SwePub - search: WFRF:(Rusz Jan)

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1.	Krizek, Filip, et al. (author) Atomically sharp domain walls in an antiferromagnet 2022 In: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 8:13 Journal article (peer-reviewed)abstract The interest in understanding scaling limits of magnetic textures such as domain walls spans the entire field of magnetism from its physical fundamentals to applications in information technologies. Here, we explore antiferromagnetic CuMnAs in which imaging by x-ray photoemission reveals the presence of magnetic textures down to nanoscale, reaching the detection limit of this established microscopy in antiferromagnets. We achieve atomic resolution by using differential phase-contrast imaging within aberration-corrected scanning transmission electron microscopy. We identify abrupt domain walls in the antiferromagnetic film corresponding to the Néel order reversal between two neighboring atomic planes. Our work stimulates research of magnetic textures at the ultimate atomic scale and sheds light on electrical and ultrafast optical antiferromagnetic devices with magnetic field–insensitive neuromorphic functionalities.
2.	Ali, Hasan, 1985-, et al. (author) Atomic resolution energy-loss magnetic chiral dichroism measurements enabled by patterned apertures 2020 In: Physical Review Research. - College Park USA. - 2643-1564. ; 2:2 Journal article (peer-reviewed)abstract Electron energy-loss magnetic chiral dichroism (EMCD) has the potential to measure magnetic properties of the materials at atomic resolution but the complex distribution of magnetic signals in the zone axis and the overlapping diffraction discs at higher beam convergence angles make the EMCD signal acquisition challenging. Recently, the use of ventilator apertures to acquire the EMCD signals with atomic resolution was proposed. Here we give the experimental demonstration of several types of ventilator apertures and obtain a clear EMCD signal at beam semiconvergence angles of 5 mrad. To simplify the experimental procedures, we propose a modified ventilator aperture which not only simplifies the complex scattering conditions but reduces the influence of lens aberrations on the EMCD signal as compared to the originally proposed ventilator apertures. In addition, this modified aperture can be used to analyze magnetic crystals with various symmetries and we demonstrate this feature by acquiring EMCD signals on different zone axis orientations of an Fe crystal. With the same aperture we obtain EMCD signals with convergence angles corresponding to atomic resolution electron probes. After the theoretical demonstration of the EMCD signal on a zone axis orientation at high beam convergence angles, this work thus overcomes the experimental and methodological hurdles and enables atomic resolution EMCD on the zone axis by using apertures.
3.	Ali, Hasan, 1985-, et al. (author) Noise-dependent bias in quantitative STEM-EMCD experiments revealed by bootstrapping 2024 In: Ultramicroscopy. - : Elsevier. - 0304-3991 .- 1879-2723. ; 257 Journal article (peer-reviewed)abstract Electron magnetic circular dichroism (EMCD) is a powerful technique for estimating element-specific magnetic moments of materials on nanoscale with the potential to reach atomic resolution in transmission electron microscopes. However, the fundamentally weak EMCD signal strength complicates quantification of magnetic moments, as this requires very high precision, especially in the denominator of the sum rules. Here, we employ a statistical resampling technique known as bootstrapping to an experimental EMCD dataset to produce an empirical estimate of the noise-dependent error distribution resulting from application of EMCD sum rules to bcc iron in a 3-beam orientation. We observe clear experimental evidence that noisy EMCD signals preferentially bias the estimation of magnetic moments, further supporting this with error distributions produced by Monte-Carlo simulations. Finally, we propose guidelines for the recognition and minimization of this bias in the estimation of magnetic moments.
