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End date: 2018 × Start date: 2016 × Publisher: Milton Park : Taylor & Francis ×

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Now showing 1 - 10 of 75
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    Adhesion of a rigid punch to a confined elastic layer revisited
    (Milton Park : Taylor & Francis, 2017) Hensel, René; McMeeking, Robert M.; Kossa, Attila
    The adhesion of a punch to a linear elastic, confined layer is investigated. Numerical analysis is performed to determine the equivalent elastic modulus in terms of layer confinement. The size of the layer relative to the punch radius and its Poisson’s ratio are found to affect the layer stiffness. The results reveal that the equivalent modulus of a highly confined layer depends on its Poisson’s ratio, whereas, in contrast, an unconfined layer is only sensitive to the extent of the elastic film. The solutions of the equivalent modulus obtained from the simulations are fitted by an analytical function that, subsequently, is utilized to deduce the energy release rate for detachment of the punch via linear elastic fracture mechanics. The energy release rate strongly varies with layer confinement. Regimes for stable and unstable crack growth can be identified that, in turn, are correlated to interfacial stress distributions to distinguish between different detachment mechanisms.
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    Toward edges-rich MoS2 layers via chemical liquid exfoliation triggering distinctive magnetism
    (Milton Park : Taylor & Francis, 2016) Gao, Guanhui; Chen, Chi; Xie, Xiaobin; Su, Yantao; Kang, Shendong; Zhu, Guichi; Gao, Duyang; Eckert, Jürgen; Trampert, Achim; Cai, Lintao
    The magnetic function of layered molybdenum disulfide (MoS2) has been investigated via simulation, but few reliable experimental results have been explored. Herein, we developed edges-rich structural MoS2 nanosheets via liquid phase exfoliation approach, triggering exceptional ferromagnetism. The magnetic measurements revealed the clear ferromagnetic property of layered MoS2, compared to the pristine MoS2 in bulk exhibiting diamagnetism. The existence of ferromagnetism mostly was attributed to the presence of grain boundaries with abundant irregular edges confirmed by the transmission electron microscopy, magnetic force microscopy and X-ray photoelectron spectroscopy, which experimentally provided reliable evidences on irregular edges-rich states engineering ferromagnetism to clarify theoretical calculation.
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    Negative plasmon dispersion in 2H-NbS2 beyond the charge-density-wave interpretation
    (Milton Park : Taylor & Francis, 2016) Cudazzo, Pierluigi; Müller, Eric; Habenicht, Carsten; Gatti, Matteo; Berger, Helmuth; Knupfer, Martin; Rubio, Angel; Huotari, Simo
    We examine the experimental and theoretical electron-energy loss spectra in 2H-${\mathrm{Cu}}_{0.2}$NbS2 and find that the 1 eV plasmon in this material does not exhibit the regular positive quadratic plasmon dispersion that would be expected for a normal broad-parabolic-band system. Instead we find a nearly non-dispersing plasmon in the momentum-transfer range $q\lt 0.35$ Å−1. We argue that for a stoichiometric pure 2H-NbS2 the dispersion relation is expected to have a negative slope as is the case for other transition-metal dichalcogenides. The presence of Cu impurities, required to stabilize the crystal growth, tends to shift the negative plasmon dispersion into a positive one, but the doping level in the current system is small enough to result in a nearly-non-dispersing plasmon. We conclude that a negative-slope plasmon dispersion is not connected with the existence of a charge-density-wave order in transition metal dichalcogenides.
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    A new look on the two-dimensional Ising model: Thermal artificial spins
    (Milton Park : Taylor & Francis, 2016) Arnalds, Unnar B.; Chico, Jonathan; Stopfel, Henry; Kapaklis, Vassilios; Bärenbold, Oliver; Verschuuren, Marc A.; Wolff, Ulrike; Neu, Volker; Bergman, Anders; Hjörvarsson, Björgvin
    We present a direct experimental investigation of the thermal ordering in an artificial analogue of an asymmetric two-dimensional Ising system composed of a rectangular array of nano-fabricated magnetostatically interacting islands. During fabrication and below a critical thickness of the magnetic material the islands are thermally fluctuating and thus the system is able to explore its phase space. Above the critical thickness the islands freeze-in resulting in an arrested thermalized state for the array. Determining the magnetic state we demonstrate a genuine artificial two-dimensional Ising system which can be analyzed in the context of nearest neighbor interactions.
