H-T Phase Diagram of Flux Line Lattice Structure in YNi2B2C

The detailed flux line lattice (FLL) structure in YNi2B2C was investigated using small angle neutron scattering and the complete H‐T phase diagram was determined. The FLL in YNi2B2C shows a change of symmetry only in the low magnetic field region between 0.05 to 0.2 T. The observed square lattice is governed by an anisotropic Fermi velocity. Contrary to the theoretical prediction, a square lattice driven by an anisotropic superconducting gap does not appear below 5 T.