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 superconducting YBa2Cu3O7-delta single crystal. The flux line l attice was observed in fields up to 2 T at various temperatures from 2 .4 to 80 K. The integrated intensity I of the (10) reflection was calc ulated as a function of temperature for three different fields (0.8, 1 and 2 T). From these I(T) curves, the magnetic penetration depth lamb da(T) was obtained on an absolute scale. This quantity is of particula r interest, since it provides information on the nature of the pairing mechanism in the superconductor, i.e. the symmetry of the wavefunctio n of the electron pairs. The results are discussed in terms of BCS the ory (s-wave pairing) and of d-wave pairing with and without impurity s cattering.