| 1. |
- Artyukhin, Sergey, et al.
(author)
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Solitonic lattice and Yukawa forces in the rare-earth orthoferrite TbFeO3
- 2012
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In: Nature Materials. - 1476-4660. ; 11:8, s. 694-699
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Journal article (peer-reviewed)abstract
- The random fluctuations of spins give rise to many interesting physical phenomena, such as the 'order-from-disorder' arising in frustrated magnets and unconventional Cooper pairing in magnetic superconductors. Here we show that the exchange of spin waves between extended topological defects, such as domain walls, can result in novel magnetic states. We report the discovery of an unusual incommensurate phase in the orthoferrite TbFeO3 using neutron diffraction under an applied magnetic field. The magnetic modulation has a very long period of 340 angstrom at 3 K and exhibits an anomalously large number of higher-order harmonics. These domain walls are formed by Ising-like Tb spins. They interact by exchanging magnons propagating through the Fe magnetic sublattice. The resulting force between the domain walls has a rather long range that determines the period of the incommensurate state and is analogous to the pion-mediated Yukawa interaction between protons and neutrons in nuclei.
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| 2. |
- Brok, Erik, et al.
(author)
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Polarized neutron powder diffraction studies of antiferromagnetic order in bulk and nanoparticle NiO
- 2015
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 91:1
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Journal article (peer-reviewed)abstract
- In many materials it remains a challenge to reveal the nature of magnetic correlations, including antiferromagnetism and spin disorder. Revealing the spin structure in magnetic nanoparticles is further complicated by the large incoherent neutron scattering cross section from water adsorbed at the particle surfaces and by the broadening of diffraction peaks due to the finite crystallite size. Moreover, the spin structure in magnetic nanoparticles may deviate significantly from that of the corresponding bulk material because of the low-symmetry surroundings of surface atoms and the large relative surface contribution to the magnetic anisotropy. Here we explore the potential use of polarized neutron diffraction to reveal the magnetic structure in NiO bulk and nanoparticle powders by applying the XYZ-polarization analysis method. Our investigations address in particular the spin orientation in bulk NiO and platelet-shaped NiO nanoparticles with thickness from greater than 200 nm down to 2.0 nm. The advantage of the applied method is that it is able to clearly separate the structural, the magnetic, and the spin-incoherent scattering signals for all particle sizes. For platelet-shaped particles with thickness from greater than 200 nm down to 2.2 nm we find that the spin orientation deviates about 16 degrees from the primary (111) plane of the platelet-shaped particles. In the smallest particles (2.0 nm thick) we find the spins are oriented with a 30 degrees. average angle to the primary (111) plane of the particles. The results show that polarization analyzed neutron powder diffraction is a viable method to investigate magnetic order in powders of antiferromagnetic nanoparticles.
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| 3. |
- Fedrigo, Anna, et al.
(author)
-
VESPA : The vibrational spectrometer for the European Spallation Source
- 2016
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In: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 87:6
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Journal article (peer-reviewed)abstract
- VESPA, Vibrational Excitation Spectrometer with Pyrolytic-graphite Analysers, aims to probe molecular excitations via inelastic neutron scattering. It is a thermal high resolution inverted geometry time-of-flight instrument designed to maximise the use of the long pulse of the European Spallation Source. The wavelength frame multiplication technique was applied to provide simultaneously a broad dynamic range (about 0-500 meV) while a system of optical blind choppers allows to trade flux for energy resolution. Thanks to its high flux, VESPA will allow the investigation of dynamical and in situ experiments in physical chemistry. Here we describe the design parameters and the corresponding McStas simulations.
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| 4. |
- Holm, Sonja L., et al.
(author)
-
HEIMDAL : A thermal neutron powder diffractometer with high and flexible resolution combined with SANS and neutron imaging - Designed for materials science studies at the European Spallation Source
- 2016
-
In: Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 828, s. 229-241
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Journal article (peer-reviewed)abstract
- HEIMDAL will be a multi length scale neutron scattering instrument for the study of structures covering almost nine orders of magnitude from 0.01 nm to 50 mm. The instrument is accepted for construction at the European Spallation Source (ESS) and features a variable resolution thermal neutron powder diffractometer (TNPD), combined with small angle neutron scattering (SANS) and neutron imaging (NI). The instrument uses a novel combination of a cold and a thermal guide to fulfill the diverse requirements for diffraction and SANS. With an instrument length of 170 m, HEIMDAL will take advantage of the high neutron flux of the long pulse at ESS, whilst maintaining a high q-resolution due to the long flight path. The q-range coverage is up to 20 Å-1 allowing low-resolution PDF analysis. With the addition of SANS, HEIMDAL will be able to cover a uniquely broad length scale within a single instrumental set-up. HEIMDAL will be able to accommodate modern materials research in a broad variety of fields, and the task of the instrument will be to study advanced functional materials in action, as in situ and in operandi at multiple length scales (0.01-100 nm) quasi simultaneously. The instrument combines state-of-the-art neutron scattering techniques (TNPD, SANS, and NI) with the goal of studying real materials, in real time, under real conditions. This article describes the instrument design ideas, calculations and results of simulations and virtual experiments.
