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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 11 (1989), S. 1863-1863 
    ISSN: 0392-6737
    Keywords: Statistical theories of liquid structure ; Errata
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 1165-1179 
    ISSN: 0392-6737
    Keywords: Collective effects
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The interacting reference response functionX I [3](k) of three-dimensional jellium ink space was defined by Niklasson in terms of the momentum distribution of the interacting electron assembly. Here the Fourier transformF I [d](r) ofX I [d] (k) is studied for the jellium model withe 2/r interactions in dimensionalityd=1,2 and 3, in an extension of recent work by Holas, March and Tosi for the cased=3. The small-r and large-r forms ofF I [d] (r) are explicitly evaluated from the analytic behaviour of the momentum distributionn d(p). In the appendix, a model ofn d (p) is constructed which interpolates between these limits.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 155-167 
    ISSN: 0392-6737
    Keywords: Statistical theories of liquid structure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The asymptotic behaviours of particle correlation functions and the related sum rules are discussed for a layered classical plasma withe 2/r interactions in the fluid state, in dependence on the number of layers. These properties derive from consistency conditions imposed by screening on the hierarchical equations, as already treated by A. Alastuey and P. A. Martin (J. Stat. Phys.,39, 405 (1985)) for various Coulomb fluids. The main results concern i) the type of clustering of correlations needed for the validity of multipolar sum rules at various orders, ii) the proof that the pair correlation function in a finite multilayer may carry an electric dipole moment and the calculation of its partioning among the layers, and iii) the dimensionality crossover in an infinitely extended or periodically repeated multiplayer with varying interlayer spacing and wave vector.
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  • 4
    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.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 11 (1989), S. 1509-1514 
    ISSN: 0392-6737
    Keywords: Statistical theories of liquid structure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary It is shown that long-wavelength concentration fluctuations in a binary liquid metal alloy are determined by a pair interchange free energyw, which is exactly given by the sum of an elastic strain term and of the concentration-concentration direct correlation function. The latter is evaluated in an alloy of homovalent components by electron screening theory in the pseudopotential approach of Shaw and Harrison and shown to be entirely determined by nonlocal terms reflecting charge transfer to the more electronegative alloy component. Numerical calculations for the liquid Na−K alloy show strong cancellation between the two contributions tow at all concentrations, in qualitative agreement with experiment.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 15 (1993), S. 393-398 
    ISSN: 0392-6737
    Keywords: Statistical theories of liquid structure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The correlations between long-wavelength fluctuations of concentration in a liquid binary alloy are determined by a balance between an elastic-strain free energy and an Ornstein-Zernike effective interaction. The latter is extracted from thermodynamic data in the case of the Li−Pb system, which is well known to chemically order with stoichiometric composition corresponding to Li4Pb. Strong attractive interactions between concentration fluctuations near the composition of chemical ordering originate from electronic charge transfer, which is estimated from the electron-ion partial structure factors as functions of composition in the liquid alloy.
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  • 7
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    Springer
    Il nuovo cimento della Società Italiana di Fisica 18 (1996), S. 1357-1362 
    ISSN: 0392-6737
    Keywords: Boson systems ; General studies of phase transitions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Accurate analytic results are obtained for the condensate fraction and the internal energy of mesoscopic systems of non-interacting Bose particles confined in an axially symmetric harmonic trap. Earlier results for a spherical trap are recovered and numerical illustrations for an experimentally relevant case are given.
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  • 8
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    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.
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 8 (1986), S. 59-75 
    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 calcola le correlazioni strutturali tra elettroni e ioni in metalli liquidi mediante uno schema pseudoclassico basato sull’uso di potenziali efficaci. Il potenziale efficace tra elettroni è costruito con riferimento al gellio, e permette di ottenere una espressione analitica per la struttura di questo modello che è in buon accordo con i risultati di calcoli quantistici. Gli altri potenziali efficaci per il metallo liquido usano modelli noti per i noccioli ionici combinati con la relazione di Ornstein-Zernike tra struttura e compressibilità. Tale modello pseudoclassico del metallo liquido è pure risolto analiticamente. Risultati numerici sono ottenuti per la struttura del, sodio e del berillio liquidi e sono confrontati con altri risultati teorici e con dati sperimentali con attenzione particolare al fattore di struttura elettrone-elettrone misurato da Eisenberger, Marra e Brown in esperimenti di diffusione inelastica di raggi X dal berillio cristallino.
    Abstract: Резюме Рассматриваются электрон-электронные и электрон-ионные структурные корреляции в простых жидких металлах, используя эффективные парные потенциалы, чтобы включить квантовые эффекты в псевдоклассическое описание электронной жидкости. Сначала конструируется эффективный парный потенциал между одновременными флуктуациями электронной илотности из известных свойств вырожденной желеобразной модели, которые представляют правило сумм для плазмонов, соотношенуе Кимбала-Никлассона и величины Ясухара для функции распределения электронных пар. Следовательно, получается аналитическое выражение в приближении Дебая-Хюккеля для электронного структурного фактора в желе по всей области плотностей, соответствующих металлам. Полученные результаты согласуются с результатами квантовых вычислений. Затем развивается простая псевлоклассическая модель для жидкого металла: эта модель включает модель заряженных твердых сфер для нин-ионного потенциала и модель пустого «остова» для электрон-ионного потенциала, учитывая требования соотношения между длинноволновыми парциальными структурными факторами и изотермической сжимаемостью металлов, изменяются масштабы кулоновких хвостов. Предложенная модель решается аналитически, рассматривая псевдоклассическии линейный отклик для электрон-ионной связи. Приводятся численные результаты для парциальных структрурных факторов в жидком натрии и жидком бериллии. Проводится сравнение полученных результатов с данными для электрон-электронного фактора в кристалле, которые получены в экспериментах по неупругому рассеянию рентгеновского излучения Айзенбергером, Марра и Брауном.
    Notes: Summary Electron-electron and electron-ion structural correlations in simple liquid metals are treated by using effective pair potentials to incorporate quantal effects into a pseudoclassical description of the electron fluid. An effective pair potential between simultaneous electron density fluctuations is first constructed from known properties of the degenerate jellium model, which are the plasmon sum rule, the Kimball-Niklasson relation and Yasuhara’s values of the electron pair distribution function at contact. An analytic expression is theraby obtained in the Debye-Hückel approximation for the electronic structure factor in jellium over a range of density appropriate to metals, with results which compare favourably with those of fully quantal evaluations. A simple pseudoclassical model is then set up for a liquid metal: this involves a model of charged hard spheres for the ion-ion potential and an empty-core model for the electron-ion potential, the Coulombic tails being scaled as required by the relation between the long-wave-length partial structure factors and the isothermal compressibility of the metal. The model is solved analytically by a pseudoclassical linear response treatment of the electron-ion coupling and numerical results are reported for partial structure factors in liquid sodium and liquid beryllium. Contact is made for the latter system with data on the electron-electron structure factor in the crystal from inelastic X-ray scattering experiments of Eisenberger Marra and Brown.
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 9 (1987), S. 1265-1273 
    ISSN: 0392-6737
    Keywords: Diffusions in solids
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Riassunto Questo lavoro esamina la sezione d’urto per diffusione quasi elastica di neutroni da fluttuazioni di carica in cristalli di tipo fluorite in prossimità della transizione ad uno stato superionico, in relazione alle caratteristiche sperimentalmente osservate per lo spettro di diffusione quasi elastica da SrCl2, CaF2 e PbF2. Si usa il formalismo della funzione di memoria di Zwanzig e Mori nell’ambito di un’approssimazione di accoppiamento anarmonico tra modi vibrazionali del cristallo. I processi di decadimento a due modi e tre modi sono analizzati mediante argomenti basati su leggi di conservazione e singolarità di van Hove, usando curve di dispersione per vibrazioni reticolari misurate in SrCl2, CaF2, SrF2 e BaF2. L’analisi attribuisce un ruolo specifico ai modi Raman nella dinamica ionica precorrente la transizione. Gli spostamenti atomici associati ai modi dominanti sono correlati all’attivazione di disordine dinamico nel sottoreticolo anionico che porta alla transizione.
    Abstract: Резюме Эксперименты по рассеянию нейтронов на SrCl2, CaF2 и PbF2 показывают, что интенсивность и ширина когерентного квази-упругого спектра быстро ывеличиваются с температурой причем основная особенность в проинтегрированной квази-упругой интенсивности представляет пик выше точки (200) вдоль направления [100] в пространстве волновых векторов рассеяния. В этой работе используется формализм функции памяти Цванцига-Мори для анализа поперечного сечения квази-упругого рассеяния на флуктуациях плотности заряда в терминах ангармонических связей между колебательными модами кристалла. Исследуются каналы двух- и трех-мод для подтверждения квази-упругого рассеяния неитронов, основываясь на 1) сохранении энергии и импульса и аргументах сингулярности ван Хова и на 2) измеренных фононных диспеионных кривых вдоль главных направлений симметрии в SrCl2, CaF2, SrF2 и BaF2. Проведенный анализ определяет микроскопическую роль Раман-активных оптических ветвей. Собственные векторы соответствующих мод в трех-модовых каналах описывают относительные смещения двух галогенов в элементарной ячейке, наложенные на относительные смещения галогенной и щелочно-земельной компонент. Эта микроскопическая картина согласется с суперионным переходом, который связан с началом динамического беспорядка в анионной компоненте кристалла.
    Notes: Summary Neutron scattering experiments on SrCl2, CaF2 and PbF2 have shown that intensity and width of the coherent diffuse quasi-elastic spectrum increase rapidly with temperature into the fast-ion conducting phase, the main feature in the integrated quasi-elastic intensity being a peak just beyond the (200) point along the [100] direction in scattering wave vector space. The Zwanzig-Mori memory function formalism is used in this work to analyse the quasi-elastic scattering cross-section from charge density fluctuations in terms of anharmonic couplings between the vibrational modes of the crystal. The two- and three-mode channels are examined for compatibility with the quasi-elastic neutron scattering evidence, on the basis of i) energy and momentum conservation and van Hove singularity arguments and ii) measured phonon dispersion curves along the main symmetry directions in SrCl2, CaF2, SrF2 and BaF2. The analysis identifies a specific microscopic role for the Raman-active optic branches. The eigenvectors of the relevant Raman-active and partner modes in the three-mode channel describe relative displacements of the two halogens in the unit cell superposed on relative displacements of the halogen and alkaline-earth components. This microscopic picture is thus consistent with the superionic transition being associated with the onset of dynamic disorder in the anionic component of the crystal.
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