A Small Angle Neutron Scattering Study of the Vortex Matter in La 2-x Sr x CuO 4 (x=0.17)

The magnetic phase diagram of slightly overdoped La2-xSrxCuO4 (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 CuO2 planes. Moreover, it is found that the diffraction signal disappears at temperatures well below Tc, due to the melting of the vortex lattice.