R. A. Doyle

Lindemann criterion and vortex-matter phase transitions in high-temperature superconductors

The vortex-matter in superconductors is generally believed to exist in two main phases, vortex-solid and vortex-liquid. Recent investigations of the phase diagram of anisotropic high-temperature superconductors indicate, however, the existence of at least three distinct phases: relatively ordered quasi-lattice, highly-disordered entangled vortex-solid, and a liquid phase. A theoretical analysis in terms of an extended Lindemann criterion provides a quantitative description of the resulting vortex-matter phase boundaries and the behavior of the transition lines with varying anisotropy and disorder.

Quasi-static magnetic behaviour of a disc-shaped type II superconductor

Abstract The isothermal, quasi-static, magnetic behaviour, including the internal field and flux distribution profiles and the bulk magnetisation of reversible (i.e. pin-free) and irreversible disc-shaped type II superconductors, in perpendicular applied field, are obtained from a rigorous theoretical treatment which explicitly includes the constitutive B [ H rev ] relation and the vortex pinning force dependence on induction B . The iterative numerical procedure to obtain the critical state is described and calculations are made for the magnetic behaviour in the low-κ (niobium) and in the high- T c , high-κ (BSCCO) systems with various vortex pinning models. Particular calculations are also made for comparison with experimental magnetisation data on pure (almost pin-free) single crystal niobium disc specimens and for the ‘arrow-head’ effect, and for the vortex-lattice ‘melting’ transition in a BSCCO disc-shaped single crystal.

Geometrical and distributed surface barriers in Bi2Sr2CaCu2O8

Abstract The local ac magnetic response in Bi 2 Sr 2 CaCu 2 O 8 crystals of platelet and prism shapes has been measured using microscopic arrays of Hall sensors. At elevated temperatures the ac response in platelet samples is governed by the geometrical barrier whereas a distributed Bean-Livingston surface barrier is dominant for the prism-shaped sample. In the local measurements these two display qualitatively different behavior and can be individually studied. Models for the local ac response are derived for the two cases and good agreement with experiment is obtained.

TRANSPORT PROPERTIES OF BI2SR2CACU2O8 CRYSTALS WITH AND WITHOUT SURFACE BARRIERS

Large BSCCO crystals with electrical contacts positioned far from the edges are studied by transport measurements, then cut into the common narrow strip geometry, and remeasured. Instead of showing larger resistance, the narrow strip samples display a dramatic drop in the resistance, enhanced activation energies, and nonlinear characteristics due to strong surface barriers. The surface barriers also dominate the resistive drop at the first-order phase transition. Because the surface barriers are avoided in large crystals, we are able to probe the solid phase and find good agreement with the recent predictions of Bragg glass theory.

Development of non-weak link bulk YBCO grain boundaries for high magnetic field engineering applications

A non-weak link joining technique has been developed for YBCO pseudo-crystals fabricated by seeded peritectic solidification based on the formation of a liquid phase which segregates from the platelet boundaries at temperatures above /spl ap/920/spl deg/C. Electrical and magnetic measurements on these boundaries suggest that their irreversibility field can be as high as 7 T at 77 K in fully oxygenated pseudo-crystals joined along their crystallographic ab-planes which is comparable to the irreversibility behaviour of the adjacent YBCO grains.

High field behavior of artificially engineered boundaries in melt-processed YBa2Cu3O7−δ

Artificial bulk “zero-angle” boundaries parallel to the c axis have been engineered between large melt-processed YBa2Cu3O7−δ (YBCO) grains and observed to carry a transport supercurrent at fields up to at least 5 T at 77 K. The temperature and angular dependencies of the boundary resistance have exactly the same form as those of the grains, which is evidence that the grains are intimately coupled. The limiting mechanism for current transfer across these boundaries is, therefore, not a simple weak link or Josephson effect. This joining technique is extremely promising for production of macroscopic engineering artifacts.

Shear-induced vortex decoupling in Bi 2 Sr 2 CaCu 2 O 8 crystals

Simultaneous transport and magnetization studies in

Current-voltage measurements as a probe of the activation barriers for flux creep in thin films of YBa2Cu3O7−δ

{\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}

Flux pinning, surface and geometrical barriers in YNi2B2C

crystals at elevated currents reveal large discrepancies, including finite resistivity at temperatures of 40 K below the magnetic irreversibility line. This resistivity, measured at the top surface, is nonmonotonic in temperature and extremely nonlinear. The vortex velocity derived from magnetization is six orders of magnitude lower than the velocity derived from simultaneous transport measurements. The findings are ascribed to a shear-induced decoupling, in which the pancake vortices flow only in the top few

Transport properties of bulk-bicrystal grain boundaries in artificially joined large-grain YBCO

{\mathrm{CuO}}_{2}