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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 4 (1984), S. 413-427 
    ISSN: 0392-6737
    Keywords: Specific calculations and results
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Riassunto Il lavoro stabilisce una connessione tra il legame degli alogenuri alcalino-terrosi nella fase solida e nella fase molecolare, nell'ambito di un modello ionico. La conformazione strutturale di queste molecole, rivelata da esperimenti di deflessione elettrica di fasci molecolari, è collegata alla polarizzabilità elettronica dello ione metallico e al raggio ionico dell'alogeno. Varie proprietà molecolari (angolo e lunghezza di legame, energia di legame, frequenze vibrazionali) sono calcolate da leggi di forza determinate sulla fase solida. I risultati sono confrontati con dati sperimentali disponibili su molecole sia allo stato libero che in matrice solida, esaminando in alcuni sistemi scelte diverse dei parametri nelle leggi di forza che sono state usate in letteratura in studi del solido ad alte temperature e della fase liquida.
    Abstract: Резюме В этой статье устанавливается связь, в рамках ионной модели, между связью дигалогенидов щелочноземельных элементов в твердой фазе и в молекулярной фазе. В этой модели поляризуемость металлического иона и радиус галогена представляют собой параметры, которые определяют структуру этих молекул, линейную или изогнутую, как было показано в экспериментах по электрическому отклонению на молекулярных пучках. Проводятся вычисления молекулярного угла, длины связи, энергии связи и колебательных частот, учитывая законы для межионных сил, полученные из исследований твердых галогенидов щелочноземельных элементов. Полученные результаты сравниваются с экспериментальными данными для молекул, либо в свободном состоянии, либо внедренных в твердую криптоновую матрицу. Проведенный анализ обеспечивает предварительную проверку моделей межионных сил для исследования твердой и жидкой структур.
    Notes: Summary The paper establishes a connection, in the framework of an ionic model, between the binding of alkaline-earth dihalides in the solid phase and in the molecular phase. The polarizabilities of the metal ion and the halogen radius are identified in the model as the parameters which determine the structure of these molecules as either linear or bent, as revealed in electrical deflection experiments on molecular beams. Calculations of the molecular angle, the bond length, the binding energy and the vibrational frequencies are carried out with interionic-force laws taken over from studies of the solid alkaline-earth halides, the results being compared with the available experimental data on molecules either in the free state or embedded in a solid-krypton matrix. The analysis provides a preliminary test of interionic force models for hot-solid and liquid-structure studies.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 1 (1982), S. 409-421 
    ISSN: 0392-6737
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Riassunto Questo lavoro analizza dati sperimentali di diffrazione neutronica da soluzioni liquide K−KCl e Rb−RbBr a varie composizioni. Mentre la diffusione a piccoli angoli (per numeri d'onda trasferiti inferiori a circa 1 Å−1) è fortemente influenzata da effetti di schermo e di aggregazione associati agli elettroni esterni della componente metallica, l'ordine a corto raggio in questi sistemi è ben riprodotto da un modello che li considera come miscele ioniche. La struttura del sistema K−KCl ad alte concentrazioni della componente metallica è ulteriormente studiata in dettaglio da punti di vista diversi. Si mostra cosí che il sistema in questa regione di composizione può anche essere visto come un metallo espanso di cui gli ioni alogeno riducono fortemente l'ordine a corto raggio. Si discutono infine alcuni aspetti dell'ordine a corto raggio nella regione di alte concentrazioni della componente salina, associati con uno struttura secondaria nella figura di diffrazione.
    Abstract: Резюме Исследуются результаты дифракции нейтронов на жидких растворах K−KCl и Rb−RbBr при различных составах. Исследование проводится в рамках модели ионной смеси, в которой пренебрегается экранированием и агрегацией, обусловленными металлическими электронами. Эта модель объясняет главные особенности наблюденных картин дифракции для волновых чиселk порядка 1 Å−1. С разных точек зрения анализируется фазовая диаграмма в системе K−KCl. Описываются короткодействующие корреляции для ионов калия в указанной области, используя металлический радиус, выведенный из свойств чистогожидкого металла, но простая металлическая модель должна быть дополнена предположением, что существенный беспорядок вводится в структуру металла ионами галогенов. Интерпретируются особенности короткодействующего упорядочения в области с высоким содержанием соли, которые связаны с выступом у главного пика со стороны большихk на диаграмме дифракции.
