NEUTRON-DIFFRACTION BY THE FLUX-LINE-LATTICE IN YBA2CU3O7-DELTA SINGLE-CRYSTALS

Small angle neutron diffraction was used to image the mixed state in a large twinned superconducting Y-123 single crystal. The integrated in tensity I-10 of the (10)-reflection was evaluated as a function of tem perature and used to derive lambda(T) on an absolute scale, which prov ides us with information on the nature of the pairing mechanism. The r esults are compared with theoretical predictions calculated within the framework of BCS-theory or other approaches such as d-wave pairing.