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    Doping dependence and electron–boson coupling in the ultrafast relaxation of hot electron populations in Ba(Fe1–x Co x )2As2
    (Milton Park : Taylor & Francis, 2016) Avigo, I.; Thirupathaiah, S.; Ligges, M.; Wolf, T.; Fink, J.; Bovensiepen, U.
    Using femtosecond time- and angle-resolved photoemission spectroscopy we investigate the effect of electron doping on the electron dynamics in $\mathrm{Ba}{({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ in a range of $0\leqslant x\lt 0.15$ at temperatures slightly above the Néel temperature. By analyzing the time-dependent photoemission intensity of the pump laser excited population as a function of energy, we found that the relaxation times at $0\lt E-{E}_{{\rm{F}}}\lt 0.2\,\mathrm{eV}$ are doping dependent and about 100 fs shorter at optimal doping than for overdoped and parent compounds. Analysis of the relaxation rates also reveals the presence of a pump fluence dependent step in the relaxation time at $E-{E}_{{\rm{F}}}=200\,\mathrm{meV}$ which we explain by coupling of the excited electronic system to a boson of this energy. We compare our results with static ARPES and transport measurements and find disagreement and agreement concerning the doping-dependence, respectively. We discuss the effect of the electron–boson coupling on the energy-dependent relaxation and assign the origin of the boson to a magnetic excitation.
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    The properties of isolated chiral skyrmions in thin magnetic films
    (Milton Park : Taylor & Francis, 2016) Leonov, A.O.; Monchesky, T.L.; Romming, N.; Kubetzka, A.; Bogdanov, A.N.; Wiesendanger, R.
    Axisymmetric solitonic states (chiral skyrmions) were first predicted theoretically more than two decades ago. However, until recently they have been observed in a form of skyrmionic condensates (hexagonal lattices and other mesophases). In this paper we report experimental and theoretical investigations of isolated chiral skyrmions discovered in PdFe/Ir(111) bilayers two years ago by Romming et al (2013 Science 341 636). The results of spin-polarized scanning tunneling microscopy analyzed within the continuum and discrete models provide a consistent description of isolated skyrmions in thin layers. The existence region of chiral skyrmions is restricted by strip-out instabilities at low fields and a collapse at high fields. We demonstrate that the same equations describe axisymmetric localized states in all condensed matter systems with broken mirror symmetry, and thus our findings establish basic properties of isolated skyrmions common for chiral liquid crystals, different classes of noncentrosymmetric magnets, ferroelectrics, and multiferroics.
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    Spatial self-organization of macroscopic quantum states of exciton-polaritons in acoustic lattices
    (Milton Park : Taylor & Francis, 2016) Buller, J.V.T.; Cerda-Méndez, E.A.; Balderas-Navarro, R.E.; Biermann, K.; Santos, P.V.
    Exciton-polariton systems can sustain macroscopic quantum states (MQSs) under a periodic potential modulation. In this paper, we investigate the structure of these states in acoustic square lattices by probing their wave functions in real and momentum spaces using spectral tomography. We show that the polariton MQSs, when excited by a Gaussian laser beam, self-organize in a concentric structure, consisting of a single, two-dimensional gap-soliton (GS) state surrounded by one dimensional (1D) MQSs with lower energy. The latter form at hyperbolical points of the modulated polariton dispersion. While the size of the GS tends to saturate with increasing particle density, the emission region of the surrounding 1D states increases. The existence of these MQSs in acoustic lattices is quantitatively supported by a theoretical model based on the variational solution of the Gross–Pitaevskii equation. The formation of the 1D states in a ring around the central GS is attributed to the energy gradient in this region, which reduces the overall symmetry of the lattice. The results broaden the experimental understanding of self-localized polariton states, which may prove relevant for functionalities exploiting solitonic objects.
