A triadic Cantor fractal multi-layer is a stack of two different dielectric materials whose thicknesses are determined according to the triadic Cantor fractal scheme. When inserted in a rectangular waveguide, the spectral response of a triadic Cantor multi-layer can be tailored to feature a narrow single transmission peak in the waveguide single-mode frequency range, with a low insertion loss and a high rejection level of forbidden frequencies. The experimental characterization of alumina-polystyrene and plexiglass-polystyrene Cantor multi-layers inserted in a WR90 waveguide (Hewlett-Packard, USA) matches thoroughly with the results of the theoretical modeling and demonstrates that triadic Cantor multi-layers can be used to realize feasible narrow-band microwave filters.

We present a study of the vortex lattice in untwinned YBa2Cu3O7−x crystals, using a combination of muon spin rotation and neutron small angle scattering measurements. Both methods show a very sharp melting temperature consistent with a first order transition. The dependence of the melting temperature on the angle of the field with respect to the crystallographic c-axis is studied. The results are compared to thermal measurements.

We present a study of the vortex lattice in untwinned YBa2Cu3O7−x crystals, using a combination of muon spin rotation and neutron small angle scattering measurements. Both methods show a very sharp melting temperature consistent with a first order transition. The dependence of the melting temperature on the angle of the field with respect to the crystallographic c-axis is studied. The results are compared to thermal measurements.

We investigate the superconducting behaviour of Bi doped Pb. Pure lead shows type-I behaviour entering an intermediate state in a magnetic field. High dopings of Bi (>3%) lead to type-II behaviour showing a mixed state, where the magnetic field penetrates the superconductor in the form of a flux lattice. At intermediate doping, the sample shows both type-I and type-II behaviour depending on the temperature. This arises because the Ginzburg-Landau parameter κ passes through its critical value of 1/ √

Introduction. We observed Io, one of the Galilean satellites of Jupiter and the most volcanically active body in the solar system, on 16 nights in 2003 using two similar techniques for separating individual volcanoes despite the relatively low resolution imaging from NASA’s Infrared Telescope Facility (IRTF). Jupiter occultations occur once every 1.7 days and can be used to separate the flux from individual volcanos, but can only determine locations in one dimension (Spencer et al., 1990). Every six years, including 2003, the Earth passes through the orbital plane of the Galilean satellites allowing mutual satellite occultations to be observed from earth (Goguen et al., 1988). Satellite occultations are superior to Jupiter occultations in two major respects and inferior in one. First, satellites provide occultations by a knife edge rather than by the diffuse Jovian limb, and thus provide higher spatial resolution. Second, Since the satellites are comparable in size to one another, a particular region on Io will be crossed by the occulting satellite’s limb in one direction during ingress and likely a different direction during egress, enabling the computation of two dimensional images of these regions (Spencer et al., 1994). In addition, satellite occultations can occult any part of Io, while Jupiter occultations only occult the Jupiter facing hemisphere. Satellite occultations are inferior in one respect because while one half of all Jupiter occultations occur with Io in eclipse, it is extremely rare to find satellite occultations of an eclipsed Io (none were available in 2003). When Io is in eclipse, all near infrared light received is emitted by Io’s volcanos with no contamination by reflected sunlight. Dates of observations and occulting body are shown in table 1.

We present a study of the vortex lattice in untwinned YBa$_2$Cu$_3$O$_{7-x}$ crystals, using a combination of muon spin rotation and neutron small angle scattering measurements. Both methods show a very sharp melting temperature consistent with a first order transition. The dependence of the melting temperature on the angle of the field with respect to the crystallographic c-axis is studied. The results are compared to thermal measurements.

We investigate the superconducting behaviour of Bi doped Pb. Pure lead shows type-I behaviour entering an intermediate state in a magnetic field. High dopings of Bi (>3 the magnetic field penetrates the superconductor in the form of a flux lattice. At intermediate doping, the sample shows both type-I and type-II behaviour depending on the temperature. This arises because the Ginzburg-Landau parameter κ passes through its critical value of 1/√(2) with temperature.

We report on detailed small-angle neutron scattering measurements with polarization analysis from the flux-line lattice in the anisotropic superconductor YBa2Cu3O7-delta. When the field was applied at an angle to the principal axes we have observed spin-flip neutron diffraction consistent with local transverse field components. From a detailed study of the angle dependence of the magnitude of the spin-flip neutron-scattered intensity we have found that the trans verse-field components are larger than predicted by an anisotropic London model for reasonable mass anisotropy values. The transverse-field components are consistent with a reduced c-axis coherence length at low temperature.

We report on detailed small-angle neutron scattering measurements with polarization analysis from the flux-line lattice in the anisotropic superconductor ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}.$ When the field was applied at an angle to the principal axes we have observed spin-flip neutron diffraction consistent with local transverse field components. From a detailed study of the angle dependence of the magnitude of the spin-flip neutron-scattered intensity we have found that the transverse-field components are larger than predicted by an anisotropic London model for reasonable mass anisotropy values. The transverse-field components are consistent with a reduced c-axis coherence length at low temperature.

