S. H. Lloyd

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.

Observation of a re-emergence of the flux lattice in BSCCO

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.

Evidence for a square vortex lattice in Sr2RuO4 from muon-spin-rotation measurements

A muon-spin-rotation study of the flux-line lattice in Sr 2RuO4 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 D 1:2.1/ from the field distribution. This gives a value for the penetration depth of D 185.15/ nm. The temperature dependence of the penetration depth is measured.

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.

Observation of flux lattice melting in YBCO by small angle neutron scattering

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.

Muon-spin rotation (μSR) measurements on Bi2Sr2CaCu2O8+δ in the presence of columnar defects

Abstract 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.

Angular-dependent muon-spin rotation on the mixed state of the organic superconductor K-(BEDT-TTF)2CU(SCN)2

Abstract We have performed muon-spin rotation (μSR) 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 Bi 2 Sr 2 CaCu 2 O 8 + δ .

Vortex studies in heavy-ion irradiated Bi2.15Sr1.85CaCu2O8+δ probed by μSR and small-angle neutron scattering

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.

μSR studies of the vortex lattice in high‐Tc and other superconductors

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.