P. G. Kealey

Observation of a square flux-line lattice in the unconventional superconductor Sr2RuO4

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 momentum. 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 theory, 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’ sheet.

Reconstruction from Small-Angle Neutron Scattering Measurements of the Real Space Magnetic Field Distribution in the Mixed State of Sr2RuO4

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.

correction: Observation of a square flux-line lattice in the unconventionalsuperconductor Sr 2 RuO 4

This corrects the article DOI: 10.1038/24335

FLUX-LINE LATTICE STRUCTURES IN UNTWINNED YBA2CU3O7-DELTA

A small angle neutron scattering study of the flux-line lattice in a large single crystal of untwinned YBa2Cu3O 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-b anisotropy. A value for the anisotropy, the penetration depth ratio, of 1.18(2) 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 degrees to c a field dependent re-orientation of the lattice is observed around 3T.

Small-angle scattering from the vortex lattice in high-Tc and other superconductors

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 facility for plastic deformation of germanium single-crystal wafers

Abstract 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.

Small angle neutron scattering and vortex lattice dynamical phase diagram

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.

Vortex lattice structures and pairing symmetry in Sr2RuO4

Abstract Recent experimental results indicate that superconductivity in Sr2RuO4 is described by the p-wave Eu representation of the D4h point group. Results on the vortex lattice structures for this representation are presented. The theoretical results are compared with experiment.

Vortex motion in type II superconductors probed by muon spin rotation and SANS

Abstract 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.

Use of the Neutron Spin Echo Technique to Observe Flux Line Motion

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.