Direct evidence for an intrinsic square vortex lattice in the overdoped high-Tc superconductor La1.83Sr0.17CuO4+δ -: art. no. 217003

We report here the first direct observations of a well ordered vortex lattice in the bulk of a La2-xSr xCuO4+delta single crystal (slightly overdoped, x=0.17 ). Our small angle neutron scattering investigation of the mixed phase reveals a crossover from triangular to square coordination with increasing magnetic field. The existence of an intrinsic square vortex lattice has never been observed in high-temperature superconductors and is indicative of the coupling of the vortex lattice to a source of anisotropy, such as those provided by a d -wave order parameter or the presence of stripes.