| 1. |
- Castellanos-Reyes, José Ángel, et al.
(författare)
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Unveiling the impact of temperature on magnon diffuse scattering detection in the transmission electron microscope
- 2023
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 108:13
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Tidskriftsartikel (refereegranskat)abstract
- Magnon diffuse scattering (MDS) signals could, in principle, be studied with high spatial resolution in scanning transmission electron microscopy (STEM), thanks to recent technological progress in electron energy-loss spectroscopy. However, detecting MDS signals in STEM is technically challenging due to their overlap with the much stronger thermal diffuse scattering (TDS) signals. In bcc Fe at 300 K, MDS signals greater than or comparable to TDS signals have been predicted to occur under the central Bragg disk, well into a currently inaccessible energy-loss region. Therefore, to successfully detect MDS in STEM, it is necessary to identify conditions in which TDS and MDS signals can be distinguished from one another in regions outside the central Bragg disk. Temperature may be a key factor due to the distinct thermal signatures of magnon and phonon signals. In this work, we present a study on the effects of temperature on MDS and TDS in bcc Fe-considering a detector outside the central Bragg disk and a fixed convergent electron probe-using the frozen phonon and frozen magnon multislice methods. Our study reveals that neglecting the effects of atomic vibrations causes the MDS signal to grow approximately linearly up to the Curie temperature of Fe, after which it exhibits less variation. The MDS signal displays an alternating behavior due to dynamical diffraction, instead of increasing monotonically as a function of thickness. The inclusion of the effects of atomic vibrations through a complex atomic electrostatic potential causes the linear growth of the MDS signal to change to a nonlinear behavior that exhibits a predominant peak for a sample of thickness 16.072 nm at 1100 K. In contrast, the TDS signal grows more linearly than the MDS signal through the studied temperature range but still exhibits appreciable dynamical diffraction effects. An analysis of the signal-to-noise ratio (SNR) shows that the MDS signal can be a statistically significant contribution to the total scattering intensity under realizable measurement conditions and feasible acquisition times. For example, our study found that a SNR of 3 can be achieved with a beam current of 1 nA in less than 30 min for the 16.072-nm-thick bcc Fe sample at 1100 K.
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| 2. |
- Yan, Xingxu, et al.
(författare)
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Real-Space Visualization of Frequency-Dependent Anisotropy of Atomic Vibrations
- 2023
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The underlying dielectric properties of materials, intertwined with intriguing phenomena such as topological polariton modes and anisotropic thermal conductivities, stem from the anisotropy in atomic vibrations. Conventionally, X-ray diffraction techniques have been employed to estimate thermal ellipsoids of distinct elements, albeit lacking the desired spatial and energy resolutions. Here we introduce a novel approach utilizing the dark-field monochromated electron energy-loss spectroscopy for momentum-selective vibrational spectroscopy, enabling the cartographic delineation of variations of phonon polarization vectors. By applying this technique to centrosymmetric cubic-phase strontium titanate, we successfully discern two types of oxygen atoms exhibiting contrasting vibrational anisotropies below and above 60 meV due to their frequency-linked thermal ellipsoids. This method establishes a new pathway to visualize phonon eigenvectors at specific crystalline sites for diverse elements, thus delving into uncharted realms of dielectric, optical, and thermal property investigations with unprecedented spatial resolutions.
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| 3. |
- Zeiger, Paul, 1994-, et al.
(författare)
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Control of the phonon band gap with isotopes in hexagonal boron nitride
- 2023
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The isotopic mass of constituent elements of materials has a well-known effect on the energy of vibrational modes. By means of monochromated scanning transmission electron microscopy we have experimentally studied the phonon bandstructure of hexagonal BN, where a phonon band gap appears between in-plane optical phonon modes and the lower energy part of the phonon spectrum. The size of the phonon band gap can be manipulated by the isotopic mass of the boron. While in 11BN the phonon band gap is about 7 meV wide, in 10BN the gap nearly closes, being an order of magnitude smaller (below 0.5 meV). This opens exciting options for manipulating terahertz wave propagation through isotopically structured devices having otherwise no interfaces between chemically distinct components.
