Field-angle dependent vortex lattice phase diagrams in MgB2

The vortex lattice (VL) phase diagram in the superconductor MgB₂ for  H || c consists of three phases labeled F, L and I. All three phases have a triangular symmetry and differ only in their orientation relative to the MgB₂ crystalline axes.  Here, the F phase is oriented along the crystalline a-axis and the I phase is characterized by the VL oriented along the a*-axis within the hexagonal basal plane. In the intermediate L phase, the VL undergoes a continuous rotation between these two high symmetry orientations. Previously, a rotation of the magnetic field about the crystalline a-axis was shown to suppress the L phase at a tricritical point at an angle of 83° [A. Leishman et al., Phys. Rev. B 103, 094516 (2021).]. Here we are reporting on similar measurements, but rotating the field about the a*-axis. This shifts the tricritical point both to a lower critical angle and a lower magnetic field. I will discuss how this can be understood based on the anisotropic vortex-vortex interaction in MgB₂.