S.L. Lee

Unconventional angular dependence of the microscopic magnetic field distribution in single crystal Bi2Sr2CaCu2O8 from muon-spin rotation studies

Abstract The angular dependence of the microscopic magnetic field distribution in single-crystal Bi 2 Sr 2 CaCu 2 O 8 has been measured for a range of applied fields using the muon-spin rotation technique. The results show significant deviations from the predictions of the anisotropic 3D model, which may reflect the extremely anisotropic, quasi-2D nature of the flux structure. The temperature dependence of the field distribution at 400 mT has also been measured for a range of orientations, showing disagreement with the predictions of a simple two-fluid model.

μSR investigation of the superconducting properties of YNi2B2C

Abstract Muon spin rotation (μSR) has been used to investigate the mixed state of the recently discovered superconductor YNi2B2C. The width of the μSR-lineshape at low temperature gives a measure of the superconducting penetration depth λ(T = 0), while the field dependence of the linewidth allows an estimate of the superconducting coherence length ξ(T = 0).

Observation of the Flux-Line Lattice by Neutron Diffraction and Muon-Spin Rotation

Small-angle neutron scattering and muon spin rotation have been used to obtain direct information about flux structures in the mixed state within the bulk of crystals of superconductors, including BSCCO-2212, niobium and YBCO. In highly anisotropic BSCCO, we see evidence of vortex fluctuations, melting, and decomposition of flux lines into ‘pancake’ vortices, which give a more twodimensional vortex structure. Our results from pure niobium show no sign of flux-lattice melting — in disagreement with recent claims. In YBCO the flux lattice is much more stable than in BSCCO, due to a much lower anisotropy, but it shows interesting flux structures as a function of field direction.

Neutron scattering from the flux-line lattice in Bi2Sr2CaCu2O8 + y

Abstract Neutron small-angle diffraction has been used to investigate the flux-line lattice structure within single crystals of the high-temperature superconductor Bi 2.15 Sr 1.95 CaCu 2 O 8 + x . The diffracted intensity goes rapidly to zero as the magnetic field or the temperature is increased. Melting at low fields as a function of temperature coincides with the appearence of finite resistance within the superconducting state. At low temperatures the diffracted intensity disappears in fields greater than ∼ 70 mT, probably due to the decomposition of the flux-line lattice into randomly pinned 2d “pancake” vortices.

Small angle neutron diffraction studies of vortex structures in high temperature superconductors

Abstract We have used neutron scattering to provide direct information about flux structures in the bulk of crystals of the superconductor Bi 2 Sr 2 CaCu 2 O 8 . Its extremely high effective mass anisotropy, makes the flux lattice susceptable to melting and also to decomposition into ‘pancake’ vortices, which would give a more two-dimensional vortex structure. At low temperatures and fields the scattered intensity is consistent with a three dimensional flux-line structure. At higher fields and temperatures, the scattering from the flux lattice dissapears well below T c . We can associate this dissappearance with the above changes in the vortex structure. We compare the neutron scattering results with macroscopic measurements of magnetisation.

Observation of magnetic flux line structures in superconductors by small-angle neutron diffraction

We describe the recent uses of the technique of small-angle neutron diffraction to investigate flux-line structures within the bulk of superconductors in the mixed state. Despite the small signal in superconductors with a long penetration depth, useful results have been obtained in both High-Tc and heavy-fermion superconductors. These can give information about the perfection of the flux lattice, the values of characteristic lengths, the influence of crystal anisotropy and defects on the flux lattice structure and orientation, and on temperature and flux lattice melting effects.