We report a detailed neutron diffraction study of both pinned and moving magnetic Flux Line Lattice (FLL) in NbTa and PbIn samples. In NbTa, the FLL presents a hexagonal lattice even in the absence of current, meanwhile PbIn presents a strongly dislocated phase. In PbIn, dislocations are eliminated by the application of transport current in agreement with theoretical predictions. On the other hand, the absence of curvature of the flux lines for subcritical currents confirm the presence of surface pinning of the FLL.

We have measured an angle-dependent contribution to the equilibrium magnetization of a YBa_2Cu_3O_{7-d} single crystal with columnar defects created by 5.8GeV Pb ions. This contribution manifests itself as a jump in the equilibrium torque signal, when the magnetic field crosses that of the defects. The magnitude of the signal, which is observed in a narrow temperature interval less than 2K and for fields up to about twice the irradiation field, is used to estimate the the energy gained by vortex pinning on the defects. The vanishing of the effective pinning energy at a temperature below Tc is attributed to its renormalization by thermal fluctuations.

Small-angle neutron scattering (SANS) is an extremely powerful probe of the vortex state in type II superconductors. The technique may be further enhanced by the use of polarised neutrons and the application of the neutron spin-echo method. We discuss some recent applications of these techniques to the study of both conventional and unconventional superconducting materials, and describe the unique information which SANS can provide on the vortex state.

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.

There are several ways of investigating flux lines with polarised neutrons. We shall describe our recent work on IN 15 at ILL, investigating spatially varying fields perpendicular to the fluxoid axes, which are present with tilted fields in anisotropic superconductors. These field components may be detected by polarisation analysis of neutrons diffracted by the flux line lattice, and allow a detailed investigation of flux structure in such materials as YBCO and NbSe(2). A further application of polarised neutrons is the use of neutron spin-echo techniques to measure the speed of moving flux lines. We have recently demonstrated this in an Nb-Ta alloy on instruments IN11 and IN15, by measuring the energy change of neutrons diffracted by flux lines, which are moving under the influence of a transport current. We shall also comment on others' work on the search for interference terms between flux lattice and nuclear diffraction signals, and the use of neutron depolarisation for investigation of flux distributions in superconductors. (C) 1999 Elsevier Science B.V. All rights reserved.

A small angle neutron scattering study of the flux-line lattice in a large untwinned single crystal of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ is presented. In fields parallel to the $c$ axis, diffraction spots are observed corresponding to four orientations of a hexagonal lattice, distorted by the $a\ensuremath{-}b$ anisotropy. A value for the anisotropy, the penetration depth ratio, of ${\ensuremath{\lambda}}_{a}/{\ensuremath{\lambda}}_{b}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.18\left(2\right)$ 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\ifmmode^\circ\else\textdegree\fi{} to $c$, a field dependent reorientation of the lattice is observed around 3 T.

The relationship between carrier concentration, structure and composition in Bi2Sr2CaCu2O8+δ is amongst the most complex of the high-Tc superconducting cuprates. In this paper a number of structural subtleties of the system are discussed based on the results of high-resolution X-ray scattering measurements from single crystals. It is found that the annealing of an oxygen-rich crystal significantly changes the character of certain diffuse scattering features. Accompanying analysis of composition and measurements of Tc suggest these changes may be the signature of oxygen ordering which is influential to the superconductivity.

A neutron small-angle scattering study of the flux-line lattice in heavily twinned YBa2Cu3O7-x 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.

The properties of the incommensurate modulation of Bi-2212 have been investigated using X-ray scattering to observe in situ the response of the structure to high temperature (up to 450 degrees C). The results demonstrate the temperature independent nature of the incommensurability, and several pinning mechanisms which would offer explanations for this are discussed. The strong influence exerted by oxygen diffusion over the structure is also highlighted, and the results clearly indicate the existence of two distinct oxygen diffusion processes. However, it is also found that the wavevector of the modulation is in no way a continuous function of oxygen content; a result contrary to the widespread assumptions in the literature. Only after prolonged annealing was evidence for a structural transition observed, involving the appearance of reflections with an apparently new wavevector of q approximate to 0.14b* + c*.

The atomic struture of the Bi2Sr2Can−1CunOy high-Tc superconductors is distinguished by incommensurate modulations, but their implications for superconductivity are as yet uncertain. Recent studies have suggested a strong invlovement due to the modification of the Cu site environment, and a strong electron lattice interaction would be expected to produce distinct structural changes in the vicinity of Tc. This paper presents the first detailed study of the temperature response of the incommensurate modulation in a high quality single crystal of Bi2Sr2CaCu2Oy. The x-ray scattering measurements show no evidence of any changes in either the period or the amplitude of the modulation over the temperature range studied (T=200−18K). The results limit any possible structural involvement to distortions with a local ordering of less than 100Åin size. A very small but reproducible decrease in the intensity of Bragg and satellite reflections was noted at ∼140-120K which is suggestive of a minor change in electron distribution.