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    Topological Weyl semimetals in the chiral antiferromagnetic materials Mn3Ge and Mn3Sn
    (Milton Park : Taylor & Francis, 2017) Yang, Hao; Sun, Yan; Zhang, Yang; Shi, Wu-Jun; Parkin, Stuart S.P.; Yan, Binghai
    Recent experiments revealed that Mn3Sn and Mn3Ge exhibit a strong anomalous Hall effect at room temperature, provoking us to explore their electronic structures for topological properties. By ab initio band structure calculations, we have observed the existence of multiple Weyl points in the bulk and corresponding Fermi arcs on the surface, predicting antiferromagnetic Weyl semimetals in Mn3Ge and Mn3Sn. Here the chiral antiferromagnetism in the Kagome-type lattice structure is essential to determine the positions and numbers of Weyl points. Our work further reveals a new guiding principle to search for magnetic Weyl semimetals among materials that exhibit a strong anomalous Hall effect.
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    Internal structure of hexagonal skyrmion lattices in cubic helimagnets
    (Milton Park : Taylor & Francis, 2016) McGrouther, D.; Lamb, R.J.; Krajnak, M.; McFadzean, S.; McVitie, S.; Stamps, R.L.; Leonov, A.O.; Bogdanov, A.N.; Togawa, Y.
    We report the most precise observations to date concerning the spin structure of magnetic skyrmions in a nanowedge specimen of cubic B20 structured FeGe. Enabled by our development of advanced differential phase contrast (DPC) imaging (in a scanning transmission electron microscope (STEM)) we have obtained high spatial resolution quantitative measurements of skyrmion internal spin profile. For hexagonal skyrmion lattice cells, stabilised by an out-plane applied magnetic field, mapping of the in-plane component of magnetic induction has revealed precise spin profiles and that the internal structure possesses intrinsic six-fold symmetry. With increasing field strength, the diameter of skyrmion cores was measured to decrease and accompanied by a nonlinear variation of the lattice periodicity. Variations in structure for individual skyrmions across an area of the lattice were also studied utilising a new increased sensitivity DPC detection scheme and a variety of symmetry lowering distortions were observed. To provide insight into fundamental energetics we have constructed a phenomenological model, with which our experimental observations of spin profiles and field induced core diameter variation are in good agreement with predicted structure in the middle of the nanowedge crystal. In the vicinity of the crystal surfaces, our model predicts the existence of in-plane twisting distortions which our current experimental observations were not sensitive to. As an alternative to the requirement for as yet unidentified sources of magnetic anisotropy, we demonstrate that surface states could provide the energetic stabilisation needed for predomination over the conical magnetic phase.
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    Bioenergy production and sustainable development: Science base for policymaking remains limited
    (Milton Park : Taylor & Francis, 2016) Robledo‐Abad, Carmenza; Althaus, Hans‐Jörg; Berndes, Göran; Bolwig, Simon; Corbera, Esteve; Creutzig, Felix; Garcia‐Ulloa, John; Geddes, Anna; Gregg, Jay S.; Haberl, Helmut; Hanger, Susanne; Harper, Richard J.; Hunsberger, Carol; Larsen, Rasmus K.; Lauk, Christian; Leitner, Stefan; Lilliestam, Johan; Lotze‐Campen, Hermann; Muys, Bart; Nordborg, Maria; Ölund, Maria; Orlowsky, Boris; Popp, Alexander; Portugal‐Pereira, Joana; Reinhard, Jürgen; Scheiffle, Lena; Smith, Pete
    The possibility of using bioenergy as a climate change mitigation measure has sparked a discussion of whether and how bioenergy production contributes to sustainable development. We undertook a systematic review of the scientific literature to illuminate this relationship and found a limited scientific basis for policymaking. Our results indicate that knowledge on the sustainable development impacts of bioenergy production is concentrated in a few well‐studied countries, focuses on environmental and economic impacts, and mostly relates to dedicated agricultural biomass plantations. The scope and methodological approaches in studies differ widely and only a small share of the studies sufficiently reports on context and/or baseline conditions, which makes it difficult to get a general understanding of the attribution of impacts. Nevertheless, we identified regional patterns of positive or negative impacts for all categories – environmental, economic, institutional, social and technological. In general, economic and technological impacts were more frequently reported as positive, while social and environmental impacts were more frequently reported as negative (with the exception of impacts on direct substitution of GHG emission from fossil fuel). More focused and transparent research is needed to validate these patterns and develop a strong science underpinning for establishing policies and governance agreements that prevent/mitigate negative and promote positive impacts from bioenergy production.