151 research outputs found
Virtual Engineering in Skills Acquisition and Development in the Career of the Rotorcraft Engineer
As the complexity of engineering systems grows, engineers increasingly need to be able to use a range of tools in order to reduce the costs, and associated risks, as they work in the various phases of the engineering life-cycle. In order to help engineers operate successfully within this product lifecycle, there have been significant developments in modelling simulation tools. Integrating these tools in a Virtual Engineering (VE) environment allows engineers to examine the potentially conflicting requirements of the different phases of the life-cycle, to develop a co-ordinated approach to requirements capture and product design through to identifying potential costly problems that could occur later in the development and operations phases. Technical skills development to use these tools is key to this process. This paper presents the experiences, learning outcomes and lessons learned in the development and implementation of bespoke rotorcraft engineering training programmes. The programmes were designed using a Problem Based Learning (PBL) framework where knowledge and skills are gained through solving problems. Four cases studies are presented in the paper, demonstrating how this PBL/VE approach can be used in the training programmes. Consideration of the future use of VE tools is provided together with future challenges for their successful application
A M\"ossbauer study of the magneto-structural coupling effect in SrFeAs and SrFeAsF
In the present paper, we report a comparison study of SrFeAs and
SrFeAsF using M\"ossbauer spectroscopy. The temperature dependence of the
magnetic hyperfine field is fitted with a modified Bean-Rodbell model. The
results give much smaller magnetic moment and magneto-structural coupling
effect for SrFeAsF, which may be understood as due to different inter-layer
properties of the two compounds.Comment: 4 pages, 2 figures,conference ICAME2011, to be appear in Hyperfine
Interaction
Proximate Kitaev quantum spin liquid behaviour in a honeycomb magnet.
Quantum spin liquids (QSLs) are topological states of matter exhibiting remarkable properties such as the capacity to protect quantum information from decoherence. Whereas their featureless ground states have precluded their straightforward experimental identification, excited states are more revealing and particularly interesting owing to the emergence of fundamentally new excitations such as Majorana fermions. Ideal probes of these excitations are inelastic neutron scattering experiments. These we report here for a ruthenium-based material, α-RuCl3, continuing a major search (so far concentrated on iridium materials) for realizations of the celebrated Kitaev honeycomb topological QSL. Our measurements confirm the requisite strong spin-orbit coupling and low-temperature magnetic order matching predictions proximate to the QSL. We find stacking faults, inherent to the highly two-dimensional nature of the material, resolve an outstanding puzzle. Crucially, dynamical response measurements above interlayer energy scales are naturally accounted for in terms of deconfinement physics expected for QSLs. Comparing these with recent dynamical calculations involving gauge flux excitations and Majorana fermions of the pure Kitaev model, we propose the excitation spectrum of α-RuCl3 as a prime candidate for fractionalized Kitaev physics.Research using ORNL’s HFIR and SNS facilities was sponsored by the US Department of Energy, Office of Science, Basic Energy Sciences (BES), Scientific User Facilities Division. A part of the synthesis and the bulk characterization performed at ORNL was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (C.A.B. and J.-Q.Y.). The work at University of Tennessee was funded in part by the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4416 (D.G.M. and L.L.). The work at Dresden was in part supported by DFG grant SFB 1143 (J.K. and R.M.), and by a fellowship within the Postdoc-Program of the German Academic Exchange Service (DAAD) (J.K.). D.L.K. is supported by EPSRC Grant No. EP/M007928/1. The collaboration as a whole was supported by the Helmholtz Virtual Institute ‘New States of Matter and their Excitations’ initiative.This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/nmat460
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Excitations in the field-induced quantum spin liquid state of α-RuCl3
The celebrated Kitaev quantum spin liquid (QSL) is the paradigmatic example of a topological magnet with emergent excitations in the form of Majorana Fermions and gauge fluxes. Upon breaking of time-reversal symmetry, for example in an external magnetic field, these fractionalized quasiparticles acquire non-Abelian exchange statistics, an important ingredient for topologically protected quantum computing. Consequently, there has been enormous interest in exploring possible material realizations of Kitaev physics and several candidate materials have been put forward, recently including α-RuCl3. In the absence of a magnetic field this material orders at a finite temperature and exhibits low-energy spin wave excitations. However, at moderate energies, the spectrum is unconventional and the response shows evidence for fractional excitations. Here we use time-of-flight inelastic neutron scattering to show that the application of a sufficiently large magnetic field in the honeycomb plane suppresses the magnetic order and the spin waves, leaving a gapped continuum spectrum of magnetic excitations. Our comparisons of the scattering to the available calculations for a Kitaev QSL show that they are consistent with the magnetic field induced QSL phase.The work at ORNL’s Spallation Neutron Source and the High Flux Isotope Reactor was supported by the United States Department of Energy (US-DOE), Office of Science - Basic Energy Sciences (BES), Scientific User Facilities Division. Part of the research was supported by the US-DOE, Office of Science - BES, Materials Sciences and Engineering Division (P.K., C.A.B. and J-Q.Y.). D.M. and P.K. acknowledge support from the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4416. The work at Dresden was in part supported by DFG grant SFB 1143 (J.K. and R.M.). J.K. is supported by the Marie Curie Programme under EC Grant agreements No.703697
Normal-State Spin Dynamics and Temperature-Dependent Spin Resonance Energy in an Optimally Doped Iron Arsenide Superconductor
The proximity of superconductivity and antiferromagnetism in the phase
diagram of iron arsenides, the apparently weak electron-phonon coupling and the
"resonance peak" in the superconducting spin excitation spectrum have fostered
the hypothesis of magnetically mediated Cooper pairing. However, since most
