The magnetic phase diagram of slightly overdoped La 2-x Sr x CuO 4 (x=0.17) is characterised by a field-induced hexagonal to square transition of the vortex lattice at low fields (~ 0.4 Tesla) [R. Gilardi et al., Phys. Rev. Lett.88, 217003 (2002)]. Here we report on a small angle neutron scattering study of the vortex lattice at higher fields, that reveals no further change of the coordination of the square vortex lattice up to 10.5 Tesla applied perpendicular to the CuO 2 planes. Moreover, it is found that the diffraction signal disappears at temperatures well below Tc, due to the melting of the vortex lattice.
Magnetic field dependence of the basal-plane superconducting anisotropy in YBa2Cu4O8from small-angle neutron scattering measurements of the vortex lattice