Generic First-Order Orientation Transition of Vortex Lattices in Type II Superconductors

The first-order transition of vortex lattices (VLs) observed in various superconductors with fourfold symmetry is explained microscopically by quasi-classical Eilenberger theory combined with nonlocal London theory. This transition is intrinsic in the generic successive VL phase transition due to either gap anisotropy or Fermi velocity anisotropy. This is also suggested by the electronic states around vortices. Ultimately, the origin of this phenomenon is attributed to somewhat hidden frustrations of a spontaneous symmetry-broken hexagonal VL in the underlying fourfold crystalline symmetry.