Systematic Studies of the Square-Hexagonal Flux Line Lattice Transition in $\mathrm{Lu}({\mathrm{Ni}}_{1-\mathit{x}}{\mathrm{Co}}_{\mathit{x}}{)}_2{B}_{2}C$: The Role of Nonlocality

We have studied, using small angle neutron scattering, the flux line lattice square to hexagonal symmetry transition in single crystal $\mathrm{Lu}({\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{B}}_2\mathrm{C}$. Low Co concentrations $(x<\mathrm{}0.1\mathrm{})$, which reduce the mean free path and increase the coherence length, also move the structural transition to higher fields than in the undoped system. These data, quantitatively understood within the framework of a theory that includes nonlocal corrections to the London model due to the Fermi surface anisotropy, can be modeled using a simple ratio of the nonlocality range to the intervortex spacing.