| 1. |
- Azimi Mousolou, Vahid, et al.
(författare)
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Magnon-magnon entanglement and its quantification via a microwave cavity
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:22
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Tidskriftsartikel (refereegranskat)abstract
- Quantum magnonics is an emerging research field, with great potential for applications in magnon based hybrid systems and quantum information processing. Quantum correlation, such as entanglement, is a central resource in many quantum information protocols that naturally comes about in any study toward quantum technologies. This applies also to quantum magnonics. Here, we investigate antiferromagnetic coupling of two ferromagnetic sublattices that can have two different magnon modes. We show how this may lead to experimentally measurable bipartite continuous-variable magnon-magnon entanglement. The entanglement can be fully characterized via a single squeezing parameter or, equivalently, entanglement parameter. The clear relation between the entanglement parameter and the Einstein, Podolsky, and Rosen (EPR) function of the ground state opens up for experimental quantification magnon-magnon continuous-variable entanglement and EPR nonlocality. We propose a practical experimental realization to measure the EPR function of the ground state, in a setting that relies on magnon-photon interaction in a microwave cavity.
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| 2. |
- Borisov, Vladislav, et al.
(författare)
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Heisenberg and anisotropic exchange interactions in magnetic materials with correlated electronic structure and significant spin-orbit coupling
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 103:17
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Tidskriftsartikel (refereegranskat)abstract
- The Dzyaloshinskii-Moriya (DM) interaction, as well as symmetric anisotropic exchange, are important ingredients for stabilizing topologically nontrivial magnetic textures, such as, e.g., skyrmions, merons, and hopfions. These types of textures are currently in focus from a fundamental science perspective and they are also discussed in the context of future spintronics information technology. While the theoretical understanding of the Heisenberg exchange interactions is well developed, it is still a challenge to access, from first principles theory, the DM interaction as well as the symmetric anisotropic exchange, which both require a fully-relativistic treatment of the electronic structure, in magnetic systems where substantial electron-electron correlations are present. Here, we present results of a theoretical framework which allows to compute these interactions in any given system and demonstrate its performance for several selected cases, for both bulk and low-dimensional systems. We address several representative cases, including the bulk systems CoPt and FePt, the B20 compounds MnSi and FeGe as well as the low-dimensional transition metal bilayers Co/Pt(111) andMn/W(001). The effect of electron-electron correlations is analyzed using dynamical mean-field theory on the level of the spin-polarized T -matrix + fluctuating exchange (SPTF) approximation, as regards the strength and character of the isotropic (Heisenberg) and anisotropic (DM) interactions in relation to the underlying electronic structure. Our method can be combined with more advanced techniques for treating correlations, e.g., quantum Monte Carlo and exact diagonalization methods for the impurity solver of dynamical mean-field theory. We find that correlation-induced changes of the DM interaction can be rather significant, with up to fivefold modifications in the most distinctive case.
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| 3. |
- Grenz, Pascal Jona, et al.
(författare)
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Adlayer influence on Dirac-type surface state at W(110)
- 2021
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 33:28
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Tidskriftsartikel (refereegranskat)abstract
- In a combined experimental and theoretical study, we investigated how Fe and Co adlayers on W(110) affect the Dirac-type surface state (DSS). Angle-resolved photoelectron spectroscopy data show an increase in binding energy of 75meV and 107meV for Fe and Co, respectively. In order to identify the origin of the energy shift we performed first-principles calculations of the surface electronic structure. The inward surface relaxation of the uncovered W(110) surface is lifted by the adlayers. This structural change is one reason of the energy shift of the DSS. Furthermore, the Fe and Co adlayers change the surface potential, which results in an additional energy shift of the DSS.
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| 4. |
- Malik, Rameez Saeed, 1987-, et al.
(författare)
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Ultrafast magnetization dynamics in the half-metallic Heusler alloy Co2FeAl
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:10
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Tidskriftsartikel (refereegranskat)abstract
- We report on optically induced, ultrafast magnetization dynamics in the Heusler alloy Co2FeAl, probed by time-resolved magneto-optical Kerr effect. Experimental results are compared to results from electronic structure theory and atomistic spin-dynamics simulations. Experimentally, we find that the demagnetization time (tau(M)) in films of Co2FeAl is almost independent of varying structural order, and that it is similar to that in elemental 3d ferromagnets. In contrast, the slower process of magnetization recovery, specified by tau(R), is found to occur on picosecond time scales, and is demonstrated to correlate strongly with the Gilbert damping parameter (alpha). Based on these results we argue that for Co2FeAl the remagnetization process is dominated by magnon dynamics, something which might have general applicability.
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| 5. |
- Miranda, I. P., et al.
