When a type-II superconductor is subjected to a magnetic field it is threaded by vortices, each carrying one quantum of magnetic flux. In materials with low vortex pinning to defects, the vortices will arrange themselves into a regular array known as the vortex lattice (VL) due to their mutual repulsion. The VL depends sensitively on the anisotropy of the screening current plane, and in many cases undergoes a structural phase transition as the magnetic field and/or temperature is varied.
I will discuss our recent studies of the VL in MgB2, where one observes an unprecedented degree of metastability in connection with a second order (continuous) VL rotation transition. This phenomenon represents a novel kind of collective vortex behavior, which cannot be understood from the single VL domain free energy or from vortex pinning. Rather, we speculate that it is due to VL domain jamming, reminiscent of behavior observed for colloids or granular materials.
To better understand the metastable VL phases in MgB2 we have studied their kinematic as well as their structural properties using small-angle neutron scattering (SANS). Using a stop-motion technique we imaged the VL as it was driven from the metastable phase to the ground state by a controlled number of small-amplitude ac magnetic field cycles either parallel or perpendicular to the dc field.
Our results show a dichotomy in the behavior for the metastable configurations induced by crossing the equilibrium, second order phase transition in different directions. For a metastable state induced by super heating, the VL returns to the ground state through a continuous domain rotation. In contrast, in the super cooled case, the transition to the ground state takes on a first order nature with VL ground state domains that nucleate and grow at their final orientation. In the latter case the metastable VL volume fraction may be determined, and is found to follow a power law with an exponent that increases with increasing AC field amplitude. Both metastable and ground state configurations show correlations along the field (vortex) direction that are comparable to the sample thickness. Finally, spatially resolved measurements (scanning-SANS) show a spatial variation in the VL domain population on length scales of the order 100 mm.
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