222 research outputs found
ESS Instrument Construction Proposal CAMEA
No full textWe propose the construction of a highly innovative spectrometer – CAMEA – offering Continuous Angular and Multiple Energy Analysis. Combining indirect time-of flight with multiple consecutive analyser arrays, this instrument will provide massive flux on the sample and strongly enhanced efficiency in detecting neutrons scattered in the horizontal scattering. The combination yields a spectrometer with completely unprecedented performance - with gains from 2 up to 4 orders of magnitude compared to current state of the art.LQMThis work is carried out as part of the CAMEA work project. Members of this project are: Paul G. Freeman, Henrik M. Ronnow (EPFL), Christof Niedermayer, Fanni Jurányi, Márton Markó (PSI) Kim Lefmann, Jonas Okkels Birk, Mads Bertelsen (University of Copenhagen) Niels Bech Christensen, Jacob Larsen (Technical University of Denmark
Benching Mark CAMEA Against Present Inelastic Neutron Spectrometers
No full textThe CAMEA concept will be compared to the world leading spectrometers to grade performance.LQMThis work is carried out as part of the CAMEA work project. Members of this project are: Paul G. Freeman, Henrik M. Ronnow (EPFL), Christof Niedermayer, Fanni Jurányi, Márton Markó (PSI) Kim Lefmann, Jonas Okkels Birk, Mads Bertelsen (University of Copenhagen) Niels Bech Christensen, Jacob Larsen (Technical University of Denmark
Ungewöhnliche magnetische Eigenschaften von niedrigdimensionalen Antiferromagneten BaNi2V2O8 und BaCu2V2O8
No full textThis work explores the magnetic properties of two novel compounds BaCu2V2O8 and BaNi2V2O8 which both provide rare physical realizations of model magnetic systems that are characterized by unconventional magnetic behaviour at finite temperatures. BaNi2V2O8 has an ideal honeycomb crystal structure where S=1 nickel magnetic ions are arranged on well isolated honeycomb layers. The nearest neighbour magnetic ions within the plane interact with each other via the dominant antiferromagnetic exchange interaction while the interaction between the honeycomb planes is found to be very weak. The spins develop long-range magnetic order in their ground state and lie entirely within the honeycomb plane forming collinear arrangement so that each spin is antiparallel to its 3 nearest neighbours. The crystal and magnetic structure of BaNi2V2O8 imply that this compound is a two-dimensional planar antiferromagnet. The magnetic properties of this compound were explored using DC susceptibility, powder and single crystal inelastic neutron scattering measurements as well as single crystal neutron diffraction measurements whose results were analysed using different theoretical approaches. The single crystal measurements at base temperature reveal a spin-wave magnetic excitation spectrum which disperses within the honeycomb plane but is completely dispersionless in the out-of-plane direction confirming the strongly two-dimensional magnetic behaviour of this compound. The dispersion of the magnetic excitation spectrum at base temperature was analysed using linear spin-wave theory which allowed the Hamiltonian of BaNi2V2O8 to be solved. The critical phenomena in BaNi2V2O8 were explored by performing the neutron diffraction measurements at finite temperatures both below and above the ordering temperature TN. The extracted critical exponents reveal that BaNi2V2O8 behaves as a two-dimensional antiferromagnet over the whole explored temperature range displaying the crossovers from 2D Ising like to 2D XY and then to 2D Heisenberg magnetic behaviour with increasing temperature. The second compound is BaCu2V2O8 which has tetragonal symmetry where the magnetic
S=1/2 Cu^{2+} ions form screw chains along the c-axis. The DC susceptibility data reveal a non-magnetic ground state and were fitted well by the dimer-model which allows the magnetic intra- and interdimer exchange constants to be estimated. The single crystal inelastic neutron scattering measurements of BaCu2V2O8 reveal that the magnetic excitation spectrum is gapped and characterized by a high ratio of the gap to the bandwidth which equals 3.6 indicating a strong dimerization. The magnetic excitations disperse along the chain direction but are completely dispersionless within the tetragonal plane implying that BaCu2V2O8 is a strongly dimerized 1D chain where the dimers are coupled together along the c-axis. The magnetic excitation spectrum was analysed using the