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| 4. |
- Zeiger, Paul M., 1994-, et al.
(författare)
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Lessons from the harmonic oscillator : Reconciliation of the frequency-resolved frozen phonon multislice method with other theoretical approaches
- 2023
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 108:9
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Tidskriftsartikel (refereegranskat)abstract
- We compare the frequency-resolved frozen phonon multislice (FRFPMS) method [introduced in P. M. Zeiger and J. Rusz, Phys. Rev. Lett. 124, 025501 (2020)] with other theoretical approaches used to account for the inelastic scattering of high-energy electrons, namely, the first-order Born approximation and the quantum excitation of phonons model. We show that these theories lead to similar expressions for the single inelastically scattered intensity as a function of momentum transfer for an anisotropic quantum harmonic oscillator in a weak phase object approximation of the scattered waves, except for a too small smearing of the scattering potential by the effective Debye-Waller factor (DWF) in the FRFPMS method. We propose that this issue can be fixed by including an explicit DWF smearing into the potential and demonstrate numerically that in any realistic situation, a FRFPMS approach revised in this way correctly accounts for the single inelastically scattered intensity and the correct elastic scattering intensity. Furthermore, our simulations illustrate that the only requirement for such a revised FRFPMS method is the smallness of mean-squared displacements for all atomic species in all frequency bins. The analytical considerations for the FRFPMS method also explain the 1/ω2 scaling of FRFPMS spectra observed by P. M. Zeiger and J. Rusz [Phys. Rev. B 104, 104301 (2021)] by the use of classical statistics in the molecular dynamics simulation. Moreover, we find that the FRFPMS method inherently adds the contributions of phonon loss and gain within each frequency bin. Both of these issues related to the frequency scaling can be fixed by a system-independent postprocessing step.
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| 5. |
- Zeiger, Paul Michel, 1994-
(författare)
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Modeling Vibrational Electron Energy Loss Spectroscopy with the Frequency-Resolved Frozen Phonon Multislice Method
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aberration correctors, improved monochromators, and better detectors have enabled exciting research with nanometer- and Ångstrom-scale resolution in the Scanning Transmission Electron Microscope (STEM). However the interaction of high-energy electrons with the many-body system of the sample is quite complex and hinders interpretation of experiments. Therefore measurements often need to be informed by extensive modeling of the beam-sample interaction.In this thesis, we report the development of a model for the computer simulation of vibrational Electron Energy Loss Spectroscopy (EELS) in the STEM, which we call the Frequency Resolved Frozen Phonon Multislice (FRFPMS) method. We motivate the development of the method by reviewing the field of vibrational EELS from the instrumental advances, which enabled it, over experimental progress to a detailed consideration of other theories of vibrational EELS. In the process, we identify the need for a method, which is able to take into account many of the complicating factors of the scattering process, such as multiple elastic interactions, and is computationally feasible today, even for extended structure models.After a brief overview of necessary computational methods, we showcase that the FRFPMS method satisfies this need by discussing several papers we have published on the method. We demonstrate that the FRFPMS method produces results, which agree very well with published experimental and also theoretical results, both for momentum-resolved as well as high spatial resolution vibrational EELS. Furthermore we compare the FRFPMS method with the Quantum Excitations of Phonons model and the first-order Born approximation for a simple model system. The FRFPMS method matches the predictions of the other theories provided that two small modifications are introduced, which modify the temperature and energy-loss dependent scaling as well as the large momentum-transfer behavior of the modelled cross section.We then apply such revised FRFPMS method to simulations of shifts of optical phonon frequencies in hBN as a function of isotopic composition. The FRFPMS results are in very good agreement with experiments performed by collaborators, demonstrating that vibrational EELS is capable of detecting such isotopic shift of phonon frequencies in a momentum-resolved fashion. In another application of our method we focus on simulating atomically-resolved STEM-EELS experiments on SrTiO3, which allow to interpret a subtle asymmetry in experimental large detector off-axis vibrational STEM-EELS maps as result of sensitivity to the direction of the phonon eigenvector. This enables imaging of anisotropic displacements of atoms as a function of vibrational frequency.
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