H. A. Hoff

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 .

Ion implanted, outdiffusion produced diamond thin films

A thin film, produced by carbon ion implantation and outdiffusion at the temperature of irradiation, has been confirmed to be diamond using micro‐Raman spectroscopy and electron diffraction. The Raman spectra contained a definitive diamond peak shifted slightly from that of natural diamond. This shift may be due to residual strain and is consistent with the broad linewidth. Fragments of the film over a micron in size were examined with transmission electron microscopy and found to be untwinned, single crystals. The film had been produced on a polycrystalline copper substrate. The carbon ions were at an ion energy of 120 keV and the irradiation was carried out to a high dose.

Correlation of ‘‘twins’’ observed by optical microscopy and transmission electron microscopy in YBa2Cu3O7−x superconductors

By using transmission electron microscopy and optical microscopy on the same specimens, the patterns of light‐ and dark‐contrast lines seen in reflected polarized light were shown to be an interference pattern due to the variable spacing of suboptical microtwins. These microtwins are mostly [110] reflection twins. The [110] twinning was observed to be cyclic and occasionally pseudotetragonal because of the progressive cycling of contact twin domains. Within a domain, and occasionally in a whole grain, the [110] reflection twins occurred as polysynthetic lamellae. The morphology of the domain structure can be explained from the theory of martensitic transformation.

Phase segregation of cerium in Nd2−xCexCuO4−y

Abstract We report phase segregation based on cerium content in thick (>70μm) single crystals and a polycrystalline sample of Nd 2− x Ce x CuO 4− y with nominal composition x =0.15. The crystals showed segregation into sharply defined sheets of materia l with different cerium concentrations. These sheets in the a-b -plane of the crystals were apparent when polished cross-sections were viewed with crossed polarizers in visible light. The different color of each layer was correlated with cerium compositions by electron microprobe analysis. AC susceptibility showed a strong, well defined transition at 24.5 K and a second, weaker transition at 20 K. The appearance of the 20 K transition was shown to be associated with a layer of material with reduced cerium content. A polycrystalline sample also displayed small intra-grain regions with reduced cerium content.

Experimental evidence of resputtering of the yttria layer in a YBa2Cu3O7−x/Y2O3/YBa2Cu3O7−x trilayer film

Using transmission electron microscopy and energy dispersive x‐ray spectroscopy analysis, the microstructure and composition of a YBa2Cu3O7−x/Y2O3/YBa2Cu3O7−x trilayer film deposited on MgO by off‐axis sputtering at 670 °C and 100 mTorr was investigated. The YBa2Cu3O7−x layer in contact with the substrate was found to be mainly single phase. However, the top YBa2Cu3O7−x layer was embedded with 5–10 nm crystalline Y2O3 (yttria) particles, which disturbed the local microstructure. The top YBa2Cu3O7−x layer also had barium copper oxide particles covering its surface. The partial decomposition of the top YBa2Cu3O7−x layer may have been due to resputtering of the yttria layer which locally altered the deposition conditions. In particular, the oxygen pressure in the vicinity of the growing film may have increased due to sputtering of the yttria layer by plasma gas atoms and the sputtered target atoms. As a result, deposition occurred under conditions away from the thermodynamic stability line.

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.

Orthorhombic to tetragonal phase transition in Y1Ba2Cu3O7−x

Quenching experiments were performed to study the orthorhombic to tetragonal phase transition in Y1Ba2Cu3O7−x. The quenched and unquenched specimens were characterized by x‐ray diffraction and optical microscopy. The phase transition temperature was found to be dependent on the environment of the sealed specimens during quenching, and could be as high as 1173 K (900 °C). The phase transition is suggested to be of second order.

Columnar growth in combustion deposited diamond thin films

The morphology of the nucleation and growth surfaces of oxygen‐acetylene combustion deposited diamond thin films has been studied using scanning electron microscopy. The diameters of both nucleation cells and growth surface grains have been measured and analyzed statistically. The general shape of the distributions is found to change from nucleation to growth surface for each film. The frequency distribution of cells is generally normal, whereas, the distribution for the growth surface is found to be always lognormal. The change in shape of the distributions from normal for the nucleation surface to lognormal for the growth surface can be explained from a stochastic theory of growth.

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.