Magnetic vortex lattice in HgBa₂CuO_4+δ observed by small-angle neutron scattering

We report the direct observation of the magnetic vortex lattice in the model high-temperature superconductor HgBa₂CuO_4+δ. Using small-angle neutron scattering on high-quality crystals, we observe two equal domains of undistorted triangular vortex lattices well aligned with the tetragonal crystallographic axes. The signal decreases rapidly with increasing magnetic field and vanishes above 0.4 T, which we attribute to a crossover from a three-dimensional to a two-dimensional vortex system, similar to previous results for the more anisotropic compound Bi_2.15Sr _1.95CaCu₂O_8+δ. Our result indicates that a triangular vortex lattice (with or without distortion) at low magnetic fields is a generic property of cuprates with critical temperatures above 80 K.