theoretical model for the 1D alternating chain which allowed the Hamiltonian of BaCu2V2O8 to be solved. The results were verified by comparison with theoretical simulations of the spectra using this Hamiltonian. The strong dimerization of BaCu2V2O8 makes this compound a good candidate for the observation and detailed investigation of strongly correlated phenomena at finite temperatures. The thermal behaviour of the magnetic excitations in BaCu2V2O8 were explored by analysing of the temperature dependence of their lineshape at finite temperatures over a wide temperature range. The analysis was performed using a new fitting function and revealed the extraordinary coherence of the magnetic excitations in BaCu2V2O8 at finite temperatures which manifests itself in an asymmetric thermal lineshape broadening in contrast to the Lorentzian lineshapes observed in conventional magnets. The observed results were verified by comparison with the results of theoretical simulations of the magnetic excitations at finite temperature in BaCu2V2O8 which were performed using both numerical and analytical theoretical approaches.In dieser Arbeit werden die magnetischen Eigenschaften von zwei neuen Verbindungen untersucht, BaNi2V2O8 und BaCu2V2O8. Beide Substanzen zeichnen sich durch ihren besonderen Modellcharakter aus der schon bei endlicher Temperatur zu ungewöhnlichen magnetischen Verhalten führt. BaNi2V2O8 kristallisiert mit einer bienenwaben Struktur in der sich die magnetischen, S=1 Ni^{2+} - Ionen anordnen. Die bienenwaben Ebenen sind voneinander gut getrennt. In der Ebene besteht zwischen den magnetischen Ionen eine dominante antiferromagnetische Wechselwirkung zu den nächsten Nachbarn, die Wechselwirkung zwischen den Ebenen ist sehr schwach. Die Momente zeigen im Grundzustand langreichweitige Ordnung. Sie sind vollständig in der bienenwaben Ebene angeordnet mit einer kollinearen Ausrichtung, jeder Spin ist antiparallel zu den drei nächsten Nachbarn ausgerichtet. Die chemische und magnetische Struktur von BaNi2V2O8 kann als zwei-dimensionaler (2D), planarer Antiferromagnet verstanden werden. Die magnetischen Eigenschaften dieser Verbindung wurden mit statischen Magnetisierungsmessungen, sowie inelastischer Neutronenstreuung an Pulver- und Einkristallproben als auch mit elastischer Neutronenstreuung an Einkristallen untersucht. Die Ergebnisse werden mit verschiedenen theoretischen Ansätzen analysiert. Die inelastischen Messungen mit Neutronenstreuung bei Basistemperatur zeigen Dispersion in der bienenwaben Ebene, während senkrecht zu den Ebenen keine Dispersion beobachtet wird. Dies zeigt den stark zwei - dimensionalen Charakter dieser Verbindung. Die Dispersion der magnetischen Anregungen bei Basistemperatur wird anhand von linearer Spin - Wellen Theorie untersucht, damit kann der Hamiltonian von BaNi2V2O8 bestimmt werden. Das kritische Verhalten von BaNi2V2O8 wird mit Neutronenstreuung unter- und oberhalb der Ordnungstemperatur TN untersucht. Die dabei bestimmten kritischen Exponenten zeigen, dass sich BaNi2V2O8 über den gesamten untersuchten Temperaturbereich wie ein zwei - dimensionaler Antiferromagnet verhält mit Übergängen bei steigender Temperatur von 2D - Ising zu 2D - XY hin zu 2D - Heisenberg Verhalten. Die zweite Verbindung, BaCu2V2O8, hat eine tetragonale Symmetrie und die magnetischen S=1/2 Cu^2+ - Ionen bilden eine schraubenförmige Kette längs der c-Achse. Die statische Suszeptibilität zeigt einen unmagnetischen Grundzustand. Die Daten werden mit einem Dimer Modell gefittet, daraus kann die Dimer - Wechselwirkung und die Wechselwirkung zwischen den Dimeren bestimmt werden. Die inelastische Neutronenstreuung an Einkristallen zeigt eine Energielücke der magnetischen Anregungen. Das Verhältnis zwischen der Energielücke und der Breite des magnetischen Spektrums beträgt 3.6. Dies ist ein Hinweis auf starke Dimerisierung. Die magnetischen Anregungen zeigen Dispersion entlang der Spinketten. In der tetragonalen Ebene zeigt das Spektrum keine Dispersion. Daher ist BaCu2V2O8 eine stark dimerisierte 1D-Spinkette in der die Dimere längs der c-Achse wechselwirken. Das Spektrum der magnetischen Anregungen wird mit einem Modell für die 1D alternierende Spinkette untersucht, damit konnte die Hamilton Funktion von BaCu2V2O8 bestimmt werden. Die Ergebnisse werden verifiziert durch Vergleich der gemessenen Spektren mit Simulationen der Spektren. Die starke Dimerisierung von BaCu2V2O8 macht diese Substanz zu einem guten Modellsystem für hochkorrelierte Phänomene bei endlichen Temperaturen, sowohl in experimenteller wie auch theoretischer Hinsicht. Das thermische Verhalten der magnetischen Anregungen von BaCu2V2O8 wird untersucht mit der Linienform der Anregungen über einen großen Temperaturbereich. Die Analyse benutzt eine neue Funktion zur Anpassung der Linienform im Gegensatz zum Lorentz Verhalten, das gewöhnlich in magnetischen Substanzen gefunden wird. Es zeigt
