Superconductivity and magnetic properties of YBa2Cu3O7-δ annealing and quenching effects

Summary Crystal structure, superconductivity and magnetic properties as functions of annealing and quenching temperatures for high-T _c YBa₂Cu₃O_7-δ superconductor were investigated. It was found that the ?orthorhombic character? of the superconducting phase is affected by the quenching process through a mechanism in which both density and order of oxygen vacancies in the Cu (X, Y)-plane of the unit cell may be involved. The quenching-induced effects manifest themselves onT _c and on the behaviour of normal-state resistivity and magnetic susceptibility, whose values are analysed and discussed.

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Riassunto Sono state studiate le proprietà strutturali, superconduttive e magnetiche del composto YBa₂Cu₃O_7-δ in funzione delle condizioni di annealing e tempra. Si è trovato che il ?carattere ortorombico? della fase superconduttrice è modificato dai processi di tempra attraverso un meccanismo nel quale possono essere coinvolti sia la concentrazione delle vacanze di ossigeno che la loro distribuzione nel pianoX-Y della cella unitaria. Gli effetti indotti dalla tempra si riflettono sul valore diT _c e sull'andamento della resistività e della suscettività magnetica nello stato normale, i cui valori sono analizzati e discussi.

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Резюме Исследуются кристаллическая структура, сверхпроводящие и магнитные свойства, как функция температур отжига и закалки для высокостемпературногоT _c сверхпроводника YBa₂Cu₃O_7-δ. Получено, что ?орторомбический характер? сверхповодящей фазы изменяется в процессе закалки через механизм, в котором учитываются и плотность и порядок кислородных вакансиь в CuX-Y плоскости элементарной ячейки. Эффекты закалки в явном виде влияют наT _c и на поведение сопротивления нормальной фазы и магнитной восприимчивости, значеня которых анализируются и обсуждаются.

     

Study of interface properties in CuPc based hybrid inorganic-organic solar cells

Metal-substituted phthalocyanine thin films such as copper-phthalocyanine (CuPc) are often used as photo-active and hole transporting layers (HTLs) in fully organic photovoltaic devices. In this work, CuPc is vacuum sublimated on an electron acceptor layer of mesoporous titania (TiO(2)) for the formation of hybrid TiO(2):CuPc solar cell devices. The performance of these hybrid solar cell devices was demonstrated without and with dye sensitization at the TiO(2):CuPc interface. The charge separation and photocurrent contribution at the interfaces in these multilayer hybrid devices was studied by using a variety of optoelectrical and photophysical characterization techniques. It is important to understand the fundamental interface properties of these multilayer hybrid solar cell devices for optimized performance.     

Downward migration credit risk problem: a non-homogeneous backward semi-Markov reliability approach

International organizations evaluate credit risk and rank firms according to risk by assigning them a ‘rating’. The time evolution of a rating can be studied by means of Markov models. Some papers have outlined the problem pertaining to the unsuitable fitting of Markov processes in a credit risk environment. This paper presents a model that overcomes the problems given by the Markov rating models. It includes non-homogeneity, the downward problem and the randomness of time in the transitions of states, thus making it possible to consider the duration inside a state in a complete way. In this paper, both, the transient and asymptotic analyses are presented. The asymptotic analysis is performed by using a mono-unireducible topological structure. Moreover, a real data application is conducted using the historical database of Standard & Poor’s as the source.     

Numerical Solution of non-Homogeneous Semi-Markov Processes in Transient Case*

In this article a numerical solution for the evolution equation of a continuous time non-homogeneous semi-Markov process (NHSMP) is obtained using a quadrature method. The paper, after a short introduction to continuous time NHSMP, presents the numerical solution of the process evolution equation with a general quadrature method. Furthermore, the paper gives results that justify this approach, proving that the numerical solution tends to the evolution equation of the continuous time NHSMP. Moreover, the formulae related to some specific quadrature methods are given and a method for obtaining the discrete time NHSMP by applying a very particular quadrature formula for the discretization is shown. In this way the relation between the continuous and discrete time NHSMP is proved. Then, the problem of obtaining the continuous time NHSMP from the discrete one is considered. This problem is solved showing that the discrete process converges in law to the continuous one if the discretized time interval tends to zero. In addition, the discrete time NHSMP in matrix form is presented, and the fact that the solution to this process always exists is proved. Finally, an algorithm for solving the discrete time NHSMP is given. To illustrate the use of this algorithm for a discrete NHSMP, an example in the area of finance is presented.     

Influence of surfactant addition on the rheological properties of aqueous Welan matrices

In the present work the effects produced by the presence of two different surfactants (Abil B 8842 and Triton N 101) on the rheological properties of aqueous welan matrices are studied, both in steady and in oscillatory shear conditions. Welan is an acidic microbial polysaccharide having high thermal, pH, and salt stability. At sufficiently low concentrations it forms aqueous weak gel matrices which can be profitably used to regulate the rheological properties of disperse systems and improve their stability. Different systems are examined, having the same polysaccharide concentration (0.25 wt%) and different surfactant concentrations (up to 40 wt%, far beyond the range of practical interest for emulsion preparation). All the systems exhibit marked shear-thinning properties which can be described quite satisfactorily by the Cross equation. The concentration dependence of the zero-shear-rate viscosity as well as the mechanical spectra confirm that, in the concentration range considered, the aqueous welan systems are typically weakly structured fluids. The influence of both surfactants is examined in detail by comparing the behavior of the different classes of systems. Both surfactants reduce the polymer contribution at low shear, whereas an opposite action is exerted at high concentration and shear. These contrasting effects are ascribed to the different structural features of the polymer matrix under low stresses and high shear conditions, respectively. Received: 6 February 2000 Accepted: 1 November 2000     

Experimental and theoretical UV characterizations of acetylacetone and its isomers

Cryogenic matrix isolation experiments have allowed the measurement of the UV absorption spectra of the high-energy non-chelated isomers of acetylacetone, these isomers being produced by UV irradiation of the stable chelated form. Their identification has been done by coupling selective UV-induced isomerization, infrared spectroscopy, and harmonic vibrational frequency calculations using density functional theory. The relative energies of the chelated and non-chelated forms of acetylacetone in the S0 state have been obtained using density functional theory and coupled-cluster methods. For each isomer of acetylacetone, we have calculated the UV transition energies and dipole oscillator strengths using the excited-state coupled-cluster methods, including EOMCCSD (equation-of-motion coupled-cluster method with singles and doubles) and CR-EOMCCSD(T) (the completely renormalized EOMCC approach with singles, doubles, and non-iterative triples). For dipole-allowed transition energies, there is a very good agreement between experiment and theory. In particular, the CR-EOMCCSD(T) approach explains the blue shift in the electronic spectrum due to the formation of the non-chelated species after the UV irradiation of the chelated form of acetylacetone. Both experiment and CR-EOMCCSD(T) theory identify two among the seven non-chelated forms to be characterized by red-shifted UV transitions relative to the remaining five non-chelated isomers.     

Zero-resistance state and specific-heat anomaly at 110 K in the Y1.2Ba0.8CuO4−y superconductor

Summary The transition to the zero-resistance state at 110 K has been observed in multiphasic samples of Y_1.2Ba_0.8CuO_4−y nominal composition. The intrinsic nature of this high-temperature superconducting state is confirmed by the jump atT _c in the specific-heat curve.