Structural stability of the square flux line lattice in YNI2B2C and LuNi2B2C studied with small angle neutron scattering

We have studied the flux line lattice in YNi2B2C and LuNi2B2C, the nonmagnetic end members of the borocarbide superconductors using small angle neutron scattering and transport. For fields, H parallel to c, we find a square symmetric lattice which disorders rapidly above H/H-c2 similar to 0.2, well below the ''peak effect'' at H/H-c2 = 0.9. The results for H/H-c2 < 0.2 can be understood within the collective pinning model, and are controlled by the tilt modulus c(44). For H/H-c2 > 0.2, the disordering appears to be associated with the field dependence of the shear modulus, C-66.