sich - bei endlichen Temperaturen - eine außerordentlich hohe Kohärenz der magnetischen Anregungen, die sich in einer asymmetrischen, Linienform manifestiert. Diese Ergebnisse werden verifiziert durch den Vergleich theoretischer Spektren bei endlicher Temperatur, die mit numerischen und analytischen Methoden gewonnen werden
Diffuse spin waves and classical spin liquid behavior in the kagomé antiferromagnet chromium jarosite, KCr<sub>3</sub>(OD)<sub>6</sub>(SO4)<sub>2</sub>
No full textThe dynamics of the S = 3/2 kagomé antiferromagnet chromium jarosite, KCr3(OD)6(SO4)2, was studied using high-resolution neutron time-of-flight spectroscopy on a polycrystalline sample with a nearly stoichiometric magnetic lattice [2.8(2)% Cr vacancies]. Neutron spectroscopy reveals diffuse spin wave excitations in the ordered phase with an incomplete gap and significant finite-lifetime broadening as well as a pronounced kagomé zero mode. Using linear spin wave theory, we estimate the exchange couplings. The system is highly two dimensional with the leading nearest-neighbor coupling being J1=0.881 meV. Above TN, diffuse excitations from the classical spin liquid regime dominate. We model the Q response and energy response separately and show that in both the ordered phase and the classical spin liquid regime they are strongly coupled.</p
Spin-fluctuation mediated superconductivity and magnetic order in the cuprate La1.88Sr0.12CuO4
No full textHigh-temperature superconductivity in cuprates emerges as one out of many electronic phases when doping the antiferromagnetic Mott insulator La2CuO4 away from half lling. The description of the superconducting phase is therefore complicated by intertwined electronic orders that compete with superconductivity. However, it is possible that the tendency towards additional ordering phenomena is a necessary condition for unconventional superconductivity to develop. Indeed most superconductors discovered throughout the last 29 years, including also the recently discovered class of iron-based superconductors, show a very rich electronic phase diagram. A common feature that characterizes both cuprates, heavy fermions, and iron pnictides is the proximity to magnetic order. Therefore, the idea of spin- uctuation mediated pairing is a popular paradigm proposed for unconventional superconductivity. A ngerprint of the pairing mechanism is found in the superconducting gap symmetry. Therefore the study of gap symmetries constitutes one of the most important parts of resolving the superconducting puzzle. This thesis consists of a theoretical and an experimental part. In the theoretical part, we address spin- uctuation mediated pairing in the weak-coupling limit by a systematic study of the one-band Hubbard model. We consider the problem of spin- uctuation mediated pairing in a paramagnetic system, as well as a system in which spin-density wave order is wellestablished. In addition, we describe how disorder locally aects the spin susceptibility and its implications on the local superconducting gap. In the experimental part, we use neutron scattering experiments to investigate magnetic order and low-energy spin uctuations in the cuprate superconductor La1:88Sr0:12CuO4. This crystal is superconducting with Tc = 27 K and shows incommensurate magnetic order as well as spin uctuations which are "stripe-like" and distinct from the antiferromagnetic order of the parent compound La2CuO4. Yet some remnant features of the original La2CuO4 spin order remain. For instance, we nd that despite the glassy nature of the low-energy spin uctuations in La1:88Sr0:12CuO4, the spectrum supports small anisotropy gaps similar to the observations of anisotropy gaps in the parent compound as well as stripe-ordered La1:875Ba0:125CuO4.LQ
Neutron optics and Monte Carlo simulations in NMI3
No full textAtomic & Nuclear Physics; Particle & High Energy Physics; Udgivelsesdato: Januar
Special Issue:Workshop on the Monte Carlo Simulation of Neutron Scattering Instruments MCNSI7
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