High-frequency linear AC response of a pinned vortex lattice

We report an extensive investigation of the complex penetration depth lambd a(AC) for a transverse field in the mixed state for an untwinned YBaCuO cry stal and conventional superconductors (PbIn, Nb, V). A broad frequency rang e (Omega/2 pi = 1 kHz-10 MHz) is used to cover the crossover Ohm(p) rom pin ned to viscous vortex line dynamics. Skin effect conditions are realized by using thick samples d similar to 1 mm allowing a direct comparison with th eory. Bulk pinning theories rely on a local complex resistivity, which enta ils a relatively narrow pinning crossover lambda(AC) proportional to (1-i O hm/Ohm(p))(-1/2) Recent 2-mode-theories, which treat pinning as a boundary condition problem, predict a broad crossover lambda(AC) proportional to (1 + (i Ohm/Ohm(p))(1/2))(-1). Our results are fully consistent with the 2-mod es electrodynamics and show the irrelevance of bulk pinning in homogeneous samples. (C) 2000 Published by Elsevier Science B.V. All rights reserved.