Pinning anomalies in organic layered superconductor κ-(BEDT-TTF)2X {X=Cu(NCS)2, Cu[N(CN)2]Br}

Vortices in organic superconductor κ-(BEDT-TTF)2X {X=Cu(NCS)2, Cu[N(CN)2]Br} are interesting for their anisotropic character. To investigate their dynamical behavior, ac complex susceptibility and dc electrical resistance measurements were done in the presence of vortices introduced by dc magnetic fields. In the course of experiments, we found that the dc field dependence of complex susceptibility changes drastically for moderate ac magnetic fields, i.e. the shielding effect of the system (-χ') has a peak structure and the dissipation term (χ'') shows a dip. In addition to the anomalies in the susceptibility, it turned out that the electrical resistance has a dip structure under the same dc magnetic fields. These anomalies imply that the vortices are less mobile there