L.F. Schneemeyer

Anisotropic critical currents in Ba2YCu3O7 analyzed using an extended Bean model

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.

Studies of oxygen-deficient Ba2YCu3O7−δ and superconductivity Bi(Pb)SrCaCuO

Abstract Ambient temperature measurements of the crystallographic cell parameters for oxygen deficient Ba 2 YCu 3 O 7−δ prepared by gettered annealing indicate the presence of microscopic differences in the oxygen configuration at fixed δ dependent on annealing temperature. The loss of superconductivity with increasing oxygen deficiency is shown to be due to a step-like increase in length of the plane copper-apical oxygen bond, and not to the orthorhombic to tetragonal transition. The crystal structure of the 84K superconductor Bi 2 Sr 2 CaCu 2 O 8+δ is described, as is the stabilization of 110K superconductivity via partial Pb/Bi substitution with long time annealing.

A new layered cuprate structure-type, (A1−xA′x)14Cu24O41

Abstract Phases crystallizing in a new structure-type with general formula (A1−xA′x)14Cu24O41 (A = alkaline earth metal, A′ = trivalent metal) and symmetry Cccm with an extended stoichiometry range have been found in studies of the SrYCuO, BaSrBiCuO and CaLaCuO systems. Single crystal X-ray studies on several crystals grown from different alkaline earth/metal oxide-cuprate melts reveal a common orthorhombic F-centered subcell of a =11.3 A , b =13.0 A and c =3.9 A . Superstructure is observed in crystals, leading to a 7-fold increase of the c-axis and a change in symmetry to space group Cccm. As in Ba2YCu3O7, the Cu atoms are found as CuO planes and linear CuO chains. Due to shear in the planes, half of the CuO squares share edges, producing CuCu zigzag chains, similar to the planes observed in CaCu2O3. In the linear CuO chains, the CuO squares share edges as well, leading to a short CuCu contact of 2.75A. Experiments on ceramic samples indicate that the oxygen content is fixed and that the samples are semiconducting.

Ba2YCu3O7-δ: electrodynamics of crystals with high reflectivity

We present results for the reflectivity, R, of two bulk crystals of Ba/sub 2/YCu/sub 3/O/sub 7-//sub delta/ which, below the superconducting transition at T/sub c/, show behavior consistent with perfect reflectivity up to an energy of (3--4)k/sub B/T/sub c/. We interpret the reproducibility and high R values (especially Rapprox.1 below T/sub c/) as indicative of intrinsic behavior in the highly conducting crystal planes probed by the light. Results above T/sub c/ show that the charge carriers have an enhanced effective mass and low scattering rate at low frequency and temperature, indicating a strong interaction with some excitation.

Ba/sub 2/YCu/sub 3/O/sub 7-//sub delta/: Electrodynamics of crystals with high reflectivity

We present results for the reflectivity, R, of two bulk crystals of Ba/sub 2/YCu/sub 3/O/sub 7-//sub delta/ which, below the superconducting transition at T/sub c/, show behavior consistent with perfect reflectivity up to an energy of (3--4)k/sub B/T/sub c/. We interpret the reproducibility and high R values (especially Rapprox.1 below T/sub c/) as indicative of intrinsic behavior in the highly conducting crystal planes probed by the light. Results above T/sub c/ show that the charge carriers have an enhanced effective mass and low scattering rate at low frequency and temperature, indicating a strong interaction with some excitation.

Large magnetic hysteresis in a melt‐textured Y‐Ba‐Cu‐O superconductor

Melt‐textured YBa2Cu3O7−δ superconductor with a long and well‐aligned grain structure is shown to exhibit magnetic hysteresis at 77 K which is the largest ever reported for bulk polycrystalline Y‐Ba‐Cu‐O. The large ΔM resulted in a magnet‐like behavior as well as a strong suspension phenomenon. It is shown, contrary to the previous reports, that the suspension behavior observed in Y‐Ba‐Cu‐O is a generic consequence of large grain size, and not due to the presence of Ag oxide or Ag particles. There appears to be no substantial enhancement in flux pinning and Jc by these particles. Comparisons of magnetization behavior in various YBa2Cu3O7−δ samples (polycrystals, silver‐oxide doped, melt‐textured, and single crystals) indicated that for applied fields substantially larger than Hc1 the current flow that gives rise to the observed magnetization is intragranular. The pinning force and hence the critical current is roughly the same [Jc (magn)∼104 A/cm2] within a factor of ∼2 regardless of grain size, grain bou...

The structure and properties of Ba2YCu3O6

The structure of Ba2YCu3O6 has been determined by neutron diffraction powder profile analysis. The cell is tetragonal P4/mmm with a=3.8570(1)A and c=11.8194(3)A. The cations are in a perovskite type arrangement, with Ba and Y ordered on the A site to give a cell tripled along c. The oxygens occupy only 23 of the perovskite anion sites and are ordered such that 13 of the Cu is two-fold coordinated and 23 is five-fold coordinated. This configuration can be derived from that of the superconductor Ba2YCu3O7 by removing the oxygen atoms along the b axis at the (0,12,0) position. A pressed pellet exhibited semiconductivity with a band gap of 0.21eV from 125–300K. The compound is described as Ba2YCu1+Cu2+2O6.

Critical currents in proton‐irradiated single‐crystal Ba2YCu3O7−δ

Irradiation with 3.5 MeV protons creates defects in Ba2YCu3O7−δ, which can act as strong pinning sites, comparable to those produced by neutron irradiation. Indeed protons as well as neutrons undergo momentum‐transferring collisions that result in atomic displacements; the additional electronic proton‐solid interaction does not appear to yield defects which contribute to pinning. Under optimized conditions we have obtained Jc(T→0, H=1 T)∼2×107 A/cm2 in single crystals. At 77 K the value is still Jc∼1.7×105 A/cm2, decreasing only threefold in a field of 6 T. The fluence and temperature dependence of Jc suggest a model, in which spatially uncorrelated defects pin vortex cores.

Stoichiometry and superconductivity in single layer Bi2+xSr2−yCuO6±δ

An insulating polymorph with a composition close to Bi2Sr2CuO6 complicates the synthesis of the single copper layer superconductor Bi2+xSr2−yCuO6±δ. We concentrate on the region where the material is single phase in air synthesis (2.18 < x < 2.29, 1.44 < y < 1.7). The physical properties from insulating to marginally conducting as the composition moves toward x = y = 0. Like to two-layer compound, the material has a superlattice due to a buckling of the planar structure, however the superlattice coherence is degraded by defects at finite values of x and y. Density measurements indicate that the nonstoichiometry of Bi2+xSr2−y CuO6±δ is not accompanied by the formation of a large number of vacancies. The insulating polymorph, Bi2Sr2CuO6, can be suppressed by synthesis in modest oxygen pressures (above 5–10 atm). The result is a bulk superconductor (Tc ≈ 10 K).

Sharp angular sensitivity of pinning due to twin boundaries in Ba2YCu3O7

We have measured the pinning force due to twin boundaries by measuring the magnetic torque produced when a field is applied perpendicular to the c axis and rotated with respect to the ab axes. At 76 K, the torque increases roughly fourfold, when the field is within 2° of the parallel to the twin boundaries. This peak is absent at 27 K. Irradiation with 5×1016 cm−2 3.5 MeV protons increases the overall critical current and eliminates the peaks observed at 76 K.