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| 5. |
- Holm, Sonja L., et al.
(author)
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Neutron guide-split: A high performance guide bundle concept for elliptical guides
- 2015
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In: Nuclear Instruments & Methods in Physics Research. Section A: Accelerators, Spectrometers, Detectors, and Associated Equipment. - : Elsevier BV. - 0167-5087 .- 0168-9002. ; 782, s. 1-8
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Journal article (peer-reviewed)abstract
- We present a new guide-split concept for transporting cold and thermal neutrons to multiple instruments from a single beam port at a neutron facility without compromising the useful neutron brilliance notably for any of the instruments. Elliptical guides are capable of transporting an almost completely filled phase space within a large divergence (+/- 2 degrees for cold neutrons). It is therefore possible to place several secondary guides side by side pointing in slightly different directions using the end of a primary guide as a virtual source. The instruments placed at the secondary guides hence exploit different parts of the phase space transported by the primary guide. In addition, the resulting kink between the primary and secondary guide eliminates line of sight. Using ray-tracing simulations of three different set-ups (with two, four, and eight secondary guides) we show that it is possible to illuminate at least eight sample positions from one beam port with a brilliance transfer above 90% on each sample on a 150 m long instrument. This has been clone for a phase space volume comprised of an area of 1 x 1 cm(2) and a maximum divergence of +/- 0.5 degrees within a wavelength band of 425-5.75 angstrom. We show, by a full virtual experiment, an example of applying the guide-split concept to an instrument proposed for the European Spallation Source, namely a magnetism diffractometer. (C) 2015 Elsevier B.V. All rights reserved,
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| 6. |
- Jacobsen, Henrik, et al.
(author)
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Spin dynamics of the director state in frustrated hyperkagome systems
- 2021
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In: Physical Review B. - : American Physical Society. - 2469-9969 .- 2469-9950. ; 104:5
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Journal article (peer-reviewed)abstract
- We present an experimental study of the magnetic structure and dynamics of two frustrated hyperkagome compounds, Gd3Ga5O12 and Gd3Al5O12. It has previously been shown that Gd3Ga5O12 exhibits long-range correlations of multipolar directors that are formed from antiferromagnetic spins on loops of ten ions. Using neutron diffraction and reverse Monte Carlo simulations we prove the existence of similar magnetic correlations in Gd3Al5O12, showing the ubiquity of these complex structures in frustrated hyperkagome materials. Using inelastic neutron scattering we shed further light on the director state and the associated low-lying magnetic excitations. In addition, we have measured quasielastic dynamics that show evidence of spin diffusion. Finally, we present AC susceptibility measurements on both Gd3Ga5O12 and Gd3Al5O12, revealing a large difference in the low-frequency dynamics between the two otherwise similar compounds.
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| 7. |
- Kleno, Kaspar H., et al.
(author)
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Eliminating line of sight in elliptic guides using gravitational curving
- 2011
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In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 634, s. 100-103
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Conference paper (peer-reviewed)abstract
- Eliminating fast neutrons (lambda < 0.5 angstrom) by removing direct line of sight between the source and the target sample is a well established technique. This can be done with little loss of transmission for a straight neutron guide by horizontal curving. With an elliptic guide shape, however, curving the guide would result in a breakdown of the geometrical focusing mechanism inherent to the elliptical shape, resulting in unwanted reflections and loss of transmission. We present a new and yet untried idea by curving a guide in such a way as to follow the ballistic curve of a neutron in the gravitational field, while still retaining the elliptic shape seen from the accelerated reference frame of the neutron. Analytical calculations and ray-tracing simulations show that this method is useful for cold neutrons at guide lengths in excess of 100 m. We will present some of the latest results for guide optimization relevant for instrument design at the ESS, in particular an off-backscattering spectrometer which utilizes the gravitational curving, for 6.66 angstrom neutrons over a guide length of 300m. (C) 2010 Elsevier B.V. All rights reserved.
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| 8. |
- Kleno, Kaspar Hewitt, et al.
