Anisotropic flux pinning in a layered superconductor Sr2RuO4

We found peculiar flux-pinning in single crystals of Sr2RuO4, the only known layered-perovskite superconductor withtout copper. Measurements on ac susceptibility under dc magnetic fields indicate that a second dissipation peak prominently appears only when the direction of the flux lines, as well as of their induced motions, is parallel to the ab planes. The anisotropic nature of this peak effect is clearly different from that reported in high-T-c cuprates, the other class of layered perovskite superconductors. The peak effect in Sr2RuO4 is attributed to the synchronization of the flux lines to the pinning centers as the flux-line lattice softens with increasing magnetic field and temperature.