4.	Ali, Hasan, 1985-, et al. (author) Simultaneous mapping of EMCD signals and crystal orientations in a transmission electron microscope 2021 In: Scientific Reports. - : Springer Nature. - 2045-2322. ; 11 Journal article (peer-reviewed)abstract When magnetic properties are analysed in a transmission electron microscope using the technique of electron magnetic circular dichroism (EMCD), one of the critical parameters is the sample orientation. Since small orientation changes can have a strong impact on the measurement of the EMCD signal and such measurements need two separate measurements of conjugate EELS spectra, it is experimentally non-trivial to measure the EMCD signal as a function of sample orientation. Here, we have developed a methodology to simultaneously map the quantitative EMCD signals and the local orientation of the crystal. We analyse, both experimentally and by simulations, how the measured magnetic signals evolve with a change in the crystal tilt. Based on this analysis, we establish an accurate relationship between the crystal orientations and the EMCD signals. Our results demonstrate that a small variation in crystal tilt can significantly alter the strength of the EMCD signal. From an optimisation of the crystal orientation, we obtain quantitative EMCD measurements.
5.	Ali, Hasan, 1985-, et al. (author) Single scan STEM-EMCD in 3-beam orientation using a quadruple aperture 2023 In: Ultramicroscopy. - : Elsevier BV. - 0304-3991 .- 1879-2723. ; 251 Journal article (peer-reviewed)abstract The need to acquire multiple angle-resolved electron energy loss spectra (EELS) is one of the several critical challenges associated with electron magnetic circular dichroism (EMCD) experiments. If the experiments are performed by scanning a nanometer to atomic-sized electron probe on a specific region of a sample, the precision of the local magnetic information extracted from such data highly depends on the accuracy of the spatial registration between multiple scans. For an EMCD experiment in a 3-beam orientation, this means that the same specimen area must be scanned four times while keeping all the experimental conditions same. This is a non-trivial task as there is a high chance of morphological and chemical modification as well as non-systematic local orientation variations of the crystal between the different scans due to beam damage, contamination and spatial drift. In this work, we employ a custom-made quadruple aperture to acquire the four EELS spectra needed for the EMCD analysis in a single electron beam scan, thus removing the above-mentioned complexities. We demonstrate a quantitative EMCD result for a beam convergence angle corresponding to sub-nm probe size and compare the EMCD results for different detector geometries.
6.	Barthel, Juri, et al. (author) Simple model for phonon spectroscopy using fast electrons 2024 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 109:18 Journal article (peer-reviewed)abstract We propose a simple approach to simulating the phonon sector in electron energy-loss spectroscopy (EELS), as implemented in scanning transmission electron microscopy. Simplification of the problem is obtained by working with the phonon density of states (PDOS), a function of phonon energy, which is an integral over the details of the dispersion relations due to the correlated motions of the atoms. For a given PDOS, we derive a spectral distribution function, to distribute the total inelastic scattering, as calculated within the quantum excitation of phonons model, into an energy-loss/gain spectrum. The spectral distribution is obtained assuming a linear relationship between inelastic phonon scattering and atomic mean-squared displacements, a good approximation for phonon EELS with a detector covering only moderate scattering angles. We provide examples of the usefulness of the proposed approach in the modeling and interpretation of experimental phonon EELS data.
7.	Castellanos-Reyes, José Ángel, et al. (author) Unveiling the impact of temperature on magnon diffuse scattering detection in the transmission electron microscope 2023 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 108:13 Journal article (peer-reviewed)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.
8.	Herrera, Edwin, et al. (author) Quantum-well states at the surface of a heavy-fermion superconductor 2023 In: Nature. - : Springer Nature. - 0028-0836 .- 1476-4687. ; 616:7957, s. 465-469 Journal article (peer-reviewed)abstract Two-dimensional electronic states at surfaces are often observed in simple wide-band metals such as Cu or Ag (refs. (1-4)). Confinement by closed geometries at the nanometre scale, such as surface terraces, leads to quantized energy levels formed from the surface band, in stark contrast to the continuous energy dependence of bulk electron bands(2,5-10). Their energy-level separation is typically hundreds of meV (refs. (3,6,11)). In a distinct class of materials, strong electronic correlations lead to so-called heavy fermions with a strongly reduced bandwidth and exotic bulk ground states(12,13). Quantum-well states in two-dimensional heavy fermions (2DHFs) remain, however, notoriously difficult to observe because of their tiny energy separation. Here we use millikelvin scanning tunnelling microscopy (STM) to study atomically flat terraces on U-terminated surfaces of the heavy-fermion superconductor URu2Si2, which exhibits a mysterious hidden-order (HO) state below 17.5 K (ref. (14)). We observe 2DHFs made of 5f electrons with an effective mass 17 times the free electron mass. The 2DHFs form quantized states separated by a fraction of a meV and their level width is set by the interaction with correlated bulk states. Edge states on steps between terraces appear along one of the two in-plane directions, suggesting electronic symmetry breaking at the surface. Our results propose a new route to realize quantum-well states in strongly correlated quantum materials and to explore how these connect to the electronic environment.
