Louis E. Toth

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.

Superconducting current leads of YBCO and Pb-BSCCO

The authors fabricated and measured cylindrical superconducting current leads composed of a Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ (YBCO) and Bi/sub 2-x/Pb/sub x/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ (Pb-BSCCO) that were approximately 20 cm long and 1 cm/sup 2/ in cross section. A steady-state, DC, critical current of 170 (230) A at a temperature of 77 K was measured for YBCO (Pb-BSCCO) using a voltage criterion of 2*10/sup -7/ V ( rho =8*10/sup -10/ Omega -cm). This current was limited by the current-induced self-magnetic field. At a temperature of 63 K this current increased to approximately 390 (465) A. The character of the magnetic field dependence of the critical current and shape of the current-induced voltage curves are different for the two materials.

Growth of single crystals of YBa2Cu3O7

Abstract Single crystals of YBa 2 Cu 3 O 7 (1:2:3) were grown by melting mixtures of fine powders of 1:2:3, CuO and BaCuO 2 and slow cooling through the peritectic regime under oxygen. There was a fairly broad composition range in the ternary phase field in which primary crystals formation is 1:2:3. Crystals were plate shaped, millimeter and submillimeter in size, with the c -axis of the unit cell perpendicular to the flat side of the plate. The onset of the superconducting transition was in excess of 90 K. Electron microscopy showed that the as-grown superconducting crystals were heavily twinned. These results suggest that larger crystals can be grown by Czochralski or Bridgman techniques using similar compositions.

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.

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.

Electronic structure of high-Tc superconductors studied using photoelectron spectroscopy

Abstract Fundamental information about the structure of the valence band and the chemical valence states of the various constituents of the LaSrCu, YBaCu, BiSrCaCu and TlCaBaCu oxides have been obtained using photoelectron spectroscopy. These results show that the one-electron theories do not adequately describe the electronic structure of these superconductors. The atomic origins of the features observed in the valence bands have been investigated by studying photoemission resonances and changes in excitation cross-sections with photon energy. Results to date suggest that these materials have varying densities of states at the Fermi level, valence bands composed of O 2p and Cu 3d states, and display no significant changes in the band structure associated with the superconducting behaviour when the temperature is lowered below T c . In addition, the complex surface chemistry of these oxides make it essential to study the surface stoichiometry and the interaction of simple molecules. O 2 and CO are found to interact only weakly with the surfaces of the materials studied to date, while the H 2 O and CO 2 react strongly, forming hydroxides and carbonates.