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Ján Rusz

Publications and source records attributed to Ján Rusz.

At least 19 recordsLinked to original sources
arXiv2026

Thermal diffuse scattering in TEM: complex absorptive potentials compared to the frozen phonon model

In transmission electron microscopy, electrons undergo inelastic scattering primarily through phonon excitations, known as thermal diffuse scattering. To capture the inelastic scattering effects on the elastic scattering components, absorptive effects must be included in the modeling of electron propagation, accounting for the gradual depletion of the elastic channel of the electron beam. Several approaches to modeling this absorption exist. In this paper, we compare the widely used complex absorptive potentials method to the more elaborate frozen phonon model, based on correlated atomic motion and on the Einstein model of atomic motion.

cond-mat.mtrl-sci↗
arXiv2026

Vectorial Reconstruction of Magnetic Order Parameters using Electron Magnetic Linear Dichroism

Determining the orientation of compensated magnetic order with nanometer resolution remains a major challenge because conventional magnetic probes primarily detect net magnetization. Here, we establish electron magnetic linear dichroism as a quantitative probe of the spin axis in transmission electron microscopy. Explicit inclusion of vectorial core-level exchange splitting into mixed-dynamic-form-factor simulations accounting for dynamical diffraction enables the calculation of the momentum- and energy-resolved dichroic response for an arbitrary Néel-vector or magnetization orientation. The magnetic linear contribution can be separated from nonmagnetic anisotropy and exhibits a characteristic angular dependence that enables reconstruction of the spin axis from a finite set of momentum-resolved electron-energy-loss spectra. Simulations for the antiferromagnetic and ferromagnetic phases of cubic FeRh show that this sensitivity persists from parallel illumination to atomically confined probes and over a broad range of crystal orientations and thicknesses. Electron magnetic linear dichroism therefore provides a route toward nanoscale vector imaging of compensated magnetic order in antiferromagnets and related magnetic materials.

cond-mat.mes-hall↗
arXiv2026

Efficient Large-Scale STEM-EELS Simulations With Torched-TACAW

The time auto-correlation of auxiliary wave functions (TACAW) method enables efficient simulations of ultra-low-loss electron energy loss spectra (EELS) arising from vibrational and magnon excitations. In practical applications to realistic materials systems, however, TACAW calculations become challenging due to the large system sizes required for models containing defects, interfaces, impurities, or grain boundaries, as well as the substantial computational cost and data throughput associated with molecular dynamics and multislice calculations. Here we discuss a practical methodology for large-scale TACAW simulations and present torched-TACAW, a freely available implementation of the TACAW part of the described workflow for efficient STEM-EELS simulations. The overall approach combines molecular dynamics based on foundational machine-learned interatomic potentials, partitioning of elongated supercells, and on-the-fly processing of multislice outputs in order to enable near ab initio quality simulations with tractable memory use and data flow. Using rutile TiO2 as a model system, we analyze important numerical aspects of the method, including windowing and supercell partitioning, and demonstrate atomic-resolution STEM-EELS simulations for thick samples.

cond-mat.mtrl-sci↗
arXiv2026

Combining Frozen Trajectory Excitation and TACAW for in silico Time-Resolved Vibrational Electron Energy Loss/Gain Spectroscopy

Seeing that ultrafast (picosecond timescale) vibrational electron energy loss spectroscopy (EELS) should soon be experimentally realizable, we present in silico approach capable of providing insight from the computational physics perspective. We present a framework that combines frozen trajectory excitation (FTE) with time auto-correlation of auxiliary wavefunctions (TACAW) to study the time-dependent spectral response of non-equilibrium lattice dynamics in a way comparable directly to experiment - (scanning) transmission electron microscope EELS, (S)TEM-EELS. In this approach, a selected phonon excitation is first introduced into an equilibrium molecular dynamics trajectory using FTE, after which the atomic positions during subsequent relaxations are treated with short-time TACAW analysis performed at different pump-probe delays. This yields momentum- and energy-resolved electron-scattering signals bearing a phonon imprint during the relaxation process, going beyond time-dependent diffuse-scattering intensities alone. We demonstrate the approach for fcc-Ni and 3C-SiC and discuss the observed phonon mode coupling and spectral redistribution during phonon relaxation.

cond-mat.mtrl-sci↗
arXiv2026

Temperature-dependent vibrational EELS simulations with nuclear quantum effects

The Time Autocorrelation of Auxiliary Wave (TACAW) method has established a framework for modeling angle-resolved electron energy loss spectroscopy (EELS) of phonons and magnons by deriving scattering intensities from the time autocorrelation of the beam wavefunction. This approach enables efficient computation of scattering intensities while naturally accounting for dynamical diffraction and multiple-scattering effects. In the cryogenic regime, vibrational spectra are dominated by nuclear quantum effects, notably zero-point motion. To capture these effects in low-temperature vibrational EELS, we incorporate thermostatted ring polymer molecular dynamics (TRPMD) into the TACAW formalism. Our results demonstrate that nuclear quantum effects lead to significant deviations from classical molecular dynamics predictions in the vibrational spectra of silicon at low temperatures and correctly predict the nearly temperature-independent optical phonon peak intensities in silicon, consistent with the first Born approximation. The TRPMD-TACAW method provides a robust theoretical tool for probing the low-temperature limit of vibrational EELS, offering a necessary benchmark for the quantitative analysis of emerging cryogenic scanning transmission electron microscopy experiments.

cond-mat.mtrl-sci↗
arXiv2025

Magnetic properties of the Fe$_5$SiB$_2$-Fe$_5$PB$_2$ system

The magnetic properties of the compound Fe$_5$Si$_{1-x}$P$_{x}$B$_2$ have been studied, with a focus on the Curie temperature $T_\textrm{C}$, saturation magnetization $M_\textrm{S}$, and magnetocrystalline anisotropy. Field and temperature dependent magnetization measurements were used to determine $T_\textrm{