| 1. |
- Lyon, Keenan, et al.
(författare)
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Parameterization of magnetic vector potentials and fields for efficient multislice calculations of elastic electron scattering
- 2021
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Ingår i: Acta Crystallographica Section A. - : International Union Of Crystallography. - 2053-2733. ; 77, s. 509-518
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Tidskriftsartikel (refereegranskat)abstract
- The multislice method, which simulates the propagation of the incident electron wavefunction through a crystal, is a well established method for analysing the multiple scattering effects that an electron beam may undergo. The inclusion of magnetic effects into this method proves crucial towards simulating enhanced magnetic interaction of vortex beams with magnetic materials, calculating magnetic Bragg spots or searching for magnon signatures, to name a few examples. Inclusion of magnetism poses novel challenges to the efficiency of the multislice method for larger systems, especially regarding the consistent computation of magnetic vector potentials A and magnetic fields B over large supercells. This work presents a tabulation of parameterized magnetic (PM) values for the first three rows of transition metal elements computed from atomic density functional theory (DFT) calculations, allowing for the efficient computation of approximate A and B across large crystals using only structural and magnetic moment size and direction information. Ferromagnetic b.c.c. (body-centred cubic) Fe and tetragonal FePt are chosen to showcase the performance of PM values versus directly obtaining A and B from the unit-cell spin density by DFT. The magnetic fields of b.c.c. Fe are well described by the PM approach while for FePt the PM approach is less accurate due to deformations in the spin density. Calculations of the magnetic signal, namely the change due to A and B of the intensity of diffraction patterns, show that the PM approach for both b.c.c. Fe and FePt is able to describe the effects of magnetism in these systems to a good degree of accuracy.
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| 2. |
- Lyon, Keenan, et al.
(författare)
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Theory of magnon diffuse scattering in scanning transmission electron microscopy
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:21
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Tidskriftsartikel (refereegranskat)abstract
- We present a theory and a simulation of diffuse scattering due to the excitation of magnons in scanning transmission electron microscopy. The calculations indicate that magnons can present atomic contrast when detected by electron energy-loss spectroscopy using atomic-size electron beams. The results presented here indicate that the intensity of the magnon diffuse scattering in bcc iron at 300 K is 4 orders of magnitude weaker than the intensity of thermal diffuse scattering arising from atomic vibrations.
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| 3. |
- Novko, Dino, et al.
(författare)
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Ab initio study of electromagnetic modes in two-dimensional semiconductors : Application to doped phosphorene
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:11
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Tidskriftsartikel (refereegranskat)abstract
- Starting from the rigorous quantum-field-theory formalism, we derive a formula for the screened conductivity designed to study the coupling of light with elementary electron excitations and the ensuing electromagnetic modes in two-dimensional (2D) semiconductors. The latter physical quantity consists of three fully separable parts, namely, intraband, interband, and ladder conductivities, and is calculated beyond the random phase approximation as well as from first principles. By using this methodology, we study the optical absorption spectra in 2D black phosphorous, so-called phosphorene, as a function of the concentration of electrons injected into the conduction band. The mechanisms of phosphorene exciton quenching versus doping are studied in detail. It is demonstrated that already small doping levels (n similar to 10(12) cm(-2)) lead to a radical drop in the exciton binding energy, i.e., from 600 meV to 128 meV. The screened conductivity is applied to study the collective electromagnetic modes in doped phosphorene. It is shown that the phosphorene transversal exciton hybridizes with free photons to form an exciton-polariton. This phenomenon is experimentally observed only for the case of confined electromagnetic microcavity modes. Finally, we demonstrate that the energy and intensity of anisotropic 2D plasmon-polaritons can be tuned by varying the concentration of injected electrons.
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