Vortex lock-in state in a layered superconductor.

We report clear evidence for a vortex ``lock-in'' state in the organic superconductor (BEDTTTF${)}_{2}$Cu(SCN${)}_{2}$, from ac susceptibility measurements with crossed ac and dc fields. A dc field applied parallel to the conducting planes causes a rapid decrease in the screening of the ac field. Identical behavior is initially observed for dc fields applied at an angle, but the screening recovers to nearly the zero-field value when the perpendicular field component exceeds a threshold. This indicates that vortices are locked between the layers below the threshold, and move freely along the planes. Above threshold, the vortices pierce the planes, and their motion is drastically inhibited.