# Non-equilibrium structural phase transitions of the vortex lattice in MgB2

## TL;DR

This study investigates non-equilibrium phase transitions in the vortex lattice of MgB2 superconductor, revealing different transition behaviors and the role of domain nucleation and growth in metastable state relaxation.

## Contribution

It provides new insights into the kinetics and mechanisms of vortex lattice transitions, highlighting the difference between supercooled and superheated states and the activated process involved.

## Key findings

- Supercooled vortex lattice undergoes a discontinuous transition.
- Superheated vortex lattice transitions are continuous.
- Transition kinetics are governed by nucleation and domain growth.

## Abstract

We have studied non-equilibrium 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 an activated process, with an activation barrier that increases as the metastable state is suppressed, as previously reported for the supercooled vortex lattice [E. R. 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 towards 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.