Instability of a rectangular vortex lattice in a stack of two long   Josephson junctions
  We investigate the stability of a rectangular Josephson vortex lattice in a stack of two long Josephson junctions. It is shown that in an infinite stack at low velocities the lattice is unstable with respect to triangular vortex lattice formation and at high velocities the lattice may be either stable or unstable due to parametric resonance. This resonance may be "suppressed" in three ways: (i) by increase of damping in the system, (ii) by the increase of the external magnetic field, or (iii) by increase of the velocity of the lattice. We show that the finineness of the stack may provide the stability of the rectangular vortex lattice. 
A. V. Chiguinev and V. V. Kurin (Institute for Physics of   Microstructures, Nizhny Novgorod, Russia)