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

We have measured the surface impedance of thick superconductors in the mixe d 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 invali date the common interpretation of the low-frequency surface inductance in t erms of a single penetration depth. Instead, a two-mode description of vort ex dynamics, assuming free vortex flow in the bulk and surface pinning, acc ounts quantitatively for the spectrum of the complex apparent penetration d epth. (C) 1998 Elsevier Science B.V. All rights reserved.