IN THE MIXED-STATEOF TYPE-II SUPERCONDUCTORS

The surface impedance of type-II superconductors in the mixed state ha s been measured as a function of the magnetic field in the low-frequen cy limit. At the low magnetic fields close to the lower critical field the surface impedance was observed to be proportional to the vortex d ensity, whereas at higher fields a crossover to the square-root depend ence on the vortex density was observed. This behavior agrees with the two-mode electrodynamics in which the penetration of electromagnetic fields into the superconductor in the mixed state is governed by two s pace scales: (i) the skin-layer width, and (ii) the screening length o f the order of the intervortex distance arising from the elasticity of the vortex array.