Non-locality and the flux line lattice square to hexagonal symmetry transition in the borocarbide superconductors

Abstract

Using small angle neutron scattering we have studied the square to hexagonal flux line lattice symmetry transition in different members of the borocarbide superconductors. The studies were performed using samples of ErNi 2B 2C, Lu(Ni 1− x Co x ) 2B 2C with cobalt doping levels x=1.5–9% and Y 0.64Lu 0.36Ni 2B 2C. We find that the onset field of the symmetry transition can be shifted more than an order of magnitude due to changes in the range of the non-local electrodynamics. Comparing the results to transport measurements of the electronic mean free path and the superconducting coherence length we find that the transition onset follows a model by V. Kogan et al., which includes non-local corrections to the London model due to the Fermi surface anisotropy of the borocarbides.