Field-Angle Dependent First-Order Phase Transition in the MgB₂ Vortex Lattice

Using small angle neutron scattering, we studied the vortex lattice (VL) in MgB₂ as the applied magnetic field is rotated about the crystalline a*-axis. For H||c, the VL phase diagram in this material is comprised of three phases characterized by the orientation of the hexagonal VL with two crystalline axes. In the F and I phases, the VL is oriented along the a- and a*-axes respectively. In the intermediate L phase, the VL rotates continuously between these two high-symmetry directions; this rotation is driven by the competition between the π- and σ-band anisotropies that are sensitive to the applied field and temperature. As the field is rotated towards the basal plane and the π-band becomes more dominant, the intermediate L phase is gradually suppressed and eventually vanishes at a critical field-angle. Furthermore, a coexistence of the L and I phases is observed for the first time, indicating the phase transition becomes discontinuous. Our results demonstrate how the competition between anisotropies in the screening current plane can be altered by changing the angle of the applied field.