J. R. Gaines

Practical preparation of copper oxide superconductors

We present a thorough yet simple approach for synthesizing high Tc superconductors of substantial quality. The approach is accompanied by a step‐by‐step discussion of possible pitfalls as well as a correlation of the sample characteristics to the various preparation procedures. A procedure sufficiently accurate for characterizing the resistance as a function of temperature for a Tc determination is also described.

Anomalous magnetic properties of Y1−xScxBa2Cu3O7−δ

Abstract We report here anomalous magnetic behavior in the Y 1− x Sc x Ba 2 Cu 3 O 7− δ series where x varies from x = 0 to x = 0.5. With increasing Sc concentration, there is a decrease in the density of states. The occurrence of a large paramagnetic spin equivalent to 2.7 μ B per Sc and an increase in the residual resistivity as well as the temperature dependence of the resistivity. It is suggested that these effects may be caused by antisite disorder and increased tendency of Sc toward covalent bonding. Changes in the density of states and possible localization by doping with Sc are discussed.

A new experimental approach to two-dimensional percolation

Abstract By using a computer to generate random patterns and then reproducing these patterns by photolithographic techniques, we have prepared a random stick system and a bond percolation system for experimental studies of 2D percolation. The agreement between the measured conductivity exponents for these two systems supports the universality hypothesis.

Transport and structural properties of the Ho1Ba2Cu3O9−δ superconductor

The compound Ho/sub 1/Ba/sub 2/Cu/sub 3/O/sub 9-//sub delta/ has been found to be a high T/sub c/ superconductor. The onset of the superconducting transition is 88 K with zero resistance achieved at 87 K. The x-ray diffraction spectrum of this material shows it to be a single-phase perovskite similar to the Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 9-//sub delta/ compound but different from the K/sub 2/NiF/sub 4/ perovskite which is believed to be the superconducting phase for the La/sub 2(1-//sub x//sub )/ Ba/sub 2//sub x/CuO/sub 4-//sub delta/ system. Possible oxygen deficiencies in several of the unit cell planes are discussed. The room-temperature resistance, the superconducting onset temperature, and the emergence of the single phase all depend upon the sample preparation firing conditions.The compound Ho1Ba2Cu3O9−δ has been found to be a high Tc superconductor. The onset of the superconducting transition is 88 K with zero resistance achieved at 87 K. The x‐ray diffraction spectrum of this material shows it to be a single‐phase perovskite similar to the Y1Ba2Cu3O9−δ compound but different from the K2NiF4 perovskite which is believed to be the superconducting phase for the La2(1−x) Ba2xCuO4−δ system. Possible oxygen deficiencies in several of the unit cell planes are discussed. The room‐temperature resistance, the superconducting onset temperature, and the emergence of the single phase all depend upon the sample preparation firing conditions.

Field induced ferromagnetic component in semiconducting La2−xSrxCuO4−δ

Abstract We report the magnetization M as a function of magnetic field H and temperature T for the semiconducting La 2− x Sr x CuO 4− δ system. Below the antiferromagnetic ordering temperature T N , upon application of high field at small x (〈 0.02) for La 2− x Sr x CuO 4− δ , a stepwise in M(H) has been observed, suggestive of the presence of a ferromagnetic component above a critical field. Above T N , there is a sublinear behavior of M on H. Possible models are discussed for these novel phenomena.

Resistance Dependence and Thermogravimetric Analysis of the Er1Ba2Cu3O9-δ Superconductor Above Room Temperature

Recently an unprecedented interest has been focused on high Tc superconductors. Progressively higher transition temperatures have been reported for the La-Ba-Cu-O series, [1] the Y-Ba-Cu-O series, [2] and recently a flouridated version of this latter series. [3] A suggestion of superconductivity at 240 K based upon the observation of an inverse AC Josephson effect, [4] in a multiphase samples fuels the search for materials with even higher transition temperatures. Interest in the behavior of these compounds is not concentrated just in the region below room temperature. An understanding of what occurs in the sintering and annealing processes is particularly imformative towards optimizing the preparation conditions. In this paper we report resistance and thermogravimetric analysis (TGA) measurements on Er1Ba2Cu3O9-δ between room temperature and 920 C. The incorporation of oxygen into the furnace allows us to simulate the whole sintering and annealing processes. Several resistive and oxygen content features will be discussed.

Enhancement mechanisms for the far-infrared absorption of metal-smoke-insulator composites based on the clustering of metal smoke

The strong far-infrared absorption observed by many workers in metal-smoke-insulator composites has been re-investigated thoroughly using well characterised samples of Ag-smoke-Teflon composites. The absorption coefficient of well separated Ag smoke can be explained quantitatively by the Maxwell-Garnett theory modified to include the size distribution of the Ag particles and the oxide coating effects. Particle clustering is shown to have an enormous impact on the far-infrared absorption of these composites. Comparisons between the authors experimental data and certain clustering modes are made. These comparisons show that most of the experimental results are consistent with the predictions of the cluster percolation model.

Structure and the (014)/(005) X-Ray Line Intensity in 1-2-3 Superconductors

The discovery of high Tc superconductivity in the La-Ba-Cu-O system [1] initiated an accelerated interest in this and related systems. To date the highest zero resistance temperatures have been observed in compounds of the form R1Ba2Cu3O9-δ where R = Y, Nd, Sm, Eu, Gd, Ho, Er, and Lu. [2–6] The Y1Ba2Cu3O9-δ compound has been studied extensively by x-ray diffraction [7–11] neutron scattering [12, 13] and thermogravimetric analys is. [2, 4] The unit cell is found to have a sequential Ba-Y-Ba stacking, an ordered oxygen deficiency, [13] and a slight orthorhombic distortion. [7–11] The similarity of the x-ray diffraction data and lattice constants of the compounds in this series strongly indicates that these compounds have the same structure, with the specific rare earth element simply substituting for Y.

Anomalous Resistive Behavior in Er 1 Ba 2 Cu 3 O 9-δ at 2290 K

Interest in high Tc superconductivity has increased on a par with the reports of higher transition temperatures. These reports range from Tc’s near 30 K in the La-Ba-Cu-O system [1], to 90 K in the Y-Ba-Cu-O system [2] and rare earth substituted compounds of the form R1 Ba2Cu3O9-δ where R = Nd, Sm, Eu, Gd, Ho, Er, and Lu. [3–5] Recently an indication of superconductivity was observed at 240 K in Y1Ba2Cu3O9-δ by the reverse AC Josephson effect, [6] and zero resistances as high as 155 K were reported in the system Y1Ba2Cu3O9-δ :F. [7]