W.L. Lechter

Bulk superconductivity above 100 K in the TlSrCaCu system

Abstract We report the synthesis and physical properties of a new copper-oxide superconductor, TlSrCaCuoxide. We have found that samples of this material exhibit zero electrical resistance below 105 K. DC susceptibility measurements show evidence of bulk superconductivity with diamagnetic onsets at 119 K and 70 K. X-ray diffraction data are compatible with the basic perovskite structure and can be indexed to a tetragonal lattice with unit cell parameters of a = 3.805 ± 0.006 A and c = 12.14 ± 0.06 A .

One-step reaction and consolidation of Hg based high-temperature superconductors by hot isostatic pressing

Abstract A hot isostatic press (HIP) techniques has been used successfully to react and consolidate in one step the Hg based high-temperature superconductors (HTSs). Hot isostatic pressing (HIPping) overcomes some of the problems encountered in the quartz encapsulation technique typically used to make Hg based HTS samples, such as containing the high vapor pressures of Hg and avoiding explosions. The HIPping conditions for processing (i.e., range of temperature and pressure) are significantly different from those used in the quartz-encapsulation method. These differences seem to relax the stringent conditions on precursor processing required in other processing methods. Hg based HTS samples were successfully made from both nitrate and carbonate precursors, as well as from commercially available precursors. X-ray diffraction experiments indicated that the best materials were made with nitrate precursors and were composed of 1223 phase (66%), 1212 phase (33%) and a small amount of BaCuO2 (1%). We have HIPped powders and contained them in selected high-purity metal foils. We find that Au is a good containment material for processing as it reacts very little with the Hg compounds being formed. Ag, on the other hand, reacts extensively. Samples with densities as high as 97% of theoretical density were formed. This technique has been used to successfully fabricate component that may lead directly to devices.

Thin-film high Tc superconductors prepared by a simple flash evaporation technique

Thin films of Bi‐Sr‐Ca‐Cu‐O and Tl‐Ba‐Ca‐Cu‐O have been deposited on single‐crystal MgO substrates by a simple flash evaporation technique. Small pellets of the superconducting compound were formed by standard techniques, then evaporated to completion on the MgO substrates using an electron beam. The best films show an onset of superconductivity at approximately 110 K and zero resistance by 78 K. X‐ray diffraction measurements show the films to be tetragonal with nominal lattice parameters of a=3.85 A and c=30.66 A. The x‐ray data also show the films to be highly textured with the metal‐oxide planes parallel to the substrate surface.

Color indicator of cuprate superconductivity observed by polarized light microscopy

A common mineralogical technique for identifying individual crystals in a heterogeneous, polycrystalline sample involves the observation at room temperature of their colors in reflected polarized light (colors of polarization). Here we present the observation that a brownish yellow (golden) color of polarization is not only characteristic of the YBa2Cu3O7 superconductor, but of the cuprate superconductors in general and is correlated with the occurrence of superconductivity.

Optical differentiation of orthorhombic superconductors such as YBa2Cu3O7 from their tetragonal precursor material

The orthorhombic superconducting phase of YBa2Cu3O7 is shown to be differentiable from its tetragonal precursor by optical microscopy. Individual grains 1 μm in diameter or larger are found to either exhibit the color of the compensator in reflected polarized light (tetragonal material) or to change colors from the compensator (orthorhombic material). By using color differences, retained tetragonal material and the orthorhombic nature of untwinned areas of grains are identified. The advantage to producing better superconducting single crystals and sintered polycrystalline materials is discussed.

Critical current model analysis of inter- and intra-grain effects in a high density sintered TlBaCaCuO ceramic

Abstract Isothermal DC magnetization in fields up to 5 T and harmonic component (AC fields of 1.1 mT) data for a sintered Tl 2 Ba 2 Ca 2 Cu 3 O x powder sample are reported. The sample contains mainly the 2223 phase, with a diamagnetic onset at 115 ± 1 K. The initial magnetization curves at various temperatures scale to a single curve. This behaviour and the results of the harmonic analysis agree with general predictions derived from critical state current models.

Casting high-T c superconducting BiSCCO

Bi-Sr-Ca-Cu-O (BiSCCO) was melted and cast into molds in order to render the superconductor into fully dense and useful shapes. Special conditions for casting and subsequent heat treatments were required to ensure structural integrity and a high transition temperature,Tc.The history of the castings during heat treatments was studied by thermal analysis, X-ray diffraction, and microscopy.

Superconductivity above 100 K in compounds containing Hg

The results of studies of new Hg based cuprate superconductors are reported. Several members of a new family of (Hg,Tl)(Ba,Sr)Can−2CunOx high temperature superconductors have been synthesized. These compounds, which are analogs to the Hg-Ba-Ca- and Tl-Ba-Ca-layered cuprates, are multi-phased and have superconducting transition temperatures above 100 K. Incorporation of Hg appears to stabilize several of the Tl-compounds, including a double layer. Tl/Sr system, in a manner similar to the role that Pb plays in the Tl/Sr- and Bi/Sr-systems. It has been suggested that recent reports of resistive Tcs above 200 K in Hg based samples are due to the presence of free Hg. Magnetization measurements of such a sample confirm this hypothesis.

The color of polarization in high temperature superconductors

Abstract We have studied the color of polarization of many members of the class of hole carrier cuprate superconductors and found a characteristic color and a strong correlation between color and the existence of superconductivity. We have extended this study to include one member of the electron carrier cuprate superconductors and have observed the same characteristic color indicating that the presence of this color is independent of the nature of charge carriers. We have also developed a method to quantify the color through photometric techniques.

Novel technique for crystal growth of superconducting ErBa2Cu3O7

Millimeter-sized crystals of ErBa2Cu3O7 were fortuitously grown while sintering a flat disk. It was possible to separate individual crystals from the sintered mass and measure their properties. The resistive transition and onset of magnetic transition of a single crystal was 93 K and the resistive transition width was < 1 K. Flux expulsion was observed and individual crystals could be levitated by a magnetic field. Crystals showed unusual growth features. They were heavily corrugated along the part of the crystal that grew parallel to the top surface of the disk and that coincided with thec axis of the unit cell. Crystals were characterized by X-rays, electron microscopy, and optical microscopy.