Field angle dependent vortex lattice phase diagram in MgB2

Using small-angle neutron scattering we have studied the superconducting vortex lattice (VL) phase diagram in MgB2 as the applied magnetic field is rotated away from the c axis and towards the basal plane. The field rotation gradually suppresses the intermediate VL phase which exists between end states aligned with two high-symmetry directions in the hexagonal basal plane for H parallel to c. Above a critical angle, the intermediate state disappears, and the previously continuous transition becomes discontinuous. The evolution towards the discontinuous transition can be parametrized by a vanishing twelvefold anisotropy term in the VL free energy.