Nature of 45 degree vortex lattice reorientation in tetragonal   superconductors
  The transformation of the vortex lattice in a tetragonal superconductor which consists of its 45 degree reorientation relative to the crystal axes is studied using the nonlocal London model. It is shown that the reorientation occurs as two successive second order (continuous) phase transitions. The transition magnetic fields are calculated for a range of parameters relevant for borocarbide superconductors in which the reorientation has been observed. 
A. Knigavko (National Chiao Tung Univ. - NCTU), V.G. Kogan (Iowa State   Univ.), B. Rosenstein(NCTU), and T.-J. Yang(NCTU)