| 1. |
- Seifert, Tom S., et al.
(författare)
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Frequency-Independent Terahertz Anomalous Hall Effect in DyCo5, Co32Fe68, and Gd27Fe73 Thin Films from DC to 40 THz
- 2021
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Ingår i: Advanced Materials. - : John Wiley & Sons. - 0935-9648 .- 1521-4095. ; 33:14
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Tidskriftsartikel (refereegranskat)abstract
- The anomalous Hall effect (AHE) is a fundamental spintronic charge-to-charge-current conversion phenomenon and closely related to spin-to-charge-current conversion by the spin Hall effect. Future high-speed spintronic devices will crucially rely on such conversion phenomena at terahertz (THz) frequencies. Here, it is revealed that the AHE remains operative from DC up to 40 THz with a flat frequency response in thin films of three technologically relevant magnetic materials: DyCo5, Co32Fe68, and Gd27Fe73. The frequency-dependent conductivity-tensor elements sigma(xx) and sigma(yx) are measured, and good agreement with DC measurements is found. The experimental findings are fully consistent with ab initio calculations of sigma(yx) for CoFe and highlight the role of the large Drude scattering rate (approximate to 100 THz) of metal thin films, which smears out any sharp spectral features of the THz AHE. Finally, it is found that the intrinsic contribution to the THz AHE dominates over the extrinsic mechanisms for the Co32Fe68 sample. The results imply that the AHE and related effects such as the spin Hall effect are highly promising ingredients of future THz spintronic devices reliably operating from DC to 40 THz and beyond.
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| 2. |
- Ukleev, Victor, et al.
(författare)
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Observation by SANS and PNR of pure Néel-type domain wall profiles and skyrmion suppression below room temperature in magnetic [Pt/CoFeB/Ru]10 multilayers
- 2024
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Ingår i: Science and Technology of Advanced Materials. - : Taylor & Francis. - 1468-6996 .- 1878-5514. ; 25:1
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Tidskriftsartikel (refereegranskat)abstract
- We report investigations of the magnetic textures in periodic multilayers [Pt(1 nm)/(CoFeB(0.8 nm)/Ru(1.4 nm)]10 using polarised neutron reflectometry (PNR) and small-angle neutron scattering (SANS). The multilayers are known to host skyrmions stabilized by Dzyaloshinskii-Moriya interactions induced by broken inversion symmetry and spin–orbit coupling at the asymmetric interfaces. From depth-dependent PNR measurements, we observed well-defined structural features and obtained the layer-resolved magnetization profiles. The in-plane magnetization of the CoFeB layers calculated from fitting of the PNR profiles is found to be in excellent agreement with magnetometry data. Using SANS as a bulk probe of the entire multilayer, we observe long-period magnetic stripe domains and skyrmion ensembles with full orientational disorder at room temperature. No sign of skyrmions is found below 250 K, which we suggest is due to an increase of an effective magnetic anisotropy in the CoFeB layer on cooling that suppresses skyrmion stability. Using polarised SANS at room temperature, we prove the existence of pure Néel-type windings in both stripe domain and skyrmion regimes. No Bloch-type winding admixture, i.e. an indication for hybrid windings, is detected within the measurement sensitivity, in good agreement with expectations according to our micromagnetic modelling of the multilayers. Our findings using neutron techniques provide valuable microscopic insights into the rich magnetic behavior of skyrmion-hosting multilayers, which are essential for the advancement of future skyrmion-based spintronic devices.
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| 3. |
- Borregales, Manuel, et al.
(författare)
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Robust iterative schemes for non-linear poromechanics
- 2018
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Ingår i: Computational Geosciences. - Dordrecht : Springer. - 1420-0597 .- 1573-1499. ; 22:4, s. 1021-1038
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Tidskriftsartikel (refereegranskat)abstract
- We consider a non-linear extension of Biot's model for poromechanics, wherein both the fluid flow and mechanical deformation are allowed to be non-linear. Specifically, we study the case when the volumetric stress and the fluid density are non-linear functions satisfying certain assumptions. We perform an implicit discretization in time (backward Euler) and propose two iterative schemes for solving the non-linear problems appearing within each time step: a splitting algorithm extending the undrained split and fixed stress methods to non-linear problems, and a monolithic L-scheme. The convergence of both schemes are shown rigorously. Illustrative numerical examples are presented to confirm the applicability of the schemes and validate the theoretical results.
