Evidence for a striped phase of high T c superconductor at the Fermi surface

A complete Fermi surface in the normal state of superconducting Bi₂Sr₂CaCu₂O_8+δ at the optimum doping (T _c = 91K) has been measured by angle scanning photoemission spectroscopy using synchrotron radiation. The Fermi surface reveals broken segments and hot spots of high photointensity along lines of k _y = n:0:23Å^?1 in the (π; π) direction. The pseudogap observed at (π,0) in underdoped materials is one of this missing segments. The results indicate the presence of characteristic subbands of a superlattice of quantum stripes. This is formed by an ordered array of ～ 14Å large stripes.     

On the existence of pyrophosphates of tetravalent metals having a layered structure

After an accurate investigation of the thermal behaviour (TG and DTA curves) of several samples of α-Zr(HPO₄)₂·H₂O and α-Ti(HPO₄)₂·H₂O, prepared both by the refluxing and HF procedures, γ-Zr(HPO₄)₂·2 H₂O and γ-Ti(HPO₄)₂·2 H₂O, new pyrophosphate phases of tetravalent metals have been isolated.The structure of these compounds, deduced from X-ray diffraction data, density measurements and considerations of bond length and angles, arises from the packing of layers of α and γ type, identical to those present in the starting materials and held together by POP bridges.These layered M(IV)P₂O₇ are obtained after the condensation process of the HPO₄ groups present in α and γ-M(IV)(HPO₄)₂ and are stable until the transition to the respective cubic pyrophosphates takes place.The range of stability of these phases changes on going from the α to the γ phase and in part depends on the degree of crystallinity and/or the crystal size of the starting layered exchanger. The chemical implications of α- and γ-L-M(IV)P₂O₇ are examined and discussed.     

Ag(I) solid solutions in zirconium dihydrogen phosphate. Thermal behaviour and phase changes

The thermal and redox behaviours of some Ag-low-converted zirconium phosphates have been investigated by thermoanalytical and X-ray methods: the conditions for obtaining Ag solid solutions in phases are reported.

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Zusammenfassung Das thermische und Redoxverhalten einiger Zirkoniumphosphate niedriger Ag-Konversion wurde durch thermoanalytische und Röntgenmethoden studiert; über die Bedingungen zum Erhalten von Ag-Festphasenlösungen in HH-Phasen wird berichtet

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Résumé On a étudié le comportement thermique et oxydo-réducteur de quelques phosphates de zirconium faiblement converti par Ag, en se servant des méthodes d'analyse thermique et des rayons X: on communique les conditions permettant d'obtenir des solutions solides d'Ag dans les phases .

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- Ag- . HH.

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Paper in part presented at the 6th ICTA, in Bayreuth (Germany), July 1980.

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We thank Dr. M. A. Massucci, of our Institute, for helpful discussions and suggestions.     

X-ray absorption near edge structures (XANES) in simple and complex Mn compounds

The X-ray Absorption Near Edge Structures (XANES) of Mn compounds at the Mn K-edge have been measured with high resolution at the Frascati Synchrotron Radiation Facility “PULS”. Molecular effects, such as “shape resonances”, have been identified in molecular complexes. Experimental evidence of solid state (or long-range order) effects have been found in spectra of simple oxides.The application of XANES for determining the local structure and chemical bonding of Mn ion in unknown compounds is discussed.     

Modello integro-differenziale di un sistema ecologico e la sua stabilità: caso di pino cembro e nocciolaia nella loro interdipendenza

Sunto Nella presente nota si propone un modello integro-differenziale dellevoluzione di un sistema ecologico formato da due specie associate per simbiosi, prendendo lesempio di Pino cembro e Nocciolaia. Si individuano soluzioni stazionarie e si dimostrano lesistenza, lunicità e la limitatezza uniforme della soluzione globale del problema di evoluzione. Il risultato più interessante riguarda la stabilità di soluzioni stazionarie: risulta che in buone condizioni naturali ci sono tre soluzioni stazionarie, di cui la soluzione nulla è stabile, la soluzione non nulla con popolazioni meno grandi è instabile e quella con popolazioni più grandi è stabile.

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In this paper an integro-differential model of the evolution of an ecological system formed by two species in symbiosis is proposed, taking example of Pinus cembra and Nucifraga caryocatactes. Stationary solutions are found and the existence, the uniqueness and the uniform boundedness of the global solution to the evolution problem are proved. The most interesting result concerns the stability of stationary solutions: it is shown that in good natural conditions there are three stationary solutions, of which the null solution is stable, the non-null solution with smaller populations is unstable and that with greater populations is stable.

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Mathematics Subject Classification (2000) 45J05, 92D40     

Soft x-ray absorption and high-resolution powder x-ray diffraction study of superconducting CaxLa1−xBa1.75−xLa0.25+xCu3Oy system

We have studied the electronic structure of unoccupied states measured by O K-edge and Cu L-edge x-ray absorption spectroscopy (XAS), combined with crystal structure studied by high resolution powder x-ray diffraction (HRPXRD), of charge-compensated layered superconducting Ca_xLa_1−xBa_1.75−xLa_0.25+xCu₃O_y (0≤x≤0.4 and 6.4≤y≤7.3) cuprate. A detailed analysis shows that, apart from hole doping, chemical pressure on the electronically active CuO₂ plane due to the lattice mismatch with the spacer layers greatly influences the superconducting properties of this system. The results suggest chemical pressure to be the most plausible parameter to control the maximum critical temperatures (T_c^max) in different cuprate families at optimum hole density.