OBSERVATION BY NEUTRON-DIFFRACTION OF THE MAGNETIC-FLUX LATTICE IN SINGLE-CRYSTAL YBA2CU3O7-DELTA

The high-transition-temperature (high-Tc) oxide superconductors are type 2 superconductors: in an applied magnetic field, magnetic flux can enter the superconductor as quantized flux lines. In conventional type 2 superconductors, these lines form a lattice1, but in the new materials the flux lattice seems to be appreciably influenced by thermal fluctuations and may 'melt' at temperatures well below Tc (refs 2-4). Flux lines in high-Tc materials have been imaged by 'decorating' the ends of the lines with tiny magnetic particles5,6, but such experiments are limited to very low fields and indicate only the behaviour at surfaces. Here we report the observation of magnetic flux lines in the bulk of a crystal of YBa2Cu3O7-δ, using small-angle neutron diffraction. The experimental data at low temperatures confirm the existence of a vortex lattice of singly quantized flux lines. The results at higher temá-peratures show that neutron diffraction can be used to investigate the melting of the flux lattice. © 1990 Nature Publishing Group.