Vortex Lattice Symmetry and Electronic Structure in Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

We report a small angle neutron scattering study of the vortex lattice in Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ in magnetic fields of $0.5\le H\le 5$ T applied along and close to the $c$ axis. Over the entire field range, the vortices form an oblique lattice with two nearly equal lattice constants and an angle of 73° between primitive vectors. Numerical calculations suggest that variations of the superconducting order parameter near the vortex core are important in stabilizing this structure. An analysis that accounts for the fourfold symmetry of the vortex core qualitatively explains both the symmetry and the orientation of the observed vortex lattice. A quantitative explanation of our data will require calculations based on a realistic gap equation.