theories of superconductivity are based on a pairing boson of sufficient
spectral weight in the normal state, detailed knowledge of the spin excitation
spectrum above the superconducting transition temperature Tc is required to
assess the viability of this hypothesis. Using inelastic neutron scattering we
have studied the spin excitations in optimally doped BaFe1.85Co0.15As2 (Tc = 25
K) over a wide range of temperatures and energies. We present the results in
absolute units and find that the normal state spectrum carries a weight
comparable to underdoped cuprates. In contrast to cuprates, however, the
spectrum agrees well with predictions of the theory of nearly antiferromagnetic
metals, without complications arising from a pseudogap or competing
incommensurate spin-modulated phases. We also show that the temperature
evolution of the resonance energy follows the superconducting energy gap, as
expected from conventional Fermi-liquid approaches. Our observations point to a
surprisingly simple theoretical description of the spin dynamics in the iron
arsenides and provide a solid foundation for models of magnetically mediated
superconductivity.Comment: 8 pages, 4 figures, and an animatio
Local antiferromagnetic exchange and collaborative Fermi surface as key ingredients of high temperature superconductors
Cuprates, ferropnictides and ferrochalcogenides are three classes of
unconventional high-temperature superconductors, who share similar phase
diagrams in which superconductivity develops after a magnetic order is
suppressed, suggesting a strong interplay between superconductivity and
magnetism, although the exact picture of this interplay remains elusive. Here
we show that there is a direct bridge connecting antiferromagnetic exchange
interactions determined in the parent compounds of these materials to the
superconducting gap functions observed in the corresponding superconducting
materials. High superconducting transition temperature is achieved when the
Fermi surface topology matches the form factor of the pairing symmetry favored
by local magnetic exchange interactions. Our result offers a principle guide to
search for new high temperature superconductors.Comment: 12 pages, 5 figures, 1 table, 1 supplementary materia
Rapid Diagnostic Algorithms as a Screening Tool for Tuberculosis: An Assessor Blinded Cross-Sectional Study
Background: A major obstacle to effectively treat and control tuberculosis is the absence of an accurate, rapid, and low-cost diagnostic tool. A new approach for the screening of patients for tuberculosis is the use of rapid diagnostic classification algorithms.
Methods: We tested a previously published diagnostic algorithm based on four biomarkers as a screening tool for
tuberculosis in a Central European patient population using an assessor-blinded cross-sectional study design. In addition, we developed an improved diagnostic classification algorithm based on a study population at a tertiary hospital in Vienna, Austria, by supervised computational statistics.
Results: The diagnostic accuracy of the previously published diagnostic algorithm for our patient population consisting of 206 patients was 54% (CI: 47%–61%). An improved model was constructed using inflammation parameters and clinical information. A diagnostic accuracy of 86% (CI: 80%–90%) was demonstrated by 10-fold cross validation. An alternative model relying solely on clinical parameters exhibited a diagnostic accuracy of 85% (CI: 79%–89%).
Conclusion: Here we show that a rapid diagnostic algorithm based on clinical parameters is only slightly improved by
inclusion of inflammation markers in our cohort. Our results also emphasize the need for validation of new diagnostic algorithms in different settings and patient populations
Magnetism and its microscopic origin in iron-based high-temperature superconductors
High-temperature superconductivity in the iron-based materials emerges from,
or sometimes coexists with, their metallic or insulating parent compound
states. This is surprising since these undoped states display dramatically
different antiferromagnetic (AF) spin arrangements and Nel
temperatures. Although there is general consensus that magnetic interactions
are important for superconductivity, much is still unknown concerning the
microscopic origin of the magnetic states. In this review, progress in this
area is summarized, focusing on recent experimental and theoretical results and
discussing their microscopic implications. It is concluded that the parent
compounds are in a state that is more complex than implied by a simple Fermi
surface nesting scenario, and a dual description including both itinerant and
localized degrees of freedom is needed to properly describe these fascinating
materials.Comment: 14 pages, 4 figures, Review article, accepted for publication in
Nature Physic
From (pi, 0) magnetic order to superconductivity with (pi, pi) magnetic resonance in Fe1.02(Te1-xSex)
The iron chalcogenide Fe1+y(Te1-xSex) is structurally the simplest of the
Fe-based superconductors. Although the Fermi surface is similar to iron
pnictides, the parent compound Fe1+yTe exhibits antiferromagnetic order with
in-plane magnetic wave-vector (pi, 0). This contrasts the pnictide parent
compounds where the magnetic order has an in-plane magnetic wave-vector (pi,
pi) that connects hole and electron parts of the Fermi surface. Despite these
differences, both the pnictide and chalcogenide Fe-superconductors exhibit
superconducting spin resonances around (pi, pi), suggesting a common symmetry
for their superconducting order parameter. A central question in this
burgeoning field is therefore how (pi, pi) superconductivity can emerge from a
(pi, 0) magnetic instability. Here, we report that the magnetic soft mode
evolving from the (pi, 0)-type magnetic long-range order is associated with
weak charge carrier localization. Bulk superconductivity occurs only as the
magnetic mode at (pi, pi) becomes dominant upon doping. Our results suggest a
common magnetic origin for superconductivity in iron chalcogenide and pnictide
superconductors.Comment: 17 pages, 4 figure
Anisotropic three-dimensional magnetism in CaFe2As2
Inelastic neutron scattering measurements of the magnetic excitations in CaFe2As2 indicate that the spin wave velocity in the Fe layers is exceptionally large and similar in magnitude to the cuprates. However, the spin wave velocity perpendicular to the layers is at least half as large that in the layer, so that the magnetism is more appropriately categorized as anisotropic three-dimensional, in contrast to the two-dimensional cuprates. Exchange constants derived from band structure calculations predict spin wave velocities that are consistent with the experimental data
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