Search Results - (Author, Cooperation:S. W. Lovesey)

2018-10-10

American Physical Society (APS)

1098-0121

1095-3795

Physics

Surface physics, nanoscale physics, low-dimensional systems

2011-04-09

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2018-08-31

American Physical Society (APS)

1098-0121

1095-3795

Physics

Magnetism

2018-09-12

American Physical Society (APS)

1098-0121

1095-3795

Physics

Electronic structure and strongly correlated systems

1434-6036

Springer Online Journal Archives 1860-2000

Physics

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.

Electronic Resource

0392-6737

Emission, absorption and scattering of radiation

Springer Online Journal Archives 1860-2000

Physics

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.

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.

Electronic Resource

0392-6737

Neutron determination of structures

Springer Online Journal Archives 1860-2000

Physics

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.

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.

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.

Electronic Resource

0392-6737

General theories and computational techniques

Springer Online Journal Archives 1860-2000

Physics

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.

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.

Electronic Resource

1434-6036

Springer Online Journal Archives 1860-2000

Physics

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.

Electronic Resource

1434-6036

Springer Online Journal Archives 1860-2000

Physics

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.

Electronic Resource

1434-6036

Springer Online Journal Archives 1860-2000

Physics

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.

Electronic Resource

1434-6036

Springer Online Journal Archives 1860-2000

Physics

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.

Electronic Resource

1434-6036

Springer Online Journal Archives 1860-2000

Physics

Abstract The inadequacy of the standard coupled-mode theory of one dimensional paramagnets is resolved by renormalisation of the exchange coupling and time scale, in such a way that the refined theory is consistent with some static spin cumulants. With the proposed refinements the coupled-mode theory gives the exact spin dynamics in the limit of vanishing temperatures. For low temperatures the theory is studied numerically to achieve comparisons with computer simulation data that vindicate the renormalisation scheme.

Electronic Resource

1434-6036

Springer Online Journal Archives 1860-2000

Physics

Abstract Dynamic spin correlations in the spin-flop phase of a Heisenberg antiferromagnet are studied in terms of the quantities that determine the inelastic scattering of neutrons from single spin-wave states. A complete expression for the cross-section is given using linear spin-wave theory. The non-linear features of the spin dynamics which are intrinsic to the spin-flop state are studied by perturbation theory. Two limiting cases are considered, and these correspond to states where the spins are close to an antiferromagnetic configuration (weak field limit) and the almost completely field aligned configuration. Analytic expressions are given for the appropriate collisional self-energies, in the limit of low temperatures, for selected wavevectors in one and three dimensional systems.

Electronic Resource

1434-6036

Springer Online Journal Archives 1860-2000

Physics

Abstract Long wavelength density fluctuations in a classical liquid are discussed in terms of a model which has the correct small frequency behaviour and the correct high frequency limit, neglecting thermal relaxation effects. Moreover, the first three frequency moments are correct, so that the model should be well suited for the interpretation of inelastic neutron scattering experiments on supercooled liquids.

Electronic Resource

1434-6036

Springer Online Journal Archives 1860-2000

Physics

Abstract We calculate the response for scattering off a particle bound by an isotropic harmonic force and damped through hybridization with a phonon bath. An exact expression for the scattering response is studied numerically to provide a detailed picture of its dependence on wave vector, frequency, temperature and phonon hybridization strength.

Electronic Resource

1434-6036

Springer Online Journal Archives 1860-2000

Physics

Abstract We argue that polarisation analysis of scattering of an unpolarised beam from a single crystal is a useful technique for looking at crystal field excitations. A general expression for the polarisation in the scattered beam is presented and two illustrative examples worked out.

Electronic Resource

1434-6036

Springer Online Journal Archives 1860-2000

Physics

Abstract The structure of annealed and quenched models with localU(1) gauge invariance is studied in terms of the Helmholtz free energy. The first non-trivial, or one-loop, account of fluctuations in the annealed model suggests that spontaneous symmetry breaking occurs in two and three dimensions, through a first-order phase transition. Within the same approximation scheme, the quenched model displays a continuous phase transition. A more complete account of the fluctuations in the annealed model changes the nature of the transition to a continuous one, whereas spontaneous symmetry breaking is then absent with quenched disorder.

Electronic Resource

1434-6036

Springer Online Journal Archives 1860-2000

Physics

Abstract We study the influence of the non-diagnonal terms in an easy-plane Heisenberg ferromagnet on the cross-section for inelastic neutron scattering from single spin-wave states. We show that these terms modify the wavevector dependence of the cross-section for unpolarized neutrons, and the created polarization. An appropriate experiment on CsNiF3, for example, should be a good test of these predictions, and would eventually clarify some uncertainties about the appropriate Boson Hamiltonian for an easy-plane ferromagnet.

Electronic Resource

1434-6036

Springer Online Journal Archives 1860-2000

Physics

Abstract We have calculated thef-sum rule for magnetic neutron scattering off electrons. Correlations alter the cross-section significantly at intermediate scattering vectors. Field induced effects are most significant at small scattering vectors, and the sum rule diverges in the limit of zero scattering vector. This feature is attributed to the field induced coupling of neutrons to the lowest energy Landau level and the collective density oscillation (hybrid mode). Our interpretation is based on an RPA calculation of the response function for neutron scattering from a magnetized plasma. The contribution to the cross-section from the electron-phonon interaction is also assessed, and it is found to increase with decreasing scattering vector.

Electronic Resource