| 1. |
- Citrin, J., et al.
(författare)
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Ion temperature profile stiffness : non-linear gyrokinetic simulations and comparison with experiment
- 2014
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 54:2, s. 023008-
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Tidskriftsartikel (refereegranskat)abstract
- Recent experimental observations at JET show evidence of reduced ion temperature profile stiffness. An extensive set of nonlinear gyrokinetic simulations are performed based on the experimental discharges, investigating the physical mechanism behind the observations. The impact on the ion heat flux of various parameters that differ within the data-set are explored. These parameters include the safety factor, magnetic shear, toroidal flow shear, effect of rotation on the magnetohydrodynamic equilibrium, R/L-n, beta(e), Z(eff), T-e/T-i, and the fast-particle content. While previously hypothesized to be an important factor in the stiffness reduction, the combined effect of toroidal flow shear and low magnetic shear is not predicted by the simulations to lead to a significant reduction in ion heat flux, due both to an insufficient magnitude of flow shear and significant parallel velocity gradient destabilization. It is however found that nonlinear electromagnetic effects due to both thermal and fast-particle pressure gradients, even at low beta(e), can significantly reduce the ion heat flux, and is a key factor in explaining the experimental observations. A total of four discharges are examined, at both inner and outer radii. For all cases studied, the simulated and experimental ion heat flux values agree within reasonable variations of input parameters around the experimental uncertainties.
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| 2. |
- Citrin, J., et al.
(författare)
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Nonlinear stabilization of tokamak microturbulence by fast ions
- 2013
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 111:15, s. 155001-
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Tidskriftsartikel (refereegranskat)abstract
- Nonlinear electromagnetic stabilization by suprathermal pressure gradients found in specific regimes is shown to be a key factor in reducing tokamak microturbulence, augmenting significantly the thermal pressure electromagnetic stabilization. Based on nonlinear gyrokinetic simulations investigating a set of ion heat transport experiments on the JET tokamak, described by Mantica et al., this result explains the experimentally observed ion heat flux and stiffness reduction. These findings are expected to improve the extrapolation of advanced tokamak scenarios to reactor relevant regimes.
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| 3. |
- Glawion, S., et al.
(författare)
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Two-Spinon and Orbital Excitations of the Spin-Peierls System TiOCl
- 2011
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 107:10, s. 107402-
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Tidskriftsartikel (refereegranskat)abstract
- We combine high-resolution resonant inelastic x-ray scattering with cluster calculations utilizing a recently derived effective magnetic scattering operator to analyze the polarization, excitation energy, and momentum-dependent excitation spectrum of the low-dimensional quantum magnet TiOCl in the range expected for orbital and magnetic excitations (0-2.5 eV). Ti 3d orbital excitations yield complete information on the temperature-dependent crystal-field splitting. In the spin-Peierls phase we observe a dispersive two-spinon excitation and estimate the inter- and intradimer magnetic exchange coupling from a comparison to cluster calculations.
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| 4. |
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| 5. |
- Koethe, T. C., et al.
(författare)
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Transfer of spectral weight and symmetry across the metal-insulator transition in VO2
- 2006
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 97:11
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed study of the valence and conduction bands of VO2 across the metal-insulator transition using bulk-sensitive photoelectron and O K x-ray absorption spectroscopies. We observe a giant transfer of spectral weight with distinct features that require an explanation which goes beyond the Peierls transition model as well as the standard single-band Hubbard model. Analysis of the symmetry and energies of the bands reveals the decisive role of the V 3d orbital degrees of freedom. Comparison to recent realistic many body calculations shows that much of the k dependence of the self-energy correction can be cast within a dimer model.
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| 6. |
- M. A. Hossain, B. Bohnenbuck, Y. D. Chuang, M. W. Haverkort, I. S. Elfimov, A.Tanaka, A. G. Cruz Gonzalez, Z. Hu, H.-J. Lin, C. T. Chen, R. Mathieu, Y. Tokura, Y. Yoshida, L. H. Tjeng, Z. Hussain, B. Keimer, G. A. Sawatzky and A. Damascelli
(författare)
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Mott versus Slater-type metal-insulator transition in Mn-substituted Sr3Ru2O7
- 2012
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 86, s. 041102(R)-
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Tidskriftsartikel (refereegranskat)
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| 7. |
- de Groot, Frank M. F., et al.
(författare)
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2p x-ray absorption spectroscopy of 3d transition metal systems
- 2021
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Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 249
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Tidskriftsartikel (refereegranskat)abstract
- This review provides an overview of the different methods and computer codes that are used to interpret 2p x-ray absorption spectra of 3d transition metal ions. We first introduce the basic parameters and give an overview of the methods used. We start with the semi-empirical multiplet codes and compare the different codes that are available. A special chapter is devoted to the user friendly interfaces that have been written on the basis of these codes. Next we discuss the first principle codes based on band structure, including a chapter on Density Functional theory based approaches. We also give an overview of the first-principle multiplet codes that start from a cluster calculation and we discuss the wavefunction based methods, including multi-reference methods. We end the review with a discussion of the link between theory and experiment and discuss the open issues in the spectral analysis.
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| 8. |
- Lüder, J., et al.
(författare)
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Theory of X-ray spectroscopy of strongly correlated systems
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- X-ray absorption spectroscopy measured at the L-edge of transition metals (TMs) is a powerful element-selective tool providing direct information about the correlation effects in the 3d states. The theoretical modelling of the 2p to 3d excitation processes remains a challenge for contemporary ab initio electronic structure techniques, due to strong core-hole and multiplet effects influencing the spectra. In this work we present a realisation of the method combining the density functional theory with multiplet ligand field theory, proposed in Phys. Rev. B 85, 165113 (2012). The core of this approach is the solution of the single-impurity Anderson model (SIAM), parameterised from first principles.In our implementation, we adopt the dynamical mean-field theory and utilize the local Hamiltonian and the hybridisation function, projected onto TM 3d states, in order to construct the SIAM. We show that the current method can be used as an alternative to the construction of the Wannier functions. The developed computational scheme is applied to calculate the L-edge spectra for several TM monoxides. An excellent agreement between the theory and experiment is found for all studied systems. The possible extensions of the method as well as its limitations are discussed.
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