Small-Angle Neutron Scattering Studies of Metastable Vortex Lattice States in MgB2

Superconductivity remains a rich and active field over a century after its discovery. When exposed to a magnetic field, type II superconductors become threaded with a periodic array of vortices. This vortex lattice can reveal properties of the host material as well as display novel physics of its own. In the type II superconductor MgB2, the vortex lattice exhibits several different phases and an unexpectedly robust metastability. Small-angle neutron scattering (SANS) provides an ideal probe to study the vortex lattice and search for the mechanism that stabilizes these metastable states.

To this end, dynamic, structural, and doping studies were performed using SANS. An AC magnetic field can be used to induce vortex motion and drive the transition from the metastable state to the equilibrium state. Reversing the metastable preparation method from supercooling to superheating across the phase boundary changed the nature of the transition pathway from discontinuous to continuous. For the supercooled vortex lattice, the metastable to equilibrium state transition can be analyzed with an activated behavior, and can be described in terms of a single parameter based on the AC amplitude. Measurements of the longitudinal correlation lengths revealed that the vortex lattice remained highly ordered along the direction of the vortices throughout the entire transition. The distribution of vortex lattice domain orientations was examined using spatially-resolved SANS, and an upper limit of 100 μm was found for the domain size. Finally, doping with non-magnetic carbon resulted in substantial changes to the vortex lattice phase diagram, while doping with magnetic manganese yielded a phase diagram and metastability that was qualitatively similar to that of pure MgB2.