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| 4. |
- Both, Jakub W., et al.
(författare)
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Iterative Methods for Coupled Flow and Geomechanics in Unsaturated Porous Media
- 2017
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Ingår i: Poromechanics VI. - : American Society of Civil Engineers (ASCE). - 9780784480779 ; , s. 411-418
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Konferensbidrag (refereegranskat)abstract
- This work concerns the linearization of a three-field discretization of generalized Biot's equations describing coupled fluid flow and mechanical deformation in unsaturated porous media. The model of interest employs the effective stress based on the so-called equivalent pore pressure and can be interpreted as linear mechanics nonlinearly coupled with Richards' equation. As linearization, we apply simultaneously the L-scheme and the Fixed Stress Splitting scheme, which have been established and analyzed for Richards' equation and the linear Biot's equations, respectively. Numerical results demonstrate robustness and mesh independent convergence rates, whereas the popular, locally convergent Newton's method does not display robust convergence for the numerical examples we present.
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| 5. |
- Both, Jakub Wiktor, et al.
(författare)
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Robust fixed stress splitting for Biot’s equations in heterogeneous media
- 2017
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Ingår i: Applied Mathematics Letters. - Amsterdam, Netherlands : Elsevier. - 0893-9659 .- 1873-5452. ; 68, s. 101-108
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Tidskriftsartikel (refereegranskat)abstract
- We study the iterative solution of coupled flow and geomechanics in heterogeneous porous media, modeled by a three-field formulation of the linearized Biot's equations. We propose and analyze a variant of the widely used Fixed Stress Splitting method applied to heterogeneous media. As spatial discretization, we employ linear Galerkin finite elements for mechanics and mixed finite elements (lowest order Raviart Thomas elements) for flow. Additionally, we use implicit Euler time discretization. The proposed scheme is shown to be globally convergent with optimal theoretical convergence rates. The convergence is rigorously shown in energy norms employing a new technique. Furthermore, numerical results demonstrate robust iteration counts with respect to the full range of Lame parameters for homogeneous and heterogeneous media. Being in accordance with the theoretical results, the iteration count is hardly influenced by the degree of heterogeneities.
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| 6. |
- Görtz, Morgan, et al.
(författare)
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On the convergence rate of the dirichlet-neumann iteration for coupled poisson problems on unstructured grids
- 2020
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Ingår i: Finite Volumes for Complex Applications IX - Methods, Theoretical Aspects, Examples, FVCA 2020. - Cham : Springer International Publishing. - 2194-1009 .- 2194-1017. - 9783030436506 ; 323, s. 355-363
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Konferensbidrag (refereegranskat)abstract
- We consider thermal fluid structure interaction with a partitioned approach, where typically, a finite volume and a finite element code would be coupled. As a model problem, we consider two coupled Poisson problems with heat conductivities $$\lambda _1$$, $$\lambda _2$$ in one dimension on intervals of length $$l:1$$ and $$l:2$$. Hereby, we consider linear discretizations on arbitrary meshes, such as finite volumes, finite differences, finite elements. For these, we prove that the convergence rate of the Dirichlet-Neumann iteration is given by $$\lambda _1l_2/\lambda _2l_1$$ and is thus independent of discretization and mesh.
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| 7. |
- Haw, Shu Chih, et al.
(författare)
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Unusual mixed spin-state of Co3+ in the ground state of LaSrCoO4: Combined high-pressure and high-temperature study
- 2020
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388.
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Tidskriftsartikel (refereegranskat)abstract
- The nature of the non-magnetic to paramagnetic transition of Co3+ oxide LaCoO3 was strongly disputed in the literature for many decades from a low-spin (LS) state below 20 K and to a mixed LS state and high-spin (HS) state or an intermediate-spin (IS) state above 100 K. In this context, the layered perovskite LaSrCoO4 is more favorable for a Jahn-Teller-active IS state because of an elongated distortion, but has been scarcely studied with experimental X-ray spectroscopies as a function of temperature or external pressure. Here, our Co-L2,3 X-ray absorption spectroscopic study indicates a mixture of 40% HS-Co3+ and 60% LS-Co3+ for LaSrCoO4 against 25% HS-Co3+ and 75% LS-Co3+ for LaCoO3 at 300 K and ambient pressure (AP). At 10 K, we observed a sizable magnetic-circular-dichroism signal and a clear HS state of the magnetic Co3+ ion from the Co-L2,3 edge of LaSrCoO4. This result demonstrates that the HS state is already populated in the ground state versus a pure LS ground state in LaCoO3. A quantitative change of quantum number of the spin of the Co3+ ion of LaSrCoO4 as a function of pressure and temperature investigated systematically with Co-Kβ X-ray emission experiments firmly demonstrates not only a mixed state of LS/HS at 300 K and AP but also a presence of the pure LS-Co3+ and HS-Co3+ states only under high pressure and high temperature, respectively.
