Second ac screening step as a probe for the first-order melting transition in layered vortex matter at intermediate temperatures

We present a new probe for the first-order transition for layered vortex matter: A second step in the screening of an ac field that is independent of the frequency and amplitude of the excitation. This second step is observed in the intermediate temperature and field ranges where detecting the jump in induction associated with the transition is rather elusive with standard magnetometry techniques. We observe this second step following a novel experimental protocol where the screening of a locally-generated ac field is remotely detected in another region of the sample. The coincidence of the typical temperature of the second step in direct and remote measurements strongly supports this feature is a probe of the first-order transition. This nonlocal effect detected at distances of thousands of vortex lattice spacings away indicates that a very efficient mechanism propagates the change in rigidity of the structure from the more (liquid) to the less (solid) symmetric vortex phases.