[en] At environmentally realistic atom concentrations ²³⁷Pu tracer was used to examine Pu uptake by unialgla cultures of Thalassiosira pseudonana (live and dead), Thalassiosira sp., and Platymonas sp., as well as by glass particles. Live or dead cells and glass particles accumulated Pu at similar rates, indicating that initial uptake was a passive phenomenon. Uptake was strongly affected by the nature of particle surfaces, but much less by composition of the media. Cells from rapidly growing cultures took up more Pu, than did those from late log or senescent cultures. Acid-washed glass took up more Pu, faster, than did unwashed glass. Cells accumulated more Pu from uv-treated seawater than from seawater untreated or enriched with f/50-level EDTA or f/50 vitamins; from complete f/50 medium uptake was even less, as with f/50 trace metals + EDTA. After a shot uptake Pu was 25% removable in tracer-free media, but after 3 days of uptake none was removable in seawater or exometabolite media, and only 15% in f/50-level EDTA. The data support the hypothesis that Pu in marine environments associates with suspended particles that could act as vertical vectors for this element

[en] One of the intriguing questions involving the copper containing high temperature superconductors is the relationship between the hole concentration and the transition temperature, T_c. The authors address this question in terms of a proposed master curve on a T_c vs hole concentration plot. This curve shows a maximum at 90K at a hole concentration of approximately 0.2-0.22 holes per sheet copper for the 1-2-3 system. In this work the authors studied the Nd_1+xBa_2-xCu₃O_y and YBa₂Cu_3-xZn_xO_y systems to check the validity of this master curve. Both Nd and Zn substitutions lower T_c, Nd by reducing the number of holes, consistent with the model given by the curve. The effect of Zn substitution is ambiguous, since both the holes are increased and the Cu-O planes are perturbed. Compensation doping, using both Nd and Zn, shows that the major effect of Zn is to disrupt the Cu-O planes

[en] Superconductivity shown to be related to the electron deficiency (hole concentration) in the copper-oxygen layer. Resistivity and Hall data for the La-Sr-Cu-oxide system is compared with that of YBa₂Cu₃O₇. The results are consistent with recent non-phonon theories of oxide superconductivity based on strongly correlated electrons or dynamic peroxide formation

[en] The electron deficiency (hole concentration) in the La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub δ/ system is determined by a chemical method. Our results show a direct correlation between T/sub c/ and hole concentration. The hole concentration equals the Sr concentration to about x = 0.15. For x>0.15, the hole concentration decreases and oxygen vacancies are formed. These results strongly support an all-electronic mechanism for superconductivity in this system

[en] We have prepared a series of samples in the La_2-xSr_xCuO_4-δ system and studied the superconducting and transport properties as a function of composition. A unique wet chemical analytical technique is used to determine the electron deficiency (hole concentration) in the copper-oxygen layer, which we define as a [Cu - O]⁺ complex. From this analysis we obtain a value for the oxygen stoichiometry and contrary to previously published results, we find few if any oxygen vacancies for 0≤ x ≤ 0.15. The [Cu -0]⁺ concentration is approximately equal to the Sr concentration and the Hall number to about x = 0.15. Conduction is therefore due to holes in a sigle band. For x > 0.15 the [Cu - 0]⁺ decreases and the number of oxygen vacancies increases. The Hall effect is qualitatively different in this range. It does not measure the number of carriers but rather indicates hole and electron conduction or other exotic behavior. A strong correlation between the [Cu - 0]⁺ concentration and T_c is shown. For YBa₂Cu₃O_9-δ, an unusual T dependence is found for the Hall number. The origin can be two carrier conduction with unusual temperature dependences of the conductivity parameters. Alternatively, some novel conduction mechanism related to the novel pairing in these high T_c materials may be responsible. 2 refs

[en] There are at present two copper oxide systems which are superconducting at temperature high compared to the conventional phonon mediated non-oxide superconductors. These systems are La_2-x(Ba,Sr)_xCuO_4-δ and YBa₂Cu₃O_9-δ. In the La_2-xSr_xCuO_4-δ system, the Sr concentration can be changed over a wide range forming solid solutions system and preserving the K₂NiF₄ structure. In the YBa₂Cu₃O_9-δ system, changes in Y and Ba concentration result in the formation of other phases

No abstract available

[en] The compensation doped system Nd_(1+x)-cCA_cBa_2-x Cu₃O_y and Nd_1+xBa_2-xCu_3-zZn_zO_y are studied to elucidate both the dependence of T_c on the Cu - O plane hole concentration and the effect of disorder on the planes. Nd substitution for Ba reduces T_c and the plane holes. The study is discussed in this paper. Ca substitution compensates for Nd, raising T_c and the plane holes. Zn has a small effect on the hole concentration. More importantly, Zn causes disorder on the Cu-O plane, suppressing T_c. The choice of these Nd systems is very important, because contrary to the YBa₂Cu_3-xAn_xO_y based systems, the Hall number can be unambiguously interpreted and is in agreement with the plane hole concentration derived from chemical analysis and the method of Tokura et al. for dividing the total holes into mobile plane holes and local chain holes

[en] One of the fundamental questions involving the high-temperature copper oxide containing superconductors is to understand the relationship between hole concentration and the superconducting transition temperature, T/sub c/. We help resolve this question by a study of Nd-, La-, Ga-, Ca-, and Zn-substituted YBa₂Cu₃O/sub y/ with controlled oxygen stoichiometry. The total hole concentration is determined by a chemical method and trends in the mobile hole concentration by Hall measurements. By considering T/sub c/ to be a function of the mobile sheet holes only, there is a maximum in T/sub c/ (92 K) at a sheet-hole concentration of about 0.20--0.25 per Cu site

[en] Gas chromatography/mass spectrometry was used to analyze the oxygen evolved from normal and ¹⁸O enriched YBa₂Cu₃O_6.98 upon dissolution in normal and ¹⁸O enriched acid. From the ¹⁸O/¹⁶O ratio the authors show that the oxide phase is the source of all the evolved oxygen. Their results indicate that some of the lattice oxygens are in a state other than O^2-, probably O^- or peroxide. the fact that they are unable to chemically detect peroxide suggests that the radical O^- could possibly be the species released from the oxide