Neutron triple-axis spectrometry

Résumé

Triple-axis spectrometry (TAS, T also resolved as "three", S also resolved as "spectroscopy") is a technique used in inelastic neutron scattering. The instrument is referred to as triple-axis spectrometer (also called TAS). It allows measurement of the scattering function at any point in energy and momentum space physically accessible by the spectrometer. History The triple-axis spectrometry method was first developed by Bertram Brockhouse at the National Research Experimental NRX reactor at the Chalk River Laboratories in Canada. The first results from the prototype triple-axis spectrometer were published in January 1955 and the first true triple-axis spectrometer was built in 1956. Bertram Brockhouse shared the 1994 Nobel prize for Physics for this development, which allowed elementary excitations, such as phonons and magnons, to be observed directly. The Nobel citation was "for pioneering contributions to the development of neutron scattering techniques for studies

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https://en.wikipedia.org/wiki/Neutron_triple-axis_spectrometry

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Neutron scattering, the irregular dispersal of free neutrons by matter, can refer to either the naturally occurring physical process itself or to the man-made experimental techniques that use the natu

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A thorough experimental characterization of a multiplexing backend with multiple energy analysis on a cold-neutron triple axis spectrometer (cTAS) is presented. The prototype employs two angular segments (2 theta-segments) each containing five vertically scattering analyzers (energy channels), which simultaneously probe an energy transfer range of 2 meV at the corresponding two scattering angles. The feasibility and strength of such a vertically scattering multiple energy analysis setup is clearly demonstrated. It is shown, that the energy resolution near the elastic line is comparable to the energy resolution of a standard cTAS. The dispersion relation of the antiferromagnetic excitations in MnF2 has been mapped out by performing constant energy transfer maps. These results show that the tested setup is virtually spurion free. In addition, focusing effects due to (mis)matching of the instrumental resolution ellipsoid to the excitation branch are clearly evident. (C) 2016 Elsevier B.V. All rights reserved.

Elsevier2016

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This thesis is devoted to the investigation of static and dynamic properties of two different sets of quantum magnets with neutron scattering techniques and the help of linear spin wave theory. Both systems are copper-based with spin-1/2, which makes them ideal to study the interplay between purely quantum and semi-classical effects. I start with the analysis of the antiferromagnet SeCuO3, which has a canted spins structure. Through careful inelastic neutron scattering experiments on thermal and cold triple-axis spectrometers, I demonstrate that this compound exhibits three primary types of excitations that are intrinsically opposite : spin waves (magnons), singlet to triplet excitations (triplons), and fractional spins excitations (spinons). Such a strong coexistence and interdependence of these collective excitations has not been observed yet, thereby the quantification and description of the excitations in SeCuO3 leads the way to further theoretical work on multi-excitation spin systems, as well as the existence of quantum effects in high dimensional systems.\ My second project is on the extraction of the magnetic structure of three members of the A(BO)Cu4(PO4)4 chiral family, namely (A; B) = (Ba; Ti), (Sr; Ti) and (Pb; Ti), from spherical neutron polarimetry measurements. I prove that the first two compounds exhibit a highly non-collinear magnetic structure, with the Cu spins forming clusters of 'two-in--two-out' arrangements on each structural unit. This structure is stabilised by the presence of a strong Dzyaloshinskii-Moriya interaction, and explains the observation of magnetoelectric effects as emerging from quadrupole moments. The analysis of the latter compound did not lead to the confirmation of its magnetic structure due to strong nuclear-magnetic interference. I conclude this thesis by the investigation of the magnetic excitation spectrum of some members of the (A; B) family, probed by inelastic neutron scattering measurements. Indeed, its particular crystallographic structure makes it an ideal playground to study tetramerisation effects on the two dimensional square lattice. Additionally, the aforementioned Dzyaloshinskii-Moriya interaction ensures the presence of a structural gap, which competes with the quantum one emerging from tetramerisation effects. Using linear spin wave theory, I describe (Ba; Ti) as a chequerboard system with almost equal intra- and inter-plaquette couplings, with weak quantum effects. I also provide a qualitative description of (Pb; Ti), which exhibits similar physics, and conclude by presenting the first results on the highly symmetric compound (K; Nb), which shows hints of a strong quantum behaviour.

EPFL2021

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CAMEA—A novel multiplexing analyzer for neutron spectroscopy

Felix Groitl, Henrik Moodysson Rønnow, Christian Rüegg

The analyzerdetector system continuous angle multiple energy analysis will be installed on the cold-neutron triple-axis spectrometer RITA-2 at SINQ, PSI. CAMEA is optimized for efficiency in the horizontal scattering plane enabling rapid and detailed mapping of excitations. As a novelty the design employs a series of several sequential upward scatteringanalyzer arcs. Each arc is set to a different, fixed, final energy and scattersneutrons towards position sensitive detectors. Thus, neutrons with different final energies are recorded simultaneously over a large angular range. In a single data-acquisition many entire constant-energy lines in the horizontal scattering plane are recorded for a quasi-continuous angular coverage of about 60°. With a large combined coverage in energy and momentum, this will result in a very efficient spectrometer, which will be particularly suited for parametric studies under extreme conditions with restrictive sample environments (high field magnets or pressure cells) and for small samples of novel materials. In this paper we outline the concept and the specifications of the instrument currently under construction.

American Institute of Physics2016

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EPFL2021