Small-angle neutron scattering study of the flux-line lattice in a single crystal of Bi_2.15Sr_1.95CaCu₂O_8+x (invited)

A flux-line lattice (FLL) was observed in a single crystal of Bi _2.15Sr_1.95CaCu₂O_8+x (BSCCO) using small-angle neutron scattering methods. The sample has a superconducting transition at 85 K. The flux-line lattice is observed to melt, evidenced by the rapid disappearance of diffracted intensity as the temperature is increased above a field-dependent melting temperature. Diffracted intensity due to the vortex lattice also falls off as the applied field is increased. It is believed that this is a manifestation of the transition of the three-dimensional flux lines into two-dimensional pancake vortices. The Bragg intensity of the FLL peak is inversely proportional to the fourth power of the London penetration depth (λ_L). Hence, the temperature (T) dependence of the order parameter can be measured quite accurately from the intensity of the Bragg spots at different temperatures. In BSCCO with an applied field of 50 mT, the measured T dependence appears linear. The low-T behavior is of great interest for an understanding of the underlying mechanism for superconductivity in these materials.