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MacFarlane WA Bobroff J Alloul H Mendels P Blanchard N Collin G Marucco J 《Physical review letters》2000,85(5):1108-1111
We present a study of the spin dynamics of magnetic defects induced by Li substitution of the plane Cu in the normal state of YBa2Cu3O6+x. The fluctuations of the coupled Cu magnetic moments in the vicinity of Li are probed by near-neighbor 89Y and 7Li NMR spin lattice relaxation. The data indicate that the magnetic perturbation fluctuates as a single entity with a correlation time tau which scales with the local static susceptibility. This behavior is reminiscent of the low T Kondo state of magnetic impurities in conventional metals. Surprisingly it extends well above the "Kondo" temperature for the underdoped pseudogapped case. 相似文献
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From the analysis of recent thermodynamic data, we show that whereas oxygen vacancies are present in the whole homogeneity range of WO3?x, crystallographic shear planes are present only for large departures from stoichiometry (x > 0.012). 相似文献
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Cristau HJ Cellier PP Spindler JF Taillefer M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(22):5607-5622
Mild, efficient, copper-catalyzed N-arylation procedures for nitrogen heterocycles, amides, carbamates, and C-arylation procedures for malonic acid derivatives have been developed that afford high yields of arylated products with excellent selectivity. The N-arylation of imidazole with aryl bromides or iodides was found to be greatly accelerated by inexpensive, air-stable catalyst systems, combining catalytic copper salts or oxides with a set of structurally simple chelating ligands. The reaction was shown to be compatible with a broad range of aryl halides, encompassing sterically hindered, electron-poor, and electron-rich ones, providing the arylated products under particularly mild conditions (50-82 degrees C). The lower limit in ligand and catalyst loading and the scope of Ullmann-type condensations catalyzed by complexes bearing those ligands with respect to the nucleophile class have also been investigated. Chelating Schiff base Chxn-Py-Al (1c) generates a remarkably general copper catalyst for N-arylation of pyrrole, indole, 1,2,4-triazole, amides, and carbamates; and C-arylation of diethyl malonate, ethyl cyanoacetate, and malononitrile with aryl iodides under mild conditions (50-82 degrees C). The new method reported here is the most successful to date with regard to Ullmann-type arylation of some of these nucleophiles. 相似文献
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[reaction: see text] An efficient method for the synthesis of diaryl ethers under particularly mild conditions is described. Inexpensive ligands were found to greatly accelerate the Ullmann-type coupling of aryl bromides or iodides with phenols. A series of diaryl ethers were obtained with excellent yields in acetonitrile in the presence of Cs(2)CO(3) and catalytic copper(I) oxide. The reaction tolerates substrates with unfavorable substitution patterns, such as sterically hindered coupling partners or electron-rich aryl halides. 相似文献
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Julien M Feher T Horvatic M Berthier C Bakharev ON Segransan P Collin G Marucco J 《Physical review letters》2000,84(15):3422-3425
Doping the high- T(c) superconductor YBa2Cu3O6.7 with 1.5% of nonmagnetic Zn impurities in CuO2 planes is shown to produce a considerable broadening of 63Cu NMR spectra, as well as an increase of low-energy magnetic fluctuations detected in 63Cu spin-lattice relaxation measurements. A model-independent analysis demonstrates that these effects are due to the development of staggered magnetic moments on many Cu sites around each Zn and that the Zn-induced moment in the bulk susceptibility might be explained by this staggered magnetization. Several implications of these enhanced antiferromagnetic correlations are discussed. 相似文献
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Bobroff J Alloul H Ouazi S Mendels P Mahajan A Blanchard N Collin G Guillen V Marucco JF 《Physical review letters》2002,89(15):157002
We use 89Y NMR in YBa(2)Cu(3)O(6+y) in order to evaluate with high sensitivity the distribution of hole content p in the CuO2 planes. For y=1 and y=0.6, this hole doping distribution is found narrow with a full width at half maximum smaller than Deltap=0.025. This rules out any large static phase separation between underdoped and optimally doped regions in contrast with the one observed by STM in Bi2212 and by NQR in LaSrCuO. This establishes that static electronic phase separation is not a generic feature of the cuprates. 相似文献
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Jean-Francis Marucco Bertrand Poumellec Jacques Gautron Philippe Lemasson 《Journal of Physics and Chemistry of Solids》1985,46(6):709-717
By means of thermogravimetric measurements, it has been possible to obtain information on the nature of the intrinsic and extrinsic defects of TiO2, NbO2 and solid solutions NbyTi1?yO2±x Pure TiO2 is an oxygen-deficient oxide The main defects are oxygen vacancies, doubly ionized V..o or singly ionized V.o, and interstitial titanium Ti3i NbO2 is a metal-deficient oxide The main defects are neutral niobium vacancies. The solid solutions NbyTi1?yO2±x may be divided into two groups If y > 0 04, the behavior is analogous to that of NbO2; with the same defects, but the width of the homogeneity range decreases with the titanium content and Nb0 04Ti0 96O2 is a stoichiometnc oxide If y < 004, the oxides are both metal deficient and oxygen deficient according to the oxygen partial pressure. We have particularly studied the solution Ti0 995Nb0 005O2 In the oxygen-deficient domain, the main defects are assumed to be neutral or singly ionized oxygen vacancies In the metal-deficient domain, the main defects are metal vacancies V4Ti From these results we have deduced the nature of intrinsic defects in TiO2 to be Schottky defects: 2V..O + V4T1. 相似文献