9.	Li, Zhuo, et al. (author) Atomic Structure and Electron Magnetic Circular Dichroism of Individual Rock Salt Structure Antiphase Boundaries in Spinel Ferrites 2021 In: Advanced Functional Materials. - : John Wiley & Sons. - 1616-301X .- 1616-3028. ; 31:21 Journal article (peer-reviewed)abstract Spinel ferrites are an important class of materials, whose magnetic properties are of interest for industrial applications. The antiphase boundaries (APBs) that are commonly observed in spinel ferrite films can hinder their applications in spintronic devices and sensors, as a result of their influence on magnetic degradation and magnetoresistance of the materials. However, it is challenging to correlate magnetic properties with atomic structure in individual APBs due to the limited spatial resolution of most magnetic imaging techniques. Here, aberration-corrected scanning transmission electron microscopy and electron energy-loss magnetic chiral dichroism are used to measure the atomic structure and electron magnetic circular dichroism (EMCD) of a single APB in NiFe2O4 that takes the form of a rock salt structure interlayer and is associated with a crystal translation of (1/4)a[011]. First principles density functional theory calculations are used to confirm that this specific APB introduces antiferromagnetic coupling and a significant decrease in the magnitude of the magnetic moments, which is consistent with an observed decrease in EMCD signal at the APB. The results provide new insight into the physical origins of magnetic coupling at an individual defect on the atomic scale.
10.	Luo, Yang, et al. (author) Three-dimensional and temperature-dependent electronic structure of the heavy-fermion compound CePt2In7 studied by angle-resolved photoemission spectroscopy 2020 In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 101:11 Journal article (peer-reviewed)abstract The three-dimensional and temperature-dependent electronic structures of the heavy-fermion superconductor CePt2In7 are investigated. Angle-resolved photoemission spectroscopy using variable photon energy establishes the existence of quasi-two- and three-dimensional Fermi surface topologies. Temperature-dependent 4d-4f on-resonance photoemission spectroscopies data reveal that heavy quasiparticle bands begin to form at a temperature well above the characteristic (coherence) temperature T+. The emergence of low-lying crystal electric field excitation may be responsible for the "relocalization" or the precursor to the establishment of heavy electrons coherence in heavy-fermion compounds. These findings provide critical insight into understanding the hybridization in heavy-fermion systems.
11.	Lyon, Keenan, et al. (author) Parameterization of magnetic vector potentials and fields for efficient multislice calculations of elastic electron scattering 2021 In: Acta Crystallographica Section A. - : International Union Of Crystallography. - 2053-2733. ; 77, s. 509-518 Journal article (peer-reviewed)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.
12.	Lyon, Keenan, et al. (author) Theory of magnon diffuse scattering in scanning transmission electron microscopy 2021 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:21 Journal article (peer-reviewed)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.
