Small angle neutron scattering study of the magnetic flux-line lattice in single crystal 2H-NbSe2

Anisotropy of the superconducting state in Sr2RuO4

Despite intense studies the exact nature of the order parameter in superconducting Sr2RuO4 remains unresolved. We have used small-angle neutron scattering to study the vortex lattice in Sr2RuO4 with the field applied close to the basal plane, taking advantage of the transverse magnetization. We measured the intrinsic superconducting anisotropy between the c axis and the Ru-O basal plane (~60), which greatly exceeds the upper critical field anisotropy (~20). Our result imposes significant constraints on possible models of triplet pairing in Sr2RuO4 and raises questions concerning the direction of the zero spin projection axis.

Effect of fluctuations on vortex lattice structural transitions in superconductors

The rhombic-to-square transition field H(square)(T) for cubic and tetragonal materials in fields along [001] is evaluated using the nonlocal London theory with the account of thermal vortex fluctuations. Unlike extended Ginzburg-Landau models, our approach shows that the line H(square)(T) and the upper critical field H(c2)(T) do not cross due to strong fluctuations near H(c2)(T) which suppress the square anisotropy induced by the nonlocality. In increasing fields, fluctuations cause a reentrance of the rhombic vortex lattice, in agreement with recent neutron scattering data on borocarbides.

Elastic moduli of vortex lattices within nonlocal London model

Vortex lattice (VL) elastic response is analyzed within the nonlocal London model which holds for high-kappa clean superconductors. The squash modulus vanishes at the field H( square) where VL undergoes a square-to-rhombus transition. For H>H( square), where the square VL is stable, the rotation modulus turns zero at H = H(r), indicating VL instability to rotations. The shear modulus depends on the shear direction; the dependence is strong in the vicinity of H( square) where the square VL is soft with respect to the shear along [110]. The H dependences of the moduli are evaluated for LuNi(2)B(2)C.

Surface Pinning as a Determinant of the Bulk Flux-Line Lattice Structure in Copper Oxide Superconductors

Direct knowledge of crystal defects and their perturbation of magnetic flux lines is essential to understanding pinning and to devising approaches that enhance critical currents in superconductors with high critical temperatures (T(c)). Atomic force microscopy was used to simultaneously characterize crystal defects and the magnetic flux-line lattice in single crystals of Bi(2)Sr(2)CaCu(2)O(8). Images show that surface defects, which are present on all real samples, pin the flux-line lattice. Above a critical height, the pinning interaction is sufficiently strong to form grain boundaries in the bulk flux-line lattice. These results elucidate the structure of the defects that pin flux lines and demonstrate that surface pinning, through the formation of grain boundaries, can determine the bulk flux-line lattice structure in high-T(c) materials. The implications of these results to the bulk flux-line lattice structure observed in previous experiments and to enhancing critical currents are discussed.