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We have used a variety of microscopic techniques to reveal the structure and motion of flux line arrangements, when the flux lines in low Tc type II superconductors are caused to move by a transport current. Using small-angle neutron scattering by the flux line lattice (FLL), we are able to demonstrate directly the alignment by motion of the nearest-neighbour FLL direction. This tends to be parallel to the direction of flux line motion, as had been suspected from two-dimensional simulations. We also see the destruction of the ordered FLL by plastic flow and the bending of flux lines. Another technique that our collaboration has employed is the direct measurement of flux line motion, using the ultra-high-resolution spectroscopy of the neutron spin-echo technique to observe the energy change of neutrons diffracted by moving flux lines. The μSR technique gives the distribution of values of magnetic field within the FLL. We have recently shown that one can perform μSR measurements while the FLL is moving. Such measurements give complementary information about the local speed and orientation of the FLL motion. We conclude by discussing the possible application of this technique to thin film superconductors.

Recent experimental results indicate that superconductivity in Sr2RuO4 is described by the p-wave E_u representation of the D_{4h} point group. Results on the vortex lattice structures for this representation are presented. The theoretical results are compared with experiment., Comment: 4 pages, 3 figures, M2S-HTSC-VI proceedings

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.

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.

We have measured the diffracted neutron scattering intensities from the square magnetic flux lattice in the perovskite superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, which is thought to exhibit $p$-wave pairing with a two-component order parameter. The relative intensities of different flux lattice Bragg reflections over a wide range of field and temperature have been shown to be inconsistent with a single component Ginzburg-Landau theory but qualitatively agree with a two-component $p$-wave Ginzburg-Landau theory.

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.

There are several ways of investigating#ux lines with polarised neutrons. We shall describe our recent work on IN15 at ILL, investigating spatially varying "elds perpendicular to the #uxoid axes, which are present with tilted "elds in anisotropic superconductors. These "eld components may be detected by polarisation analysis of neutrons di!racted by the #ux line lattice, and allow a detailed investigation of #ux 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 #ux lines. We have recently demonstrated this in an Nb}Ta alloy on instruments IN11 and IN15, by measuring the energy change of neutrons di!racted by #ux lines, which are moving under the in#uence of a transport current. We shall also comment on others’ work on the search for interference terms between #ux lattice and nuclear di!raction signals, and the use of neutron depolarisation for investigation of #ux distributions in superconductors. ( 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 phenomenon of superconductivity continues to be of considerable scientific and practical interest. Underlying this phenomenon is the formation of electron pairs, which in conventional superconductors do not rotate about their centre of mass (‘s -wave’ pairing; refs 1, 2). This contrasts with the situation in high-temperature superconductors, where the electrons in a pair are believed to have two units of relative angular momentum (‘d -wave’ pairing; ref. 3 and references therein). Here we report small-angle neutron-scattering measurements of magnetic flux lines in the perovskite superconductor Sr2RuO4 (ref. 4), which is a candidate for another unconventional paired electron state—‘p -wave’ pairing, which has one unit of angular momentum5,6,7. We find that the magnetic flux lines form a square lattice over a wide range of fields and temperatures, which is the result predicted by a recent theory8,9 of p -wave superconductivity in Sr2RuO4. This theory also indicates that only a fraction of the electrons are strongly paired and that the orientation of the square flux lattice relative to the crystal lattice will determine which parts of the three-sheet Fermi surface of this material are responsible for superconductivity. Our results suggest that superconductivity resides mainly on the ‘γ’ sheet9.

We report a neutron diffraction study of the induced moments at the metamagnetic transition observed at low temperature in the bilayer perovskite Sr3Ru2O7. This compound is a close relative of the unconventional superconductor Sr2RuO4.

We have used a variety of microscopic techniques to reveal the structure and motion of flux line arrangements, when the flux lines in low T c type II superconductors are caused to move by a transport current. Using small-angle neutron scattering by the flux line lattice (FLL), we are able to demonstrate directly the alignment by motion of the nearest-neighbor FLL direction. This tends to be parallel to the direction of flux line motion, as had been suspected from two-dimensional simulations. We also see the destruction of the ordered FLL by plastic flow and the bending of flux lines. Another technique that our collaboration has employed is the direct measurement of flux line motion, using the ultra-high-resolution spectroscopy of the neutron spin-echo technique to observe the energy change of neutrons diffracted by moving flux lines. The muon spin rotation (μSR) technique gives the distribution of values of magnetic field within the FLL. We have recently succeeded in performing μSR measurements while the FLL is moving. Such measurements give complementary information about the local speed and orientation of the FLL motion. We conclude by discussing the possible application of this technique to thin film superconductors.

