  The vortex lattice (VL) in the high-kappa superconductor YBa2Cu3O7, at 2 K
and with the magnetic field parallel to the crystal c-axis, undergoes a
sequence of transitions between different structures as a function of applied
magnetic field. However, from structural studies alone, it is not possible to
determine precisely the system anisotropy that governs the transitions between
different structures. To address this question, here we report new small-angle
neutron scattering measurements of both the VL structure at higher
temperatures, and the field- and temperature-dependence of the VL form factor.
Our measurements demonstrate how the influence of anisotropy on the VL, which
in theory can be parameterized as nonlocal corrections, becomes progressively
important with increasing magnetic field, and suppressed by increasing the
temperature towards Tc. The data indicate that nonlocality due to different
anisotropies play important roles in determining the VL properties.
