First order transition of vortex lattices (VL) observed in various
superconductors with four-fold 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 or Fermi velocity anisotropies. This is also suggested by the
electronic states around vortices. Ultimate origin of this phenomenon is
attributed to some what hidden frustrations of a spontaneous symmetry broken
hexagonal VL on the underlying four-fold crystalline symmetry.Comment: 4 pages, 5 figures, some typos are correcte