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 fl ux lines, as well as of their induced motions, is parallel to the ab p lanes. The anisotropic nature of this peak effect is clearly different from that reported in high-T-c cuprates, the other class of layered p erovskite superconductors. The peak effect in Sr2RuO4 is attributed to the synchronization of the flux lines to the pinning centers as the f lux-line lattice softens with increasing magnetic field and temperatur e.