(author)
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Systematic performance study of common neutron guide geometries
- 2012
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In: Nuclear Instruments & Methods in Physics Research. Section A: Accelerators, Spectrometers, Detectors, and Associated Equipment. - : Elsevier BV. - 0167-5087 .- 0168-9002. ; 696, s. 75-84
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Journal article (peer-reviewed)abstract
- In this report, we present the results from a systematic benchmarking of four different long neutron guide geometries: elliptic, parabolic, ballistic (piecewise linearly focusing/defocusing), and straight, for various wavelength, divergence restriction, and guide length settings. In this work, we mapped relevant parts of the neutron phase space to show where advanced guide geometries have significant transport advantages over simple guide geometries. The primary findings are that the elliptic and parabolic geometries perform almost equally well, and they are considerably superior to the other geometries, except for low-divergence, cold neutrons. In addition, it was observed that transporting thermal neutrons more than 100 m using elliptic guides was possible with only a 10% loss in the phase space density for divergences up to +/- 0.5 degrees, which enables the construction of very long thermal neutron instruments. Our work will allow instrument designers to use tabulated, standard geometries as a starting point for optimising the guide required for the particular instrument. (C) 2012 Elsevier B.V. All rights reserved.
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| 9. |
- Lefmann, Kim, et al.
(author)
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Optimal shape of a cold-neutron triple-axis spectrometer
- 2011
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In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 634, s. 1-6
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Conference paper (peer-reviewed)abstract
- We have performed a McStas optimization of the primary spectrometer for a generic 40 m long, cold-neutron triple-axis spectrometer with a doubly focusing monochromator. The optimal design contains an elliptically focusing guide, a virtual source point before a low-grade PG monochromator, and non-equidistant focusing at the monochromator. The flux at 5 meV shows a gain factor 12 over the "classical" design with a straight 12 x 3 cm(2), m=2 guide and a vertically focusing PG monochromator. In addition, the energy resolution was found to be improved. This unexpectedly large design improvement agrees with the Liouville theorem and can be understood as the product of many smaller gain factors, combined with a more optimal utilization of the beam divergence within the guide. Our results may be relevant for a possible upgrade of a number of cold-neutron triple-axis spectrometers and for a possible triple-axis spectrometer at the European Spallation Source. (C) 2010 Elsevier B.V. All rights reserved.
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| 10. |
- Lefmann, Kim, et al.
(author)
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Simulation of a suite of generic long-pulse neutron instruments to optimize the time structure of the European Spallation Source
- 2013
-
In: Review of Scientific Instruments. - : AIP Publishing. - 1089-7623 .- 0034-6748. ; 84:5
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Journal article (peer-reviewed)abstract
- We here describe the result of simulations of 15 generic neutron instruments for the long-pulsed European Spallation Source. All instruments have been simulated for 20 different settings of the source time structure, corresponding to pulse lengths between 1 ms and 2 ms; and repetition frequencies between 10 Hz and 25 Hz. The relative change in performance with time structure is given for each instrument, and an unweighted average is calculated. The performance of the instrument suite is proportional to (a) the peak flux and (b) the duty cycle to a power of approximately 0.3. This information is an important input to determining the best accelerator parameters. In addition, we find that in our simple guide systems, most neutrons reaching the sample originate from the central 3-5 cm of the moderator. This result can be used as an input in later optimization of the moderator design. We discuss the relevance and validity of defining a single figure-of-merit for a full facility and compare with evaluations of the individual instrument classes. (C) 2013 AIP Publishing LLC.
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| 11. |
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| 12. |
- Nocerino, Elisabetta, et al.
(author)
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Structural Evolution and Onset of the Density Wave Transition in the CDW Superconductor LaPt2Si2 Clarified with Synchrotron XRD
-
Other publication (other academic/artistic)abstract
- The quasi-2D Pt-based rare earth intermetallic material LaPt2Si2 has attracted attention as it exhibits strong interplay between charge density wave (CDW) and and superconductivity (SC). However, the most of the results reported on this material come from theoretical calculations, preliminary bulk investigations and powder samples, which makes it difficult to uniquely determine the temperature evolution of its crystal structure and, consequently, of its CDW transition. Therefore, the published literature around LaPt2Si2 is often controversial. In this paper, we clarify the complex evolution of the crystal structure, and the temperature dependence of the development of density wave transitions, in good quality LaPt2Si2 single crystals, with high resolution synchrotron X-ray diffraction data. According to our findings, on cooling from room temperature LaPt2Si2 undergoes a series of subtle structural transitions which can be summarised as follows: second order commensurate tetragonal (P4/nmm)-to-incommensurate structure followed by a first order incommensurate-to-commensurate orthorhombic (Pmmn) transition and then a first order commensurate orthorhombic (Pmmn)-to-commensurate tetragonal (P4/nmm). The structural transitions are accompanied by both incommensurate and commensurate superstructural distortions of the lattice. The observed behavior is compatible with discommensuration of the CDW in this material.
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| 13. |
- Sandberg, Lise Orduk, et al.