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| 8. |
- Hedman, Linnea, et al.
(författare)
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Receiving support to quit smoking and quit attempts among smokers with and without smoking related diseases : Findings from the EUREST-PLUS ITC Europe Surveys
- 2018
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Ingår i: Tobacco Induced Diseases. - Heraklion : European Publishing. - 1617-9625. ; 16
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Tidskriftsartikel (refereegranskat)abstract
- INTRODUCTION Having a chronic disease either caused or worsened by tobacco smoking does not always translate into quitting smoking. Although smoking cessation is one of the most cost-effective medical interventions, it remains poorly implemented in healthcare settings. The aim was to examine whether smokers with chronic and respiratory diseases were more likely to receive support to quit smoking by a healthcare provider or make a quit attempt than smokers without these diseases.METHODS This population-based study included a sample of 6011 adult smokers in six European countries. The participants were interviewed face-to-face and asked questions on sociodemographic characteristics, current diagnoses for chronic diseases, healthcare visits in the last 12 months and, if so, whether they had received any support to quit smoking. Questions on smoking behavior included nicotine dependence, motivation to quit smoking and quit attempts in the last 12 months. The results are presented as weighted percentages with 95% confidence intervals (CI) and as adjusted odds ratios with 95% CI based on logistic regression analyses.RESULTS Smokers with chronic respiratory disease, those aged 55 years and older, as well as those with one or more chronic diseases were more likely to receive smoking cessation advice from a healthcare professional. Making a quit attempt in the last year was related to younger age, high educational level, higher motivation to quit, lower nicotine dependence and having received advice to quit from a healthcare professional but not with having chronic diseases. There were significant differences between countries with smokers in Romania consistently reporting more support to quit as well as quit attempts.CONCLUSIONS Although smokers with respiratory disease did indeed receive smoking cessation support more often than smokers without disease, many smokers did not receive any advice or support to quit during a healthcare visit.
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| 9. |
- Kumar, Kundan, et al.
(författare)
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Formal upscaling and numerical validation of unsaturated flow models in fractured porous media
- 2020
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Ingår i: Journal of Computational Physics. - : Elsevier. - 0021-9991 .- 1090-2716. ; 407, s. 1-21
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we consider a mathematical model for describing flow in an unsaturated porous medium containing a fracture. Both the flow in the fracture as well as in the matrix blocks are governed by Richards' equation coupled by natural transmission conditions. Using formal asymptotics, we derive upscaled models as the limit of vanishing epsilon, the ratio of the width and length of the fracture. Our results show that the ratio of porosities and permeabilities in the fracture to matrix determine, to the leading order of approximation, the appropriate effective model. In these models the fracture is a lower dimensional object for which different transversally averaged models are derived depending on the ratio of the porosities and permeabilities of the fracture and respective matrix blocks. We obtain a catalogue of effective models which are validated by numerical computations. (C) 2019 Published by Elsevier Inc.
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| 10. |
- Landa-Marban, David, et al.
(författare)
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An Upscaled Model for Permeable Biofilm in a Thin Channel and Tube
- 2020
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Ingår i: Transport in Porous Media. - : Springer. - 0169-3913 .- 1573-1634. ; 132:1, s. 83-112
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we derive upscaled equations for modeling biofilm growth in porous media. The resulting macroscale mathematical models consider permeable multi-species biofilm including water flow, transport, detachment and reactions. The biofilm is composed of extracellular polymeric substances (EPS), water, active bacteria and dead bacteria. The free flow is described by the Stokes and continuity equations, and the water flux inside the biofilm by the Brinkman and continuity equations. The nutrients are transported in the water phase by convection and diffusion. This pore-scale model includes variations in the biofilm composition and size due to reproduction of bacteria, production of EPS, death of bacteria and shear forces. The model includes a water-biofilm interface between the free flow and the biofilm. Homogenization techniques are applied to obtain upscaled models in a thin channel and a tube, by investigating the limit as the ratio of the aperture to the length epsilon of both geometries approaches to zero. As epsilon gets smaller, we obtain that the percentage of biofilm coverage area over time predicted by the pore-scale model approaches the one obtained using the effective equations, which shows a correspondence between both models. The two derived porosity-permeability relations are compared to two empirical relations from the literature. The resulting numerical computations are presented to compare the outcome of the effective (upscaled) models for the two mentioned geometries.
