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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