VORTEX PINNING AND LOCK-IN EFFECT IN A LAYERED SUPERCONDUCTOR WITH LARGE INPLANE ANISOTROPY

We use ac susceptibility to study the vortex pinning force anisotropy and the magnetic lock-in effect in the organic superconductor (TMTSF)( 2)ClO4, which is believed to have an in-plane anisotropy of gamma(ba) similar to 10 and a maximum out-of-plane anisotropy gamma(ca) similar to 100. Our measurements show only weak effects of the in-plane anisot ropy. The pinning force for Josephson vortices (parallel to the conduc ting planes) is nearly independent of their orientation, except for a small but narrow peak (full width at half maximum similar or equal to 6 degrees) when the vortices are parallel to the TMTSF stacks (a axis) . The pinning force initially decreases as the vortices unlock from th e layers, contrary to the behavior previously observed in the organic superconductor (BEDT-TTF)(2)Cu(SCN)(2). The lock-in threshold field is only weakly dependent on the initial angle of the Josephson vortices in the ab plane.