    Notes: Summary Neutron diffraction patterns from K−KCl and Rb−RbBr liquid solutions at various compositions are examined in a ionic-mixture model which neglects screening and aggregation due to the metallic electrons. The main features of the observed diffraction patterns for wave numberk roughly above 1 Å−1 are accounted for by the model. The approach to the metal-rich end of the phase diagram is analysed in detail from different viewpoints in the K−KCl system. Short-range correlations of the potassium ions are described in this region by a metallic radius deduced from properties of the pure liquid metal, but a simple expanded-metal model must be supplemented by the assumption that considerable disorder is introduced in its structure by the halogen ions. Features of short-range ordering in the salt-rich region that are implied by a shoulder on the high-k side of the main peak in the diffraction pattern are also commented upon.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 3 (1984), S. 730-738 
    ISSN: 0392-6737
    Keywords: Studies of specific liquid structures
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Riassunto Il lavoro discuta la struttura di soluzioni di cloruro di potassio in potassio liquido a concentrazionex=0.2, contrastando i risultati di un modello teorico con dati di diffrazione neutronica. Si esaminano, in particolare, lo schermo elettronico di fluttuazioni ioniche di grande lunghezza d'onda e la natura dell'ordine ionico a corto raggio. Si suggeriscono ulteriori esperimenti di diffrazione nella regione di piccoli angoli di diffusione e a concentrazioni maggiori della componente salina.
    Abstract: Резюме В рамках теотической модели, вклющей данные по дифракции нейтронов, исследуется структура Ъидких растворов K0.8-(KCl)0.2. Обсуждаются электронное зкранирование длинноволновых флуктуаций плотности ионов и природа коротковолоовой упорядоченности ионов. Предлагаются дифракционные эксперименты по малоугловому рассеянию и при более высоких концентрациях (KCl).
    Notes: Summary The structure of liquid solutions of potassium chloride in potassium, at a concentrationx=0.2 of the salt component, is examined in an electron-ion model against multipattern neutron diffraction data. Electronic screening of long-wave-length ionic-density fluctuations and the nature of short-range ionic order are discussed. Further diffraction experiments in the small-angle scattering region and at higher concentrations of the salt component are suggested.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 3 (1984), S. 509-521 
    ISSN: 0392-6737
    Keywords: Localized single-particle electronic states (excluding impurities)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Riassunto Il lavoro presenta uno studio teorico di elettroni solvatati a forte diluizione in alogenuri alcalini fusi. Un calcolo autoconsistente dello stato fondamentale è confrontato con dati NMR d'interazioni iperfini e con misure di suscettività magnetica. Le bande di assorbimento ottico dovute a transizioni 1s→2p e 1s→3p sono calcolate in uno schema di Franck-Condon e i risultati sono confrontati con l'evidenza sperimentale, con attenzione particolare all'analisi dello spettro nel CsCl fuso data da Yuh e Nachtrieb. Si discutono quindi un'instabilità degli stati eccitati sotto rilassamento ionico, la vita media dello stato fondamentale nel liquido e lo spostamento dello spettro eccitonico dovuto all'elettrone solvatato.
    Abstract: Резюме Теоретически исследуются свойства сольватированных электронов при высокой степени разведения в четырех расплавленных щелочных галоидах. Самосогласованное вычисление основного состояния связанного электрона и окружающей жидкой структуры сравнивается с данными ЯМР для сверхтонких смещений и с измерениями магнитной восприимчивости. Вычисляются зоны поглощения, связанные с переходами 1s→2p и 1s→3p в схеме Франка-Кондона. Полученные результаты сравниваются с имеющимися экспериментальными данными по оптическому поглощению, причем, особое внимание уделяется анализу спектров для расплавленного CsCl. Также обсуждается неустойчивость возбужденных состояний относительно ионной релаксации и конечное время жизни основного состояния в жидкости. В заключение, оценивается смещение экситонного спектра, обусловленное сольватированным электроном.
