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μSR and small-angle neutron scattering (SANS) have been used to probe the vortex arrangement in single crystals of the high-Tc superconductor Bi2.15Sr1.85CaCu2O8+δ which have been irradiated with heavy ions to produce columnar defects. The influence of these pinning sites on the spatial arrangement of the vortices is discussed, and the results are compared with numerical simulations.

Muon-spin rotation $(\ensuremath{\mu}\mathrm{SR})$ and torque magnetometry have been used to probe the anisotropy of an untwinned single crystal of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}.$ The absence of twin planes allows the ratios of all three principal components of the superconducting effective mass tensor to be determined. The values for the in-plane anisotropy are in good agreement with values obtained by other techniques. The out-of-plane anisotropies as measured by $\ensuremath{\mu}\mathrm{SR}$ are also found to be in good agreement with other microscopic measurements, but somewhat lower than those resulting from torque measurements on this and other crystals.

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. ( 2000 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.

A muon-spin-rotation study of the flux-line lattice in is presented. For the field parallel to the crystallographic c-direction, the observed field distribution strongly indicates a square symmetry of the vortex lattice. We determine the value of the coherence length from the upper critical field and the Ginzburg-Landau parameter which is found to be from the field distribution. This gives a value for the penetration depth of . The temperature dependence of the penetration depth is measured.

A neutron small-angle scattering study of the flux-line lattice in heavily twinned YBa2Cu3O72x is presented. It is found that the diffraction signal disappears at temperatures well below Tc , 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. @S0163-1829~98!03022-7#

High resolution, neutron small angle scattering studies have been performed to investigate the low field $(Bl250\mathrm{mT})$ region of the phase diagram for the vortex lattice in the superconducting state of ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$. The data present clear evidence for a vortex lattice reorientation transition from a state with the diagonal of the rhombic unit cell along a $[110]$ direction to a $[100]$ direction. Above this transition the lattice distorts under the influence of the applied field until the apex angle becomes constant at some higher field. For $\mathbf{B}\ensuremath{\parallel}\mathbf{c}$ a square lattice configuration is formed. These experiments confirm qualitatively many of the predictions of a general model based on nonlocal corrections to the London model as applied to this and similar materials.

Using the techniques of muon spin rotation and torque magnetometry, we investigate the crossover field B{sub cr} in Bi{sub 2.15}Sr{sub 1.85}Ca{sub 1}Cu{sub 2}O{sub 8+{delta}} at which the vortex lattice becomes disordered along the field direction. It is found that B{sub cr} scales as the projection of the applied field along the perpendicular to the superconducting planes. This has the implication that a field large enough to give a disordered lattice when applied perpendicular to the planes, can give a well-ordered vortex-line lattice for angles of the field to the c axis greater than a critical value. {copyright} {ital 1998} {ital The American Physical Society}

A detailed study has been made of a transition in the flux lattice of the superconductor ${\mathrm{Bi}}_{2.15}$${\mathrm{Sr}}_{1.85}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ using the technique of muon-spin rotation. The results are in excellent agreement with a recent theoretical analysis on the low-field phase diagram of highly anisotropic superconductors. In particular, the important role of electromagnetic interactions between vortex segments is demonstrated. The form of the phase boundary suggests that the transition may be identified with a simultaneous melting and decoupling of the vortices in adjacent layers.

We review some of the assumptions made in the use of muon spin rotation in superconductors: i.e. that the muons are implanted at random positions in the flux lattice, remain static after implantation and do not appreciably affect the properties of the surrounding superconductor; also that the flux lines are straight and static, and that the observed muon rotation frequency spectrum reflects the microscopic distribution of field values. We shall show how evidence for and against the truth of these assumptions in particular cases may be obtained from the μSR results themselves or by comparison with other measurements, and how this in turn may lead to deeper understanding of flux line structure and motion in superconductors.

SYNOPSIS This paper gives details of the work done during the first phase of a 3-year project, the aims of which are to design, develop and build a controller for optimum load matching a small variable speed wind turbine. The Proven (WT2200) 2.2kW turbine is a three bladed passive pitching and coning turbine which operates downwind with passive yawing and uses a permanent magnet three phase generator. The control of the rotor speed will result in a maximisation of the energy yield and will limit transient loads on the rotor and drive train. To design a controller for the load the characteristics of the turbine generator and drive train must first be determined and a mathematical model derived. It is envisaged that the controller will also allow export of electricity to the local grid through a rectifier/inverter stage. A simple model is presented here and comparisons made with experimental results.

mu SR has been used to probe the structure and stability of the flux vortex array in the organic superconductor kappa-(BEDT-TTF)(2)Cu(SCN)(2), At temperatures below 5 K and fields below 5 mT the internal field distribution is found to closely match that expected for a three dimensional (3D) Abrikosov flux line lattice (FLL). Careful studies in this 3D-FLL regime have enabled an improved measurement of the temperature dependence of the superconducting penetration depth to be made. A linear term is found in the temperature dependence of the penetration depth, suggesting the presence of line nodes in the gap parameter and d-wave pairing.

We have performed muon-spin rotation (mu SR) measurements on the organic superconductor K-(BEDT-TTF)(2)CU(SCN)(2) as a function of the angle between the applied field and the superconducting planes. The measurements are found to have much in common with those on the anisotropic cuprate superconductor Bi2Sr2CaCu2O8 divided by delta. (C) 1998 Elsevier Science B.V. All rights reserved.

We present muon-spin rotation measurements on the cuprate superconductor Bi2Sr2CaCu2O8+δ, in the presence of columnar defects. The results are compared to measurements on the pristine material, and indicate a strong suppression of thermal fluctuations of the vortex positions in the irradiated material.

We have performed detailed SANS investigations on a large, high quality, twinned YBCO single crystal. The crystal was oriented with H 51 degrees to c to reduce the effect of twin plane pinning. The diffracted intensity was found to disappear at a temperature well below Hc2(T). The data is consistent with a Lindemann melting criterion giving a physically reasonable value forcL of 0.15 that is in good agreement with previous work. The disappearance of the diffracted intensity coincides with the irreversibility lines as measured by an inductance technique.

We have performed detailed SANS investigations of the flux lattice in BSCCO. At fields above 600G, the diffracted intensity increases as a function of temperature above 30K. This increase in intensity indicates that the flux lattice becomes more perfect at higher temperatures. We attribute this to a weakening of pinning at higher temperatures, which allows the flux lines to relax into a more ordered configuration. This decrease in pinning strength is due to thermal fluctuations smearing out the pinning wells seen by the flux pancakes. If our interpretation is correct, our work represents the first experimental evidence for this phenomenon.