(författare)
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Mechanisms behind large Gilbert damping anisotropies
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 103:22
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Tidskriftsartikel (refereegranskat)abstract
- A method with which to calculate the Gilbert damping parameter from a real-space electronic structure method is reported here. The anisotropy of the Gilbert damping with respect to the magnetic moment direction and local chemical environment is calculated for bulk and surfaces of Fe50Co50 alloys from first-principles electronic structure in a real-space formulation. The size of the damping anisotropy for Fe50Co50 alloys is demonstrated to be significant. Depending on details of the simulations, it reaches a maximum-minimum damping ratio as high as 200%. Several microscopic origins of the strongly enhanced Gilbert damping anisotropy have been examined, where in particular interface/surface effects stand out, as do local distortions of the crystal structure. Although theory does not reproduce the experimentally reported high ratio of 400% [Phys. Rev. Lett. 122, 117203 (2019)], it nevertheless identifies microscopic mechanisms that can lead to huge damping anisotropies.
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| 6. |
- Ntallis, Nikolaos, et al.
(författare)
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Connection between magnetic interactions and the spin-wave gap of the insulating phase of NaOsO3
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:13
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Tidskriftsartikel (refereegranskat)abstract
- The scenario of a metal-insulator transition driven by the onset of antiferromagnetic order in NaOsO3 calls for a trustworthy derivation of the underlying effective spin Hamiltonian. To determine the latter we rely on ab initio electronic-structure calculations, linear spin-wave theory, and comparison to experimental data of the corresponding magnon spectrum. We arrive this way to Heisenberg couplings that are less than or similar to 45 to less than or similar to 63% smaller than values presently proposed in the literature and Dzyaloshinskii-Moriya interactions in the region of 15% of the Heisenberg exchange J. These couplings together with the symmetric anisotropic exchange interaction and single-ion magnetocrystalline anisotropy successfully reproduce the magnon dispersion obtained by resonant inelastic x-ray scattering measurements. In particular, the spin-wave gap fully agrees with the measured one. We find that the spin-wave gap is defined from a subtle interplay between the single-ion anisotropy, the Dzyaloshinskii-Moriya exchange, and the symmetric anisotropic exchange interactions. The results reported here underpin the local-moment description of NaOsO3, when it comes to analyzing the magnetic excitation spectra. Interestingly, this comes about from a microscopic theory that describes the electron system as Bloch states, adjusted to a mean-field solution to Hubbard-like interactions.
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| 7. |
- Ntallis, Nikolaos, et al.
(författare)
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Majority gate for two-dimensional ferromagnets lacking inversion symmetry
- 2021
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Ingår i: Physical Review Research. - : American Physical Society. - 2643-1564. ; 3:2
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Tidskriftsartikel (refereegranskat)abstract
- The manipulation of topologically protected field configurations such as skyrmions, merons, and antimerons, already predicted and experimentally observed in noncentrosymmetric magnets, could definitely have potential applications in logic gate operations as carriers of information. Here, we present and elaborate a proof of concept on how to construct a three-input noncanonical majority gate on a kagome ferromagnet lacking inversion symmetry. By taking advantage of the existence of edge modes in a kagome magnet, it is possible to create topological excitations such as merons and antimerons at the edge of the material. Using atomistic spin dynamics simulations, we determine the precise physical conditions for the creation and annihilation of merons and antimerons and, in a second stage, we describe the majority gate functionality.
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| 8. |
- Shaw, Justin M., et al.
(författare)
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Quantifying Spin-Mixed States in Ferromagnets
- 2021
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 127:20
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Tidskriftsartikel (refereegranskat)abstract
- We quantify the presence of spin-mixed states in ferromagnetic 3D transition metals by precise measurement of the orbital moment. While central to phenomena such as Elliot-Yafet scattering, quantification of the spin-mixing parameter has hitherto been confined to theoretical calculations. We demonstrate that this information is also available by experimental means. Comparison of ferromagnetic resonance spectroscopy with x-ray magnetic circular dichroism results show that Kittel's original derivation of the spectroscopic g factor requires modification, to include spin mixing of valence band states. Our results are supported by ab initio relativistic electronic structure theory.
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| 9. |
- Streib, Simon, et al.
(författare)
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Exchange constants for local spin Hamiltonians from tight-binding models
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 103:22
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Tidskriftsartikel (refereegranskat)abstract
- We consider the mapping of tight-binding electronic structure theory to a local spin Hamiltonian, based on the adiabatic approximation for spin degrees of freedom in itinerant-electron systems. Local spin Hamiltonians are introduced in order to describe the energy landscape of small magnetic fluctuations, locally around a given spin configuration. They are designed for the linear response near a given magnetic state and, in general, are insufficient to capture arbitrarily strong deviations of spin configurations from the equilibrium. In order to achieve this mapping, we include a linear term in the local spin Hamiltonian that together with the usual bilinear exchange tensor, produces an improved accuracy of effective magnetic Weiss fields for noncollinear states. We also provide examples from tight-binding electronic structure theory, where our implementation of the calculation of exchange constants is based on constraining fields that stabilize an out-of-equilibrium spin configuration. We check our formalism by means of numerical calculations for iron dimers and chains.
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