RF-studies of vortex dynamics in isotropic type-II superconductors

We have measured the surface impedance of thick superconductors in the mixed state over a broad 2 kHz-20 MHz frequency range. The depinning cross-over is observed, but it is much broader than expected from classical theories of pinning. A striking result is the existence of size effects which invalidate the common interpretation of the low-frequency surface inductance in terms of a single penetration depth. Instead, a two-mode description of vortex dynamics, assuming free vortex flow in the bulk and surface pinning, accounts quantitatively for the spectrum of the complex apparent penetration depth. (C) 1998 Elsevier Science B.V. All rights reserved.