13.	Makino, Hitoshi, et al. (author) A study on the relationship of magnetic moments orientation in L10FePt network nanostructured film by electron energy-loss magnetic chiral dichroism using semi-core excitation spectra 2022 In: Journal of Magnetism and Magnetic Materials. - : Elsevier. - 0304-8853 .- 1873-4766. ; 558 Journal article (peer-reviewed)abstract In this study, we applied electron energy-loss magnetic chiral dichmism (EMCD), an electron counterpart of X-ray magnetic circular dichmism (XMCD), to a network nanostructured FePt L1(0) ordered alloy film to examine the relative orientation of magnetic moments between neighboring Fe and Pt atoms using the Fe-M-2,M-3, Pt-O-2,O-3, and Pt-N-6,N-7 semi-core excitation spectra with transmission electron microscopy and electron energy-loss spectroscopy. EMCD signals were successfully extracted from a large number of spectra using a dedicated data analysis procedure to obtain sufficient noise statistics. Results showed that the relative sign relation of the EMCD signals between the Fe and Pt absorption edges was consistent with that of the theoretical dielectric tensor while assuming that parallel magnetic moments exist between neighboring Fe and Pt. We believe the results of this study can be applied to alloys with different nanostructures to determine whether the spin configuration depends on the size and geometry of the nanostructures.
14.	Maksimovic, Nikola, et al. (author) Evidence for a delocalization quantum phase transition without symmetry breaking in CeCoIn5 2022 In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 375:6576, s. 76-81 Journal article (peer-reviewed)abstract The study of quantum phase transitions that are not clearly associated with broken symmetry is a major effort in condensed matter physics, particularly in regard to the problem of high-temperature superconductivity, for which such transitions are thought to underlie the mechanism of superconductivity itself. Here we argue that the putative quantum critical point in the prototypical unconventional superconductor CeCoIn5 is characterized by the delocalization of electrons in a transition that connects two Fermi surfaces of different volumes, with no apparent broken symmetry. Drawing on established theory of f-electron metals, we discuss an interpretation for such a transition that involves the fractionalization of spin and charge, a model that effectively describes the anomalous transport behavior we measured for the Hall effect.
15.	Snarski-Adamski, Justyn, et al. (author) Simulations of magnetic Bragg scattering in transmission electron microscopy 2023 In: Ultramicroscopy. - : Elsevier BV. - 0304-3991 .- 1879-2723. ; 247 Journal article (peer-reviewed)abstract We have simulated the magnetic Bragg scattering in transmission electron microscopy in two antiferromagnetic compounds, NiO and LaMnAsO. This weak magnetic phenomenon was experimentally observed in NiO by Loudon (2012). We have computationally reproduced Loudon's experimental data, and for comparison we have performed calculations for the LaMnAsO compound as a more challenging case, containing lower concentration of magnetic elements and strongly scattering heavier non-magnetic elements. We have also described thickness and voltage dependence of the intensity of the antiferromagnetic Bragg spot for both compounds. We have considered lattice vibrations within two computational approaches, one assuming a static lattice with Debye-Waller smeared potentials, and another explicitly considering the atomic vibrations within the quantum excitations of phonons model (thermal diffuse scattering). The structural analysis shows that the antiferromagnetic Bragg spot appears in between (111) and (000) reflections for NiO, while for LaMnAsO the antiferromagnetic Bragg spot appears at the position of the (010) reflection in the diffraction pattern, which corresponds to a forbidden reflection of the crystal structure. Calculations predict that the intensity of the magnetic Bragg spot in NiO is significantly stronger than thermal diffuse scattering at room temperature. For LaMnAsO, the magnetic Bragg spot is weaker than the room-temperature thermal diffuse scattering, but its detection can be facilitated at reduced temperatures.
