M. T. Wylie

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.

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.

μSR investigation of the superconducting properties of YNi2B2C

Abstract Muon spin rotation (μSR) has been used to investigate the mixed state of the recently discovered superconductor YNi2B2C. The width of the μSR-lineshape at low temperature gives a measure of the superconducting penetration depth λ(T = 0), while the field dependence of the linewidth allows an estimate of the superconducting coherence length ξ(T = 0).

Observation of the Flux-Line Lattice by Neutron Diffraction and Muon-Spin Rotation

Small-angle neutron scattering and muon spin rotation have been used to obtain direct information about flux structures in the mixed state within the bulk of crystals of superconductors, including BSCCO-2212, niobium and YBCO. In highly anisotropic BSCCO, we see evidence of vortex fluctuations, melting, and decomposition of flux lines into ‘pancake’ vortices, which give a more twodimensional vortex structure. Our results from pure niobium show no sign of flux-lattice melting — in disagreement with recent claims. In YBCO the flux lattice is much more stable than in BSCCO, due to a much lower anisotropy, but it shows interesting flux structures as a function of field direction.

Small‐angle neutron scattering study of the flux‐line lattice in a single crystal of Bi2.15Sr1.95CaCu2O8+x (invited)

A flux‐line lattice (FLL) was observed in a single crystal of Bi2.15Sr1.95CaCu2O8+x (BSCCO) using small‐angle neutron scattering methods. The sample has a superconducting transition at 85 K. The flux‐line lattice is observed to melt, evidenced by the rapid disappearance of diffracted intensity as the temperature is increased above a field‐dependent melting temperature. Diffracted intensity due to the vortex lattice also falls off as the applied field is increased. It is believed that this is a manifestation of the transition of the three‐dimensional flux lines into two‐dimensional pancake vortices. The Bragg intensity of the FLL peak is inversely proportional to the fourth power of the London penetration depth (λL). Hence, the temperature (T) dependence of the order parameter can be measured quite accurately from the intensity of the Bragg spots at different temperatures. In BSCCO with an applied field of 50 mT, the measured T dependence appears linear. The low‐T behavior is of great interest for an under...

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.

Neutron scattering from the flux-line lattice in Bi2Sr2CaCu2O8 + y

Abstract Neutron small-angle diffraction has been used to investigate the flux-line lattice structure within single crystals of the high-temperature superconductor Bi 2.15 Sr 1.95 CaCu 2 O 8 + x . The diffracted intensity goes rapidly to zero as the magnetic field or the temperature is increased. Melting at low fields as a function of temperature coincides with the appearence of finite resistance within the superconducting state. At low temperatures the diffracted intensity disappears in fields greater than ∼ 70 mT, probably due to the decomposition of the flux-line lattice into randomly pinned 2d “pancake” vortices.

Small angle neutron diffraction studies of vortex structures in high temperature superconductors

Abstract We have used neutron scattering to provide direct information about flux structures in the bulk of crystals of the superconductor Bi 2 Sr 2 CaCu 2 O 8 . Its extremely high effective mass anisotropy, makes the flux lattice susceptable to melting and also to decomposition into ‘pancake’ vortices, which would give a more two-dimensional vortex structure. At low temperatures and fields the scattered intensity is consistent with a three dimensional flux-line structure. At higher fields and temperatures, the scattering from the flux lattice dissapears well below T c . We can associate this dissappearance with the above changes in the vortex structure. We compare the neutron scattering results with macroscopic measurements of magnetisation.