    Notes: Summary Properties of solvated electrons at high dilution in four molten alkali halides are investigated theoretically. A self-consistent evaluation of the bound ground state of the electron and of the surrounding liquid structure, already developed in a previous paper, is compared with the results of NMR hyperfine shifts and magnetic-susceptibility measurements. The absorption bands associated with Franck-Condon 1s→2p and 1s→3p transitions are calculated and compared with the available data of optical absorption, with special emphasis on the detailed analysis by Yuh and Nachtrieb for molten CsCl. An instability of the excited states against ionic relaxation and the origin of a finite lifetime for the ground state are also discussed. Finally, the perturbation induced by the solvated electron in the fundamental absorption of the molten salt is estimated.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 4 (1984), S. 39-54 
    ISSN: 0392-6737
    Keywords: Studies of specific liquid structures
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Riassunto Il lavoro discute i fattori di struttura misurati nei metalli alcalini liquidi nell'àmbito della teoria di potentiziali di coppia schermati. L'inversione dei dati strutturali permette di ottenere informazioni sulla buca attrattiva nel potenziale, che è confrontata con potenziali teorici dopo un controllo del metodo d'inversione su dati di simulazione. Si analizza quindi il comportamento del fattore di struttura a piccoli numeri d'onda, in relazione all'osservazione di un termine lineare nella figura di diffrazione di raggi X.
    Abstract: Резюме Измеренные структурные факторы жидких щелочных металлов исследуются в рамках теории экранированных парных потенциалов. Из структурных данных получается информация об основной яме притяжения в эффективном парном потенциале. Полученные результаты сравниваются с целым рядом теоретических парных потенциалов в случаях натрия и калия. Предложенный метод проверяется посредством компьютерного моделирования для рубидия. Также приводятся и обсуждаются результаты для других щело ных металлов. Подробно исследуется область малоуглового рассеяния, уделяя особое внимание возможности существования линейного члена в степенном разложении структурного фактора при очень малых передаваемых импульсах. Анализируется чувствительность предложенного подхода в связи с обнаружением линейного члена в экспериментах по дифракции рентгеновских лучей.
    Notes: Summary Measured structure factors of liquid alkali metals are examined in the framework of screened pair potential theory. Information on the main attractive well in the effective pair potential is obtained from the structural data by an approximate method stemming from an optimized random-phase treatment of the indirect ion-ion attraction. The results are compared with a variety of theoretical pair potentials in the cases of sodium and potassium, after a test of the method against computer simulation data on a model for rubidium. Results for the other alkali metals are also given and discussed. The small-angle scattering region is then examined in considerable detail, with special attention to the possibility of a linear term in a series expansion of the structure factor at very small momentum transfer. Although sensitivity to both the bare electron-ion coupling and the local field factor in the screening function is demonstrated and analysed, no linear term of the magnitude reported in recent X-ray diffraction experiments is found in the present theoretical framework.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 58 (1984), S. 49-55 
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The paper presents approximate calculations of dimensionality and lattice symmetry effects in the wave vector and frequency dependent diffusional response of disordered lattices. Numerical results are given and comparatively discussed for the tracer diffusion correlation function and the quasi-elastic incoherent scattering width in triangular, simple square and simple cubic lattices, only the restriction of double occupancy avoidance being taken into account. The quasi-elastic coherent scattering width is estimated for a triangular system possessing liquid-like structural disorder by means of the simple inclusion of a spread in jump lengths around a preferred set of jumps. Qualitative contact is made with recent neutron scattering experiments on alkali-metal graphite intercalation compounds.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Journal of superconductivity 13 (2000), S. 593-601 
    ISSN: 1572-9605
    Keywords: pairing mechanism ; transition temperature ; isotope effect ; cuprates
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract An effective two-dimensional dynamic interaction is developed which incorporates screening of holes by plasmons and by optical phonons to discuss the nature of the pairing mechanism leading to superconductivity in layered mercury cuprates. The system is treated as an ionic solid containing layers of charge carriers and a model dielectric function is set up which fulfils the appropriate sum rules on the electronic and ionic polarizabilities. The static limit of the model dielectric function is used to calculate the effective hole-hole coupling strength. The values of the electron-phonon coupling strength and of the Coulomb interaction parameter indicate that the superconductor is in the strong coupling regime with effective screening of the charge carriers. The superconducting transition temperature of optimally doped HgBa2CuO4+δ is estimated as 120 K from Kresin's strong coupling theory and the energy gap ratio is substantially larger than the BCS value. The value of the isotope exponent is severely reduced below the BCS value. The implications of the model and its analysis are discussed.
    Type of Medium: Electronic Resource
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