16.	Snarski-Adamski, Justyn, et al. (author) Simulations of magnetic Bragg scattering in transmission electron microscopy 2023 In: 2023 IEEE International Magnetic Conference. - : Institute of Electrical and Electronics Engineers (IEEE). Conference paper (peer-reviewed)abstract We have modeled the magnetic Bragg scattering in two antiferromagnetic materials, NiO and LaMnAsO, using transmission electron microscopy. Experimentally, Loudon detected these weak magnetic phenomena in NiO. As a more difficult situation with a lower concentration of magnetic elements and higher concentration of heavier non-magnetic elements that significantly scatter, we did computations for the LaMnAsO compound in order to compare our computational replication of Loudon's experimental data. Additionally, we have discussed the antiferromagnetic Bragg spot's thickness and voltage dependency for both compounds. We used two computational methods, one assuming a static lattice with smeared Debye-Waller potentials and the other explicitly taking into account the atomic vibrations within the quantum excitations of phonons model (thermal diffuse scattering). According to the structural study, the antiferromagnetic Bragg spot in NiO is located between the (111) and (000) reflections. However, in LaMnAsO, it is located at the site of the (110) reflection in the diffraction pattern, which is a forbidden reflection of the crystal structure. According to calculations, the magnetic Bragg spot in NiO has an intensity that is much greater than thermal diffuse scattering at room temperature. The magnetic Bragg spot for LaMnAsO is weaker than the thermal diffuse scattering at room temperature, but its identification can be made easier at lower temperatures.
17.	Song, Jiao-Jiao, et al. (author) Relocalization of uranium 5f electrons in the antiferromagnetic heavy-fermion superconductor UPd2Al3 : Insights from angle-resolved photoemission spectroscopy 2024 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 109:20 Journal article (peer-reviewed)abstract We investigate the antiferromagnetic heavy fermion superconductor UPd2⁢Al3, employing angle-resolved photoemission spectroscopy to unravel the complex electronic structure of its U 5⁢? electrons. We observe unexpected characteristics that challenge the conventional temperature-dependent behavior of heavy fermion systems, revealing unexpected characteristics. At temperatures above the anticipated coherence temperature (?*), we observe itinerant U 5⁢? electrons at temperatures higher than previously postulated. Additionally, a previously unidentified dispersionless band emerges around 600 meV below the Fermi energy, potentially linked to spin-orbit splitting within the U 5⁢? states. Hybridization between the 5⁢? electrons and conduction band was observed with an energy dispersion of 10 meV at low temperatures, suggesting that U 5⁢? electrons near and at the Fermi surface have an itinerant nature. Temperature-dependent 5⁢?−5⁢? resonance spectra reveal that the 5⁢? electron spectrum weight increases with lowering temperature and begins to decrease at temperatures significantly higher than the Néel temperature (??). We further show that the competition between the Kondo effect and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions may be responsible for the relocalization of 5⁢? electrons, making relocalization a precursor to the establishment of magnetic order at lower temperatures. Our experiments also provide evidence that 5⁢? electrons with the same orbital are involved in both the Kondo effect and RKKY interactions, suggesting that the two coexist at lower temperatures.
18.	Song, Jiao-Jiao, et al. (author) The 4f-Hybridization Strength in CemMnIn3m+2n Heavy-Fermion Compounds Studied by Angle-Resolved Photoemission Spectroscopy 2021 In: Chinese Physics Letters. - : IOP Publishing. - 0256-307X .- 1741-3540. ; 38:10 Journal article (peer-reviewed)abstract We systemically investigate the nature of Ce 4f electrons in structurally layered heavy-fermion compounds CemMnIn3m+2n (with M = Co, Rh, Jr, and Pt, m = 1, 2, n = 0-2), at low temperature using on-resonance angle-resolved photoemission spectroscopy. Three heavy quasiparticle bands f(0), f(7/2)(1) and f(5/2)(1), are observed in all compounds, whereas their intensities and energy locations vary greatly with materials. The strong f(0) states imply that the localized electron behavior dominates the Ce 4f states. The Ce 4f electrons are partially hybridized with the conduction electrons, making them have the dual nature of localization and itinerancy. Our quantitative comparison reveals that the f(5/2)(1)-f (0) intensity ratio is more suitable to reflect the 4f-state hybridization strength.
19.	Yan, Xingxu, et al. (author) Real-Space Visualization of Frequency-Dependent Anisotropy of Atomic Vibrations 2023 Other publication (other academic/artistic)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.
