Melt‐textured growth of polycrystalline YBa2Cu3O7−δ superconductor using directional solidification created an essentially 100% dense structure consisting of long, needle‐ or plate‐shaped crystals preferentially aligned parallel to the a‐b conduction plane. The new microstructure, which completely replaces the previous granular and random structure in the sintered precursor, exhibits dramatically improved transport Jc values at 77 K of ∼17 000 A/cm2 in zero field and ∼4000 A/cm2 at H=1 T (as compared to ∼500 and ∼1 A/cm2, respectively, for the as‐sintered structure), with the severe field dependence of Jc (‘‘weak‐link’’ problem) no longer evident in the new melt‐textured material. The improvement in Jc is attributed to the combined effects of densification, alignment of crystals, and formation of cleaner grain boundaries. Microstructure and distribution of various phases present in the melt‐textured material are discussed in relation to the superconducting properties.

High critical currents in Y‐Ba‐Cu‐O superconductors

We have extended Bean’s critical state model to explicitly include anisotropic critical currents. Measurements at 30 K of the critical currents parallel to the Cu‐O planes but with vortex motion either parallel or across twin boundaries show twin boundaries are probably not an important cause of vortex pinning. In the critical state, current flow perpendicular to the Cu‐O planes is about 30 times smaller than flow parallel to these planes.

Anisotropic critical currents in Ba2YCu3O7 analyzed using an extended Bean model

We have calculated the initial magnetization curves and complete hysteresis loops for hard type‐II superconductors. The critical‐current density Jc is assumed to be a function of the internal magnetic field Hi according to Kim’s model, Jc(Hi)=k/(H0+‖Hi‖), where k and H0 are constants. As is the case for other critical‐state models, additional assumptions are that bulk supercurrent densities are equal to Jc, and that the lower critical field is zero. Our analytic solution is for an infinite orthorhombic specimen with finite rectangular cross section, 2a×2b (a≤b), in which a uniform field H is applied parallel to the infinite axis. Assuming equal flux penetration from the sides, we reduced the two‐dimensional problem to a one‐dimensional calculation. The calculated curves are functions of b/a, a dimensionless parameter p=(2ka)1/2/H0, and the maximum applied field Hm. The field for full penetration is Hp=H0[(1+p2)1/2−1]. A related parameter is H*m=H0[(1+2p2)1/2−1]. Hysteresis loops were calculated for the di...

Kim model for magnetization of type‐II superconductors

THE defects in as-grown bulk YBa2Cu3O7 have been shown to act as weak flux pinning sites, leading to a reversible magnetization1, magnetization decay2 and non-zero resistivity3 in moderate magnetic fields at temperatures near the transition temperature, Tc. These effects limit the technological utility of the as-grown material3. This is not entirely surprising, as both single crystals and polycrystalline materials are grown under conditions expected to minimize the defect density, but the weak pinning in these materials has nevertheless raised the issue of a possible intrinsic limitation to the critical current density. Here we report the attainment of a critical current density, Jc, of ∼ 6 x 105 A cm-2 at 77 K and 9 kOe, in a single crystal of Yba2Cu3O7 irradiated with fast neutrons to introduce defects. (In both irradiated and unirradi-ated crystals, the critical current density was inferred by measuring the magnetic moment resulting from an induced persistent current.) This hundredfold enhancement of Jc relative to unirradiated crystals demonstrates that artificially induced defects can act as strong flux pinning sites, and analysis of the defects should increase our understanding of flux pinning in high- Tc superconductors.

Critical currents near 106 A cm−2 at 77 K in neutron-irradiated single-crystal YBa2Cu3O7

Analysis of the many experiments on high-temperature superconductivity indicate several essential aspects of any theory. The conductivity and other transport properties as a function of disorder, temperature, and frequency point to a non-Fermi liquid-like behavior, whereas photoemission experiments and magnetic properties indicate the presence of a Fermi surface in momentum space. To reconcile this apparent contradiction, a new type of electron liquid, called a Luttinger liquid, has been postulated, and the present article aims to show the need for this postulate. Theory and experiment indicate that the suitable phenomenological electronic structure model of the CuO layers is that of the one-band Hubbard model. It is also argued that experiment clearly indicates that interlayer interactions strongly affect the superconducting transition temperature, Tc, consistent with the fact that no theoretical calculations on two-dimensional Hubbard models have resulted in the prediction of high transition temperatures, and that anyon models are not favored by experiment.

http://science.sciencemag.org/content/sci/256/5063/1526.full.pdf

Experimental constraints on the theory of high-tc superconductivity.

Magnetization measurements were performed on single crystals of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ to study the effect of fast neutron irradiation on flux pinning and the critical magnetization current. Neutron irradiation up to a fluence of 8.16\ifmmode\times\else\texttimes\fi{}${10}^{17}$ n/${\mathrm{cm}}^{2}$ systematically enhanced the magnitude and reduced the anisotropy of the critical magnetization current compared to unirradiated samples. The largest increases in the critical magnetization current occurred at 77 K. There was a slight linear decrease in the transition temperature with neutron fluence.

Enhanced critical magnetization currents due to fast neutron irradiation in single-crystal Y Ba 2 Cu 3 O 7 − δ

Magnetization measurements on single crystals of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ show strong anisotropic flux pinning with critical magnetization current densities of 1.4\ifmmode\times\else\texttimes\fi{}${10}^{6}$ A/${\mathrm{cm}}^{2}$. The critical magnetization currents fall sharply with temperature in both single-crystal and polycrystalline samples. The high critical magnetization current densities can be reduced by nearly two orders of magnitude by annealing.

Large anisotropic critical magnetization currents in single-crystal Y Ba 2 Cu 3 O 7 − δ

We present an x-ray diffraction study of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystals. High-resolution scans reveal two orthorhombic phases with different lattice constants but a common orientation indicating a phase separation into phases with $\ensuremath{\delta}\ensuremath{\approx}0.0 \mathrm{and} 0.3$. Thermal expansions of the unit-cell parameters of both phases measured between 10 and 300 K were fit using a quasiharmonic approximation giving a best-fit Debye temperature of 370\ifmmode\pm\else\textpm\fi{}5 K. The measured orthorhombicities were found to change smoothly through the superconducting transition temperature without any anomaly.

Phase constitution and thermal expansion of YBa2Cu3O7- delta single crystals.

Measurements of the temperature dependence of the infrared conductivity of the CuO{sub 2} planes in YBa{sub 2}Cu{sub 3}O{sub 7} in the superconducting state are reported. Remarkably, we find that the temperature dependence of {sigma}{sub 1}({omega},{ital T}) is independent of {omega} for {omega}{approx lt}500 cm{sup {minus}1}, and that it is the same as that of the nuclear Korringa product, 1/{ital T}{sub 1}{ital T}. These observations indicate that for the best-characterized high-temperature superconductor, YBa{sub 2}Cu{sub 3}O{sub 7}, the phenomenology of dynamic properties is fundamentally different from that of conventional superconductors.

J.Z. Liu

Papers

Enhanced critical magnetization currents due to fast neutron irradiation in single-crystal Y Ba 2 Cu 3 O 7 − δ

Large anisotropic critical magnetization currents in single-crystal Y Ba 2 Cu 3 O 7 − δ

Phase constitution and thermal expansion of YBa2Cu3O7- delta single crystals.

Dynamic properties of a high-Tc superconductor : direct evidence for non-BCS behavior

Superconductivity and structure of single crystal YBa2Cu3Ox