Fast-ionic-conductor behavior of driven lattice-gas models

We discuss driven diffusive lattice-gas systems as a model for fast ionic conductors, derive associated hydrodynamic equations and expressions for transport coefficients, and compare mean-field theory, Monte Carlo results and experimental observations. The comparison between model and real behaviours helps to understand some properties of those materials which seem to be characterized by the occurrence of nonequilibrium steady states and phase transitions. In particular, our study suggests the existence in Nature of a novel (nonequilibrium) universality class.     

First-principles study on superconductivity of the gold–indium alloy under high pressure

The superconductivity of gold–indium alloys has been investigated using first-principles calculations based on the density functional theory. At ambient pressure, the calculated superconducting transition temperature (T _c) is 0.04 μ K in pure gold, but T _c dramatically increases by substituting indium atoms for gold atoms. The gold–indium alloy having 12.5 atomic percent indium (Au_0.875In_0.125) shows T _c of 0.1 K, and Au_0.75In_0.25 marks 1.7 K. The dramatic increase in T _c owing to the alloying effect is caused by the enhancement of the electron–phonon coupling. The superconductivity of gold is predicted to be drastically weakened with increasing pressure and virtually disappear at 10 GPa, but it continues up to at least 30 GPa by the inclusion of indium atoms.     

Investigations of oxide superconductors by x-ray absorption,photoemission and cognate spectroscopies

X-ray absorption spectra, X-ray photoelectron spectra and Auger spectra of cuprate superconductors are discussed. The studies establish the absence of Cu³⁺ for all practical purposes, but point out the importance of oxygen holes. X-ray photoelectron spectra of BaBi_0.25Pb_0.75O₃ and related compounds are also examined.     

Josephson Currents and Gap Enhancement in Graph Arrays of Superconductive Islands

Evidence is reported that topological effects in graph-shaped arrays of superconducting islands can condition superconducting energy gap and transition temperature. The carriers giving rise to the new phase are couples of electrons (Cooper pairs) which, in the superconducting state, behave as predicted for bosons in our structures. The presented results have been obtained both on star and double comb-shaped arrays and the coupling between the islands is provided by Josephson junctions whose potential can be tuned by external magnetic field or temperature. Our peculiar technique for probing distribution on the islands is such that the hopping of bosons between the different islands occurs because their thermal energy is of the same order of the Josephson coupling energy between the islands. Both for star and double comb graph topologies the results are in qualitative and quantitative agreement with theoretical predictions.     

Electrode potentials of electrochemical cells with oxide-conducting solid electrolyte in chemically nonequilibrium gas mixtures

A wide range of electrodes for solid oxide electrolyte cells in chemically nonequilibrium gas media simultaneously containing oxygen and combustible gases is systematically studied. The concentration dependences are investigated for the potential response of electrodes made of conducting oxides, noble metals (platinum, gold, silver), and also composite electrodes containing dispersed oxide semiconductor phases, apart from the main conducting material. Besides, dynamic characteristics of these electrodes are investigated. The studies are performed in the temperature range from 450 to 800°C. The content of combustible gases in the mixture was varied from 10 ppm to 2%. The research results are of interest for development of sensors for combustible gases and for studying kinetics and mechanisms of catalytic oxidation reactions.     

Spin excitations in a K0.84Fe1.99Se2 superconductor as studied by Mössbauer spectroscopy

Mössbauer spectroscopy was used to probe the site-specific information of a K_0.84Fe_1.99Se₂ superconductor. A spin excitation gap, ΔE ≈5.5 meV, is observed by analyzing the temperature dependence of the hyperfine magnetic field (HMF) at the iron site within the spin wave theory. Using the simple model suggested in the literature, the temperature dependence of the HMF is well reproduced, suggesting that, below room temperature, the alkali metal intercalated iron-selenide superconductors can be regarded as ferromagnetically coupled spin blocks that interact with each other antiferromagnetically to form the observed checkerboard-like magnetic structure.     

Time-dependent perturbation theory for nonequilibrium lattice models

We develop a time-dependent perturbation theory for nonequilibrium interacting particle systems. We focus on models such as the contact process which evolve via destruction and autocatalytic creation of particles. At a critical value of the destruction rate there is a continuous phase transition between an active steady state and the vacuum state, which is absorbing. We present several methods for deriving series for the evolution starting from a single seed particle, including expansions for the ultimate survival probability in the super- and subcritical regions, expansions for the average number of particles in the subcritical region, and short-time expansions. Algorithms for computer generation of the various expansions are presented. Rather long series (24 terms or more) and precise estimates of critical parameters are presented.     

Synthesis and Rapid Densification of Nanoparticles of Barium Praseodymium Hafnium Oxide: A New Complex Perovskite

Nanoparticles (25–100 nm) of barium praseodymium hafnate (Ba₂PrHfO_5.5), a new substrate material for high temperature superconducting YBa₂Cu₃O_7- and Bi-cuprate films has been synthesized through an exothermic chemical reaction for the first time. The Ba, Hf and Pr ions required for the formation of Ba₂PrHfO_5.5 were obtained in solution by dissolving a stoichiometric mixture of BaCO₃, Pr₆O₁₁ and HfO₂ heated at 1200°C for 4 h in boiling nitric acid. By complexing the ions with citric acid and adjusting the oxidant/fuel ratio, it was possible to obtain Ba₂PrHfO_5.5 as nanoparticles in a single step chemical process. The nanoparticles thus obtained were characterized by powder X-ray diffraction, thermogravimetric analysis, differential thermal analysis, infrared spectroscopy, and surface area analysis. The Ba₂PrHfO_5.5 synthesized through the present exothermic chemical process, because of its nanoparticulate nature, could be sintered to a density of 98% in a period of just 9 min at a temperature of 1435°C in air. Scanning electron microscopic studies on the sintered Ba₂PrHfO_5.5 samples showed that densification has occurred without significant microstructural coarsening up to a sintered density of 98% of its theoretical value in a short duration.