Anisotropic vortex lattice in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

We report on small angle neutron scattering observations of the flux line lattice (FLL) in a single crystal of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. To probe the mass anisotropy ratio, ${\mathit{m}}_3$/${\mathit{m}}_1$, measurements were made as a function of angle, Θ, between the 8 kOe applied field and the crystallographic (001) axis for 0°≤Θ≤80°. With the rotation about an a/b (or y) axis, two symmetry-related distorted hexagonal FLL domains formed. Contrary to theoretical prediction, the lattices formed are consistent with a rotation of the short basis vector, ${\mathbf{a}}_1$, from the x axis by 15°, after the effects of anisotropy are removed. The mass ratio is 20±2, which is slightly lower than published values. The temperature dependence of the intensity is not conventional.