Field-Angle Dependent Vortex Lattice Phase Diagram in MgB₂

Vortex lattices (VLs) in type-II superconductors can provide unique insights into the interaction of the superconducting order parameter with the underlying crystal lattice. The structure of the VL can be particularly interesting when the applied field is perpendicular to a hexagonal crystal plane, where the six-fold rotation symmetry leads to a near isotropic energy landscape, allowing higher order terms to couple to the VL.
Such is the case for MgB₂, where a twelve-fold anisotropy causes the VL to fracture into degenerate, counter-rotated domains in the so called L phase. These domains alter the VL kinetics, which inhibits structural phase transitions and leads to robust metastable states. To explore the coupling of this anisotropy with the VL, we have performed small-angle neutron scattering (SANS) as we rotate the applied field away from the c axis. This rotation suppresses the twelve-fold anisotropy and the L phase correspondingly shrinks, eventually disappearing entirely above a critical angle.