Structural Stability of the Square Flux Line Lattice inYNI2B2Cand<…

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, $\mathbf{H}\ensuremath{\parallel}\mathbf{c}$, we find a square symmetric lattice which disorders rapidly above $H/{H}_{c2}\ensuremath{\sim}0.2$, well below the ``peak effect'' at $H/{H}_{c2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.9$. The results for $H/{H}_{c2}l0.2$ can be understood within the collective pinning model, and are controlled by the tilt modulus ${c}_{44}$. For $H/{H}_{c2}g0.2$, the disordering appears to be associated with the field dependence of the shear modulus, ${c}_{66}$.