Magnetic vortex lattice in HgBa2CuO4+δ observed by small-angle neutron scattering

We report the direct observation of the magnetic vortex lattice in the model high-temperature superconductor HgBa${}_{2}$CuO${}_{4+\ensuremath{\delta}}$. 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${}_{2.15}$Sr${}_{1.95}$CaCu${}_{2}$O${}_{8+\ensuremath{\delta}}$. 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.