(author)
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Emergent magnetic behavior in the frustrated Yb3Ga5O12 garnet
- 2021
-
In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 104:6
-
Journal article (peer-reviewed)abstract
- We report neutron scattering, magnetic susceptibility and Monte Carlo theoretical analysis to verify the short-range nature of the magnetic structure and spin-spin correlations in a Yb3Ga5O12 single crystal. The quantum spin state of Yb3+ in Yb3Ga5O12 is verified. The quantum spins organize into a short-ranged emergent director state for T < 0.6 K derived from anisotropy and near-neighbor exchange. We derive the magnitude of the nearneighbor exchange interactions 0.6 < J(1) < 0.7 K, J(2) = 0.12 K and the magnitude of the dipolar exchange interaction, D, in the range 0.18 < D < 0.21 K. Certain aspects of the broad experimental dataset can be modeled using a J(1)D model with ferromagnetic near-neighbor spin-spin correlations while other aspects of the data can be accurately reproduced using a J(1)J(2)D model with antiferromagnetic near-neighbor spin-spin correlation. As such, although we do not quantify all the relevant exchange interactions, we nevertheless provide a strong basis for the understanding of the complex Hamiltonian required to fully describe the magnetic state of Yb3Ga5O12.
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| 14. |
- Udby, L., et al.
(author)
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Analysing neutron scattering data using McStas virtual experiments
- 2011
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In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 634, s. 138-143
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Conference paper (peer-reviewed)abstract
- With the intention of developing a new data analysis method using virtual experiments we have built a detailed virtual model of the cold triple-axis spectrometer RITA-II at PSI, Switzerland, using the McStas neutron ray-tracing package. The parameters characterising the virtual instrument were carefully tuned against real experiments. In the present paper we show that virtual experiments reproduce experimentally observed linewidths within 1-3% for a variety of samples. Furthermore we show that the detailed knowledge of the instrumental resolution found from virtual experiments, including sample mosaicity, can be used for quantitative estimates of linewidth broadening resulting from, e.g., finite domain sizes in single-crystal samples. (C) 2010 Elsevier B.V. All rights reserved.
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| 15. |
- Udby, L., et al.
(author)
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Magnetic ordering in electronically phase-separated La2-xSrxCuO4+y: Neutron diffraction experiments
- 2009
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 80:1
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Journal article (peer-reviewed)abstract
- We present results of magnetic neutron diffraction experiments on the codoped superoxygenated La2-xSrxCuO4+y (LSCO+O) system with x=0.09. We find that the magnetic phase is long-range ordered incommensurate antiferromagnetic with a Neacuteel temperature T-N coinciding with the superconducting ordering temperature T-c=40 K. The incommensurability value is consistent with a hole doping of n(h)approximate to 1>8 but in contrast to nonsuperoxygenated La2-xSrxCuO4 with hole doping close to n(h)approximate to 18 the magnetic-order parameter is not field dependent. We attribute this to the magnetic order being fully developed in LSCO+O as in the spin and charge ordered "stripe" compounds La1.48Nd0.40Sr0.12CuO4 and La7/8Ba1/8CuO4.
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| 16. |
- Vickery, Anette, et al.
(author)
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A Monte Carlo Simulation of Neutron Instrument Resolution Functions
- 2013
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In: Journal of the Physical Society of Japan. - 1347-4073. ; 82, s. 037-037
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Journal article (peer-reviewed)abstract
- In this paper we demonstrate that the Monte Carlo simulation package McStas is a useful tool, not only for instrument design, but also for resolution function calculation. It has been possible to determine analytically the 4 dimensional resolution function of both a direct geometry time of flight neutron spectrometer and a triple axis spectrometer. The obtained resolution functions are directly comparable to and in agreement with the Monte Carlo simulated results. These results are a step towards incorporating a 4 dimensional resolution function into data analysis. For each instrument we show an example of the resolution function calculated at a particluar point in (Q, omega) space.
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| 17. |
- Willendrup, Peter K., et al.
(author)
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Using McStas for modelling complex optics, using simple building bricks
- 2011
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In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. - : Elsevier BV. - 0168-9002. ; 634, s. 150-155
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Conference paper (peer-reviewed)abstract
- The McStas neutron ray-tracing simulation package is a versatile tool for producing accurate neutron simulations, extensively used for design and optimization of instruments, virtual experiments, data analysis and user training. In McStas, component organization and simulation flow is intrinsically linear: the neutron interacts with the beamline components in a sequential order, one by one. Historically, a beamline component with several parts had to be implemented with a complete, internal description of all these parts. e.g. a guide component including all four mirror plates and required logic to allow scattering between the mirrors. For quite a while, users have requested the ability to allow "components inside components" or meta-components, allowing to combine functionality of several simple components to achieve more complex behaviour, i.e. four single mirror plates together defining a guide. We will here show that it is now possible to define meta-components in McStas, and present a set of detailed, validated examples including a guide with an embedded, wedged, polarizing mirror system of the Helmholtz-Zentrum Berlin type. (C) 2010 Elsevier B.V. All rights reserved.
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