Structural transition kinetics and activated behavior in the superconducting vortex lattice

Structural phase transformations are ubiquitous in solids, exhibiting common features independent of the microscopic properties of a particular material. We show that this commonality extends to phase transition kinetics associated with the superconducting vortex lattice. Using small-angle neutron scattering, we investigated the behavior of a metastable vortex lattice (VL) state in MgB₂ as it is driven towards equilibrium by an AC magnetic field. This shows an activated behavior, where the AC field amplitude and cycle count are equivalent to, respectively, an effective “temperature” and “time”. The activation barrier increases as the metastable state is suppressed, corresponding to an aging of the VL. Furthermore, we find a cross-over from a partial to a complete suppression of metastable domains depending on the AC field amplitude, which may empirically be described by a single free parameter. This represents a novel kind of collective vortex behavior, most likely governed by the nucleation and growth of equilibrium VL domains.