Nonequilibrium structural phase transitions of the vortex lattice in MgB2

We have studied nonequilibrium phase transitions in the vortex lattice in superconducting MgB2, where metastable states are observed in connection with an intrinsically continuous rotation transition. Using small-angle neutron scattering and a stop-motion technique, we investigated the manner in which the metastable vortex lattice returns to the equilibrium state under the influence of an ac magnetic field. This shows a qualitative difference between the supercooled case which undergoes a discontinuous transition and the superheated case where the transition to the equilibrium state is continuous. In both cases, the transition may be described by an activated process, with an activation barrier that increases as the metastable state is suppressed, as previously reported for the supercooled vortex lattice [Louden et al., Phys. Rev. B 99, 060502(R) (2019).] Separate preparations of superheated metastable vortex lattices with different domain populations showed an identical transition toward the equilibrium state. This provides further evidence that the vortex lattice metastability, and the kinetics associated with the transition to the equilibrium state, is governed by nucleation and growth of domains and the associated domain boundaries.