Effect of fluctuations on vortex lattice structural transitions in superconductors

The rhombic-to-square transition field H-square(T) for cubic and tetragonal materials in fields along [001] is evaluated using the nonlocal London theory with the account of thermal vortex fluctuations. Unlike extended Ginzburg-Landau models, our approach shows that the line H-square(T) and the upper critical field H-c2(T) do not cross due to strong fluctuations near H-c2(T) which suppress the square anisotropy induced by the nonlocality. In increasing fields, fluctuations cause a reentrance of the rhombic vortex lattice, in agreement with recent neutron scattering data on borocarbides.