A small angle neutron scattering study of the flux-line lattice in a large single crystal of untwinned YBa2Cu3O 7−δ is presented. In fields parallel to the c-axis, diffraction spots are observed corresponding to four orientations of a hexagonal lattice, distorted by the a-b anisotropy. A value for the anisotropy, the penetration depth ratio, of λa/λ b =1.18(2) was obtained. The high quality of the data is such that second order diffraction is observed, indicating a well ordered FLL. With the field at 33 • to c a field dependent re-orientation of the lattice is observed around 3T.PACS numbers: 74.60. Ge, 74.72.Bk, 61.12.Ex The remarkable properties of the mixed state in the cuprate high-T c superconductors are of great current interest. In particular, the expectation that high-T c superconductors have an unconventional pairing symmetry has led inevitably to the question: how does the structure of the flux-line lattice (FLL) differ between conventional and unconventional superconductors? The question has been taken up by several recent theoretical contributions [1][2][3][4] which predict a variety of interesting FLL effects all deviating from the benchmark triangular Abrikosov lattice. However, such discussions may presuppose that the crystallographic properties of the FLL are already well understood in the simplest low-field regime where unconventional effects are least prevalent. This has been far from the experimental truth. Observations require a probe sensitive to the microscopic arrangement of fluxlines. Direct imaging [5] and decoration techniques [6] all have inherent drawbacks, and muon spin rotation has not yet achieved the sophistication to resolve the most subtle effects [7]. By comparison, small angle neutron scattering (SANS) provides unrivalled insights into the crystallography of the FLL, and is the only technique capable of unequivocably resolving such questions.The demanding nature of neutron experiments requires large single crystals (masses > ∼ 200mg), and because of this YBa 2 Cu 3 O 7−δ (YBCO) has been the cuprate of choice for SANS experiments [8]. However, the materials properties of YBCO are complicated. The presence of Cu-O chains which are aligned with the crystallographic b direction render the otherwise tetragonal structure orthorhombic. Upon cooling from the growth, twin boundaries form along {110} directions separating domains of interchanged a and b axes. A strong interaction between flux-lines and these twin planes significantly influences FLL properties. There is a further effect of the chains. Although the orthorhombic distortion is only slight (≈ 1%), the electronic structure is markedly affected, and the consequence is anisotropy within the abplane of both superconducting [9,10] and normal state [11] properties. All previous SANS studies have been on heavily twinned crystals [12,13], and although observations of a pattern with four-fold symmetry were claimed to be due to unconventional d x 2 −y 2 pairing [12], it could not be discounted that alignment by twin planes, in comb...

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We have used the neutron spin-echo technique to measure the small energy change of neutrons which are diffracted by a moving vortex lattice in a low-pinning Nb-Ta superconducting sample. A transport current was passed in the mixed state to cause flux line movement. In the case of uniform motion, the flux velocity v(L) was given as expected by the values of electric and magnetic fields, via E = -v(L)wedgeB. We show that with a nonuniformly moving vortex lattice, one can measure the dispersion of the velocities, opening up new possibilities for investigating moving vortex lines.

A neutron small-angle scattering study of the flux-line lattice in heavily twinned YBa 2 Cu 3 O 7Ϫx is presented. It is found that the diffraction signal disappears at temperatures well below T c , associated with a melting of the flux lattice. The shape of the melting line is consistent with both a Lindemann criterion and the scaling expected for a vortex-glass transition with the superconducting parameters from the three-dimensional XY model. The influence of twin planes on the structure of the vortex lattice and its melting is studied by applying the field at different angles to the c axis. The results are compared with recent specific heat measurements on similar crystals.

We have measured an angle-dependent contribution to the equilibrium magnetization of a YBa 2 Cu 3 O 7Ϫ␦ single crystal with columnar defects created by irradiation with 5.8-GeV Pb ions. This contribution manifests itself as a jump in the equilibrium torque signal, when the magnetic-field direction crosses that of the defects. The magnitude of the jump, which is observed in a narrow temperature interval of less than 2 K wide, for fields up to about twice the dose equivalent field B , is used to estimate the energy gained by vortex pinning on the defects. The vanishing of the effective pinning energy at a temperature below T c is attributed to its renormalization by thermal fluctuations.

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