Abstract
The vortex lattice (VL) in MgB2 is characterized by the presence of long-lived metastable states (MSs), which arise from cooling or heating across the equilibrium phase boundaries. A return to the equilibrium configuration can be achieved by inducing vortex motion. Here we report on small-angle neutron scattering studies of MgB2, focusing on the structural properties of the VL as it is gradually driven from metastable to equilibrium states (ESs) by an AC magnetic field. Measurements were performed using initial MSs obtained either by cooling or heating across the equilibrium phase transition. In all cases, the longitudinal correlation length remains constant and comparable to the sample thickness. Correspondingly, the VL may be considered as a system of straight rods, where the formation and growth of ES domains only occurs in the two-dimensional plane perpendicular to the applied field direction. Spatially resolved raster scans of the sample were performed with apertures as small as 80 μm, corresponding to only 1.2 × 106 vortices for an applied field of 0.5 T. These revealed spatial variations in the metastable and equilibrium VL populations, but individual domains were not directly resolved. A statistical analysis of the data indicates an upper limit on the average domain size of approximately 50 μm.
Highlights
Vortices in type-II superconductors are of great interest, both from a fundamental perspective and as a limiting factor for applications where vortex motion leads to dissipation
As a result of both lattice imperfections and the finite experimental resolution, reflections are broadened in reciprocal space, and scattering will occur for a range of angles around θ = θ0
We have examined the structural properties of the vortex lattice (VL) in MgB2 as it is driven between metastable and equilibrium configurations, using an AC magnetic field to induce vortex motion
Summary
Vortices in type-II superconductors are of great interest, both from a fundamental perspective and as a limiting factor for applications where vortex motion leads to dissipation. Vortex matter presents a simple model system to examine important fundamental problems such as structure formation and transformation at the mesoscopic scale, metastable states (MSs), and non-equilibrium dynamics. The presence of metastable non-equilibrium vortex lattice (VL) phases in superconducting MgB2 is well established [6, 7]. The equilibrium VL phase diagram for this material, shown, displays three triangular configurations, denoted F, L and I, differing only in their orientation relative to the hexagonal crystalline axes [6, 8]. In the intermediate L phase (figure 1(c)), the VL rotates continuously from the a to the a* orientation, giving rise to two degenerate domain orientations. The metastability is not due to pinning, but represents a novel kind of collective vortex behavior most likely due to the presence of VL domain boundaries [7]
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