The vortex depinning transition in untwined YBaCuO using complex impedance measurements

Abstract

We present surface impedance measurement of the vortex linear response in a large untwinned YBCO crystal. The depinning spectra obtained over a broad frequency range (100 Hz-30 MHz) are those of a surface pinned vortex lattice with a free flux flow resistivity (two modes response). The critical current in the "Campbell" like regime and the flux flow resistivity in the dissipative regime are extracted. Those two parameters are affected by the first order transition, showing that this transition may be related to the electronic state of vortices.The linear ac response of a pinned vortex lattice has been observed in the 60's in conventional type II superconducting materials [1]. At low frequencies, say f 1 KHz, a small ac field has an apparent penetration characterized by a static regime without any loss (the Campbell regime). It has been recognized as a direct consequence of the vortex pinning, and disagrees with first interpretations based on thermally activated depinning [2]. Note that this linear regime is not the ohmic regime of a vortex lattice free from any pinning (the so called liquid phase). At the same time, the high frequency response (f 1-10 MHz) is that of a medium with a free resistivity (skin effect). Those two regimes are separated by the depinning frequency, and the shape of the whole spectrum allows to discriminate between different types of pinned vortex states. One of the main interest of this method is that vortex states can be studied even in the presence of a large critical current. The principle of the experiment is to measure the ac penetration length, due to the vortex response, in a small coil directly glued around the sample. The detailed experimental procedure can be found in [3]. A typical depinning spectrum is shown in the figure 1. It follows closely, over the whole frequency range, the two modes model of