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| 11. |
- List, Florian, et al.
(författare)
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Rigorous upscaling of unsaturated flow in fractured porous media
- 2020
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Ingår i: SIAM Journal on Mathematical Analysis. - : SIAM PUBLICATIONS. - 0036-1410 .- 1095-7154. ; 52:1, s. 239-276
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we consider a mathematical model for flow in an unsaturated porous medium containing a fracture. In all subdomains (the fracture and the adjacent matrix blocks) the flow is governed by Richards' equation. The submodels are coupled by physical transmission conditions expressing the continuity of the normal fluxes and of the pressures. We start by analyzing the case of a fracture having a fixed width-length ratio, called epsilon > 0. Then we take the limit epsilon -> 0 and give a rigorous proof for the convergence toward effective models. This is done in different regimes, depending on how the ratio of porosities and permeabilities in the fracture, respectively, in the matrix, scale in terms of epsilon, and leads to a variety of effective models. Numerical simulations confirm the theoretical upscaling results.
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| 12. |
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| 13. |
- Radu, Florin Adrian, et al.
(författare)
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A robust linearization scheme for finite volume based discretizations for simulation of two-phase flow in porous media
- 2015
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Ingår i: Journal of Computational and Applied Mathematics. - : Elsevier. - 0377-0427 .- 1879-1778. ; 289, s. 134-141
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Tidskriftsartikel (refereegranskat)abstract
- In this work we consider a mathematical model for two-phase flow in porous media. The fluids are assumed immiscible and incompressible and the solid matrix non-deformable. The mathematical model for the two-phase flow is written in terms of the global pressure and a complementary pressure (obtained by using the Kirchhoff transformation) as primary unknowns. For the spatial discretization, finite volumes have been used (more precisely the multi-point flux approximation method) and in time the backward Euler method has been employed. We present here a new linearization scheme for the nonlinear system arising after the temporal and spatial discretization. We show that the scheme is linearly convergent. Numerical experiments are presented that sustain the theoretical results.
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| 14. |
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| 15. |
- Reveron, Manuel Antonio Borregales, et al.
(författare)
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Iterative solvers for Biot model under small and large deformations
- 2021
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Ingår i: Computational Geosciences. - : Springer. - 1420-0597 .- 1573-1499. ; 25, s. 687-699
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Tidskriftsartikel (refereegranskat)abstract
- We considerL-scheme and Newton-based solvers for Biot model under large deformation. The mechanical deformation follows the Saint Venant-Kirchoff constitutive law. Furthermore, the fluid compressibility is assumed to be non-linear. A Lagrangian frame of reference is used to keep track of the deformation. We perform an implicit discretization in time (backward Euler) and propose two linearization schemes for solving the non-linear problems appearing within each time step: Newton's method andL-scheme. Each linearization scheme is also presented in a monolithic and a splitting version, extending the undrained split methods to non-linear problems. The convergence of the solvers, here presented, is shown analytically for cases under small deformation and numerically for examples under large deformation. Illustrative numerical examples are presented to confirm the applicability of the schemes, in particular, for large deformation.
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| 16. |
- Schmitz, Detlef, et al.
(författare)
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Soft X-Ray Magnetic Circular Dichroism of Vanadium in the Metal-Insulator Two-Phase Region of Paramagnetic V2O3 Doped with 1.1% Chromium
- 2019
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Ingår i: Physica status solidi. B, Basic research. - : Wiley-VCH Verlagsgesellschaft. - 0370-1972 .- 1521-3951.
