Structural Stability of the Square Flux Line Lattice in ${\mathrm{YNI}}_2{B}_{2}C$ and ${\mathrm{LuNi}}_2{B}_{2}C$ Studied with Small Angle Neutron Scattering

We have studied the flux line lattice in ${\mathrm{YNi}}_2{\mathrm{B}}_2\mathrm{C}$ and ${\mathrm{LuNi}}_2{\mathrm{B}}_2\mathrm{C}$, the nonmagnetic end members of the borocarbide superconductors using small angle neutron scattering and transport. For fields, $\mathbf{H}\parallel \mathbf{c}$, we find a square symmetric lattice which disorders rapidly above $H/H_{c2\mathrm{}}\sim 0.2$, well below the “peak effect” at $H/H_{c2\mathrm{}}=0.9\mathrm{}$. The results for $H/H_{c2\mathrm{}}<0.2\mathrm{}$ can be understood within the collective pinning model, and are controlled by the tilt modulus $c_{44}$. For $H/H_{c2\mathrm{}}>0.2\mathrm{}$, the disordering appears to be associated with the field dependence of the shear modulus, $c_{66}$.