20.	Yuan, Ya Hua, et al. (author) Angle-resolved photoemission spectroscopy view on the nature of Ce 4f electrons in the antiferromagnetic Kondo lattice CePd5Al2 2021 In: Physical Review B. - : American Physical Society (APS). - 2469-9969 .- 2469-9950. ; 103:12 Journal article (peer-reviewed)abstract We report an angle-resolved photoemission spectroscopy study of the antiferromagnetic Kondo lattice CePd5Al2, focusing on the quasi-two-dimensional k-space nature of its Fermi surface and, tuning photon energy to the Ce 4d-4f on-resonance transition, the hybridization of the Ce 4f state. A strong shoulder feature on the f0 peak was detected, suggesting hybridization between conduction and f bands. On-resonance spectra revealed narrow, yet hybridized quasiparticle bands with sharp peaks and ∼ 9 meV energy dispersion near the Fermi energy EF. The observed dispersive hybridized f band can be well described by a hybridization-band picture based on the periodic Anderson model (PAM). Hence, the 4f electrons in CePd5Al2 display a dual nature, with both localized and itinerant features, but with dominantly localized character.
21.	Zanfrognini, Matteo, et al. (author) Dynamical diffraction effects in STEM orbital angular momentum resolved electron energy-loss magnetic chiral dichroism 2020 In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 102:18 Journal article (peer-reviewed)abstract In this paper, we explore the properties of dynamical diffraction coefficients in orbital angular momentum resolved electron energy-loss magnetic chiral dichroism spectra, in a scanning transmission electron microscopy setup. We demonstrate that for basic zone axis geometries with fourfold or threefold symmetry the coefficients are constrained to have simplified forms. By exploiting these properties, we show how a dichroism spectrum accessible using this technique is only weakly dependent on sample thickness and, more generally, on dynamical diffraction effects. Our results indicate that in such cases it is possible to determine the orbital and spin components of atomic magnetic moments approximately from experimental spectra without the need for additional dynamical diffraction calculations.
22.	Zeiger, Paul, 1994-, et al. (author) Control of the phonon band gap with isotopes in hexagonal boron nitride 2023 Other publication (other academic/artistic)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.
23.	Zeiger, Paul, et al. (author) Efficient and Versatile Model for Vibrational STEM-EELS 2020 In: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 124:2 Journal article (peer-reviewed)abstract We introduce a novel method for the simulation of the impact scattering in vibrational scanning transmission electron microscopy electron energy loss spectroscopy simulations. The phonon-loss process is modeled by a combination of molecular dynamics and elastic multislice calculations within a modified frozen phonon approximation. The key idea is thereby to use a so-called S thermostat in the classical molecular dynamics simulation to generate frequency dependent configurations of the vibrating specimen's atomic structure. The method includes correlated motion of atoms and provides vibrational spectrum images at a cost comparable to standard frozen phonon calculations. We demonstrate good agreement of our method with simulations and experiments for a 15 nm flake of hexagonal boron nitride.
24.	Zeiger, Paul, et al. (author) Frequency-resolved frozen phonon multislice method and its application to vibrational electron energy loss spectroscopy using parallel illumination 2021 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:10 Journal article (peer-reviewed)abstract We explore the capabilities of the frequency-resolved frozen phonon multislice method introduced in [Phys. Rev. Lett. 124, 025501 (2020)] to model inelastic vibrational scattering in transmission electron microscopy. We review the method in detail and discuss advantages of using a so-called hotspot thermostat instead of the d thermostat used in our first report. We apply the method to simulate vibrational electron energy loss spectra of hexagonal boron nitride under plane wave illumination. Simulated spectroscopic information well represents the theoretical phonon band structure of the studied material, both in terms of energies as well as polarization vectors of individual phonon modes.
25.	Zeiger, Paul M., 1994-, et al. (author) Lessons from the harmonic oscillator : Reconciliation of the frequency-resolved frozen phonon multislice method with other theoretical approaches 2023 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 108:9 Journal article (peer-reviewed)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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