A facility for plastic deformation of single-crystal germanium wafers is described. It consists of a commercial tube furnace which radiates heat to evacuated quartz-glass tubes housing the tools for bending and flattening of the wafers. The facility is semi-automatic and requires minimal attention. All movements and temperature changes are done by a robot via a PLC-control system. Two nine-crystal focusing monochromators (54 × 116 and 70 × 116 mm2) made from 100 wafers with average mosaicity ∼13′ have been constructed. Summaries of the test results are presented.

We have used the technique of small-angle neutron scattering to observe magnetic flux lines directly in a ${\mathrm{Y}\mathrm{B}\mathrm{a}}_{2}{\mathrm{C}\mathrm{u}}_{3}{\mathrm{O}}_{7}$ single crystal at fields higher than previously reported. For field directions close to perpendicular to the ${\mathrm{C}\mathrm{u}\mathrm{O}}_{2}$ planes, we find that the flux lattice structure changes smoothly from a distorted triangular coordination to nearly perfectly square as the magnetic induction approaches 11 T. The orientation of the square flux lattice is as expected from recent $d$-wave theories but is $45\ifmmode^\circ\else\textdegree\fi{}$ from that recently observed in ${\mathrm{L}\mathrm{a}}_{1.83}{\mathrm{S}\mathrm{r}}_{0.17}{\mathrm{C}\mathrm{u}\mathrm{O}}_{4+\ensuremath{\delta}}$.

We report muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$ measurements on the moving vortex lattice (VL) in the type-II superconductor Pb-In, backed up by small-angle neutron scattering (SANS) observations on the same sample. We observe a motional narrowing of the $\ensuremath{\mu}\mathrm{SR}$ lineshape $p(B)$ and by SANS, alignment of the VL to the direction of vortex motion. We have calculated the $\ensuremath{\mu}\mathrm{SR}$ lineshape expected with a range of orientations of the moving VL. We demonstrate how the new $\ensuremath{\mu}\mathrm{SR}$ results give information on the moving VL which is complementary and consistent with the SANS data.

We describe how the ferromagnetic spin echo technique may be used to make a direct and microscopic measurement of the motion of flux lines in a superconductor under the influence of an applied current. Our experimental results agree well with theory and indicate that both the value and distribution of velocities of flux lines may be measured.

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 small-angle neutron-scattering (SANS) measurements of flux line properties near ${H}_{c2}$ in an ultrapure sample of niobium with weak pinning of flux in the bulk. These confirm in detail the Abrikosov picture of the flux line lattice to within 20 mK of the upper critical field line. However, it has recently been claimed [X. S. Ling et al., Phys. Rev. Lett. 86, 712 (2001)], on the basis of SANS observations of a disordering of flux lines in niobium, that the flux lattice melts at temperatures clearly separated from the upper critical field line. This discrepancy may possibly arise from differences in sample purity and pinning.

High resolution, neutron small angle scattering experiments have been performed on the flux-line lattice within the mixed state of the type-II superconductors YNi2B2C and TmNi2B2C. In the Y compound clear evidence is found for a transition at low fields from a distorted hexagonal lattice with its diagonal along a [110] direction to a [100] direction as the field is increased. At higher fields the lattice is found to be square. These results are in

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 ${\mathbf{v}}_{L}$ was given as expected by the values of electric and magnetic fields, via $\mathbf{E}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}{\mathbf{v}}_{L}\ensuremath{\wedge}\mathbf{B}$. 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.

We have measured the diffracted neutron scattering intensities from the square magnetic flux lattice in the perovskite superconductor Sr2RuO4, which is thought to exhibit p-wave pairing with a two-component order parameter. The relative intensities of different flux lattice Bragg reflections over a wide range of field and temperature have been shown to be inconsistent with a single component Ginzburg-Landau theory but qualitatively agree with a two-component p-wave Ginzburg-Landau theory.

Nature 396, 242–245 (1998) In this Letter, the X-ray Laue patterns used to give the orientation of our crystal were misinterpreted by 45°. With the magnetic field perpendicular to the RuO2 planes, the nearest-neighbour directions in the square flux-line lattice (FLL) are actually at 45° to the Ru–O–Ru directions in the crystal lattice.