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Tidskriftsartikel (refereegranskat)abstract
- V2O3 doped with 1.1% Cr is investigated at its isostructural correlation-driven metal-insulator transition near room temperature in its paramagnetic state with X-ray magnetic circular dichroism (XMCD) spectroscopy in external magnetic fields. A relative XMCD amplitude of about 2 permille is observed at the L-2,L-3 absorption edges of vanadium as expected for magnetic moment per mass values of the order of 1 J T-1 kg(-1) from magnetometry and the literature. Across the metal-insulator transition, the vanadium XMCD spectral shape significantly changes. According to atomic multiplet simulations, these changes are due to a changing orbital occupation indicating a changing phase composition. According to estimates used in this study, the dipole moment of the spin density distribution 7⟨Tz⟩ in the bulk increases such that the effective vanadium spin moment increases by a few percent with temperature in the two-phase region. Thereby, it partially compensates for the decrease in the relative XMCD amplitude due to a decreasing alignment of the paramagnetic moments. After a few minor temperature cycles, the sample is in a two-phase state in which the XMCD and X-ray linear dichroism spectra hardly depend on the temperature, and the specific electrical resistance is intermediate, showing only a weak sign of the metal-insulator transition.
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| 17. |
- Storvik, Erlend, et al.
(författare)
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On the optimization of the fixed-stress splitting for Biot’s equations
- 2019
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Ingår i: International Journal for Numerical Methods in Engineering. - : John Wiley & Sons. - 0029-5981 .- 1097-0207. ; 120:2, s. 179-194
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we are interested in efficiently solving the quasi-static, linear Biot model for poroelasticity. We consider the fixed-stress splitting scheme, which is a popular method for iteratively solving Biot’s equations. It is well known that the convergence properties of the method strongly depend on the applied stabilization/tuning parameter. We show theoretically that, in addition to depending on the mechanical properties of the porous medium and the coupling coefficient, they also depend on the fluid flow and spatial discretization properties. The type of analysis presented in this paper is not restricted to a particular spatial discretization, although it is required to be inf-sup stable with respect to the displacement-pressure formulation. Furthermore, we propose a way to optimize this parameter that relies on the mesh independence of the scheme’s optimal stabilization parameter. Illustrative numerical examples show that using the optimized stabilization parameter can significantly reduce the number of iterations.
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| 18. |
- Vasilyev, Leonid, et al.
(författare)
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On the Properties of the Parameter Space of the Generalized Continuum Transport Model for Description of Fluid Flow in Porous Networks
- 2017
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Ingår i: Transport in Porous Media. - : Springer Science and Business Media LLC. - 0169-3913 .- 1573-1634. ; 119:3, s. 673-688
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Tidskriftsartikel (refereegranskat)abstract
- Generalized transport models, such as Dual and Multiple Continua Models, Global Random Walk, Multirate Mass Transfer and Continuous Time Random Walk are widely used for description of anomalous transport in fractured and porous media. For these models the form of the parameter space is crucial for the most accurate description of anomalous effects as well as the mean transport phenomenon. Constraining of the parameter space is required for the proper interpretation of the physical properties taking place. In this study the Generalized Continuum Transport model is considered as a versatile tool for the parameter space selection as well as better quantification of anomalous (non-Fickian) transport. Different variants of the parameter space are applied to the GCT model and the breakthrough curves obtained from the pore-network models with strong anomalities are fitted. Flexibility of the model is demonstrated through its static and dynamic adaptivity to network structure and transport complexity. The beneficial results of the curve fitting are also compared with the classical models. It is thus demonstrated that the complexity of the model as well as the model parameters can be directly determined based on fine-scale simulations.
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| 19. |
- Zhou Hagström, Nanna, 1993-, et al.
(författare)
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Megahertz-rate Ultrafast X-ray Scattering and Holographic Imaging at the European XFEL
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence, and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, we present the results from the first megahertz repetition rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL. We illustrate the experimental capabilities that the SCS instrument offers, resulting from the operation at MHz repetition rates and the availability of the novel DSSC 2D imaging detector. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative examples, providing an ideal test-bed for operation at megahertz rates. Nevertheless, our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range.
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| 20. |
- Zhou Hagström, Nanna, 1993-, et al.
(författare)
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Megahertz-rate ultrafast X-ray scattering and holographic imaging at the European XFEL
- 2022
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Ingår i: Journal of Synchrotron Radiation. - : International Union of Crystallography (IUCr). - 0909-0495 .- 1600-5775. ; 29, s. 1454-1464
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Tidskriftsartikel (refereegranskat)abstract
- The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, results from the first megahertz-repetition-rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL are presented. The experimental capabilities that the SCS instrument offers, resulting from the operation at megahertz repetition rates and the availability of the novel DSSC 2D imaging detector, are illustrated. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative, providing an ideal test-bed for operation at megahertz rates. Our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range.
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