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  • 1
    Electronic Resource
    Electronic Resource
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Materials Research 16 (1986), S. 87-112 
    ISSN: 0084-6600
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 8 (2001), S. 1065-1077 
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: In the past decade, synchrotron radiation has triggered a surge in studies of the polarization dependence of X-ray beams passing through non-isotropic materials. A vast range of experimental results concerning polarization-dependent absorption (dichroism) and dispersion (birefringence, for example) are available from materials which are either magnetic or exhibit preferred directions due to the local atomic environment. This article aims to bring together the diversity of modern experiments in this field with established methods of optical calculus, in a way that highlights the simplicity of the underlying physics. A useful framework is formed when observable quantities, in the X-ray case, are related to atomic variables of the sample material. Atomic descriptions of absorption spectra with various levels of complexity are considered, and some well documented sum-rules are encountered. The framework is the most general allowed within the electric dipole approximation. By way of illustration, dichroic X-ray absorption by two materials with highly anisotropic properties and magnetic ions with different valence shells are considered; namely, a 3d-transition ion in ferrous niobate, and a lanthanide ion in dysprosium borocarbide. Both materials display interesting magnetic properties that are challenging to interpret at an atomic level of detail, and it is shown how absorption experiments can contribute to resolving some issues.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 32 (1979), S. 189-194 
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The spin and orbital components of the inelastic neutron cross-section for scattering from a magnetised, degenerate plasma are calculated treating the Coulomb interaction in the RPA. Particular attention is focussed on results for small wavevectors where effects of the magnetic field are most pronounced. It is shown that a collective mode is excited at small wavevectors with a frequency that is the square root of the sum of the squares of the plasmon and cyclotron frequencies.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 8 (1986), S. 39-51 
    ISSN: 0392-6737
    Keywords: Emission, absorption and scattering of radiation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Riassunto La funzione di risposta per lo scattering magnetico dei neutroni dagli elettroni che forma un plasma ad un componente, o modello di jellio, soggetto ad un campo magnetico fisso, è calcolata nel limite semiclassico nel quale la statistica di Boltzmann si applichi agli stati elettronici. Si dà un’espressione completa per la risposta di un plasma ideale in una forma compatta e chiusa adatta alla ricerca numerica e che include la dipendenza dai parametri elettronici come la massa efficace ed il rapporto giromagnetico. Si discutono gli effetti dovuti all’interazione di Coulomb entro i limiti dell’approssimazione di fase random. I risultati teorici sono usati per prevedere le condizioni che devono essere ottenute dall’osservazione dell’eccitazione dei neutroni di Landau e dei modi collettivi.
    Notes: Summary The response function for magnetic neutron scattering off electrons forming a one-component plasma, or jellium model, subject to a steady magnetic field, is calculated in the semi-classical limit in which Boltzmann statistics apply to the electron states. A complete expression for the response of an ideal plasma is given in a compact, closed form, amenable to numerical investigation, and including the dependence on electron parameters such as effective mass and gyromagnetic ratio. Effects due to the Coulomb interaction are discussed within the limitations of the random phase approximation. The theoretical results are used to predict the conditions that must obtain for the observation of neutron excitation of Landau and collective modes.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 6 (1985), S. 40-50 
    ISSN: 0392-6737
    Keywords: Neutron determination of structures
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Riassunto Si studiano modelli stocastici del movimento di particelle singole composti da due o più stati differenti di moto in termini della funzione di correlazione a paia che determina la sezione d’urto inelastica incoerente del neutrone. Si considerano due classi di modelli. Primo, modelli nei quali le funzioni di probabilità per stati individuale di moto sono determinati da equazioni principali; secondo, modelli non markoviani nei quali i valori di transizione dipendono dal tempo. I modelli specifici discussi includono stati nei quali le particelle sono sottoposte a movimeni diffusi e armonici, movimento in un flusso fisso di fluido, o sono stazionarie in trappole per un tempo finito.
    Abstract: Резюме Исследуются стохастические модели движения, отдельных частиц, представляющих два или более различных состояний движения, в терминах функции парной корреляции, которая определяет неупругое некогерентное поперечное сечение рассеяния нейтронов. Сначала рассматриваются модели, в котоьых функции вероятности для индивидуальных состояний движения определяются «задаюшима уравнением, а затем рассматриваются немарковские модели, в которых интенсивности переходов зависят от времени. Рассмотренные модели включают состояния, в котоьых частицы испытывают диффузионное и гармоничеслое движения, движения в стационарном потоке жидкости или постоянно захватываются на ловушки на конечное время.
    Notes: Summary Stochastic models of single-particle motion composed of two or more different states of motion are studied in terms of the pair correlation function that determines the inelastic incoherent neutron crosssection. Two classes of models are considered. First, models in which the probability functions for individual states of motion are determined by master equations and, second, non-Markovian models in which the transition rates are time dependent. Specific models discussed include states in which the particles undergo diffusive and harmonic motions, motion in a steady flow of fluid, or are stationary in traps for a finite time.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 7 (1986), S. 580-586 
    ISSN: 0392-6737
    Keywords: General theories and computational techniques
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Description / Table of Contents: Riassunto Si dimostra che l’interazione elettrostatica neutrone-elettrone, formata dai termini, di Schwinger e Foldy, porta ad una sezione d’urto nella quale la funzione di risposta è la funzione dielettrica inversa osservata nella diffusione dei raggi X e negli esperimenti di perdita degli elettroni. Le condizioni sperimentali per lo scattering fuori dal plasmone acustico nei metalli sono al momento inottenibili e, per vettori d’onda di scattering molto grandi, in cui l’intensità di diffusione è proporzionale al profilo di Compton, la diffusione è troppo debole per essere utile. Per vettori di diffusione intermedi, fuori dalla prima zona di Brillouin, le restrizioni sperimentali sono aconra notevoli anche se l’esperimento è, fattibile, ma non abbiamo calcoli rilevanti della funzione dielettrica, per modelli di struttura di banda, dai quali calcolare la sezione d’urto. I calcoli utilizzabili per la prima zona di Brillouin portano ad attendersi un responso significativo e altamente strutturato.
    Notes: Summary We demonstrate that the electrostatic neutron-electron interaction, formed by the Schwinger and Foldy terms, leads to a cross-section in which the response function is the inverse dielectric function observed in X-ray scattering and electron loss experiments. The experimental conditions for scattering off the acoustic plasmon in metals are unobtainable at present and, at large scattering wave vectors, where the scattered intensity is proportional to the Compton profile, scattering is too weak to be useful. For intermediate scattering vectors, outside the first Brillouin zone, experimental restrictions are still severe although the experiment is feasible but we do not have relevant calculations of the dielectric function, for band structure models, from which to estimate the cross-section. Calculations available for the first Brillouin zone lead us to expect a significant and highly structured response.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 37 (1980), S. 307-315 
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Low temperature theories of the dynamic properties of a one-dimensional Heisenberg magnet are studied in order to interpret the additional low-frequency mode observed, by computer simulation, in ferromagnetically coupled magnets subject to a steady field. A theory which contains energy and magnetisation fluctuations on an equal footing predicts frequencies and amplitudes that are consistent with data for the spectrum of spontaneous magnetisation fluctuations.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 58 (1985), S. 79-82 
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We clarify the relation between a simple viscoelastic theory of density fluctuations, and a recent proposal that describes the spectrum of fluctuations as the sum of three Lorentzian functions of the frequency.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 54 (1984), S. 195-199 
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Low temperature data are given for the spectra of longitudinal and transverse correlation functions of an antiferromagnetic classical heisenberg chain. The one dimensional magnet subjected to large external fields is investigated by means of a computer simulation calculation. The results are tied in with the static properties and compared with theoretical predictions.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 62 (1986), S. 413-421 
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The dynamic structure factor is calculated for a low concentration of light mass scatterers substituted in a cubic crystal matrix. A new numerical method for the exact calculation is demonstrated. We derive a local density of states for the low momentum transfer limit, and derive the shifts and widths of the oscillator peaks in the high momentum transfer limit. We discuss the limitations of an approximation which decouples the defect from the lattice.
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