Influence of quenched disorder on the square-to-rhombohedral structural transformation of the vortex lattice of type-II superconductors

A theory of structural transformations of the vortex lattice in a disordered fourfold symmetric type-II superconductor is constructed using two complementary descriptions of the vortex matter. At low temperatures we use a nonlocal London approach, while near Hc2 (T) the lowest Landau level Ginzburg-Landau model with an asymmetric high derivative term is utilized. The quenched disorder influences the location of the square-to-rhombohedral structural transition line, the simplest transformation of that kind. In the clean case the slope of the line in the T-B plane is generally negative, as thermal fluctuations favor a more symmetric square lattice. We calculated the transition line in the presence of pinning and find that the disorder, which plays an important role in low- Tc materials, makes the slope positive, as has been observed in recent experiments.