Neutron Diffraction by the Flux Line Lattice in YBa 2 Cu 3 O 7−δ Single Crystals

Small‐angle neutron diffraction was used to image the mixed state in a large superconducting YBa 2 Cu 3 O 7−δ single crystal. The flux line lattice 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 calculated as a function of temperature for three different fields (0.8, 1 and 2 T). From these I ( T ) curves, the magnetic penetration depth λ ( T ) was obtained on an absolute scale. This quantity is of particular interest, since it provides information on the nature of the pairing mechanism in the superconductor, i.e . the symmetry of the wavefunction of the electron pairs. The results are discussed in terms of BCS theory ( s ‐wave pairing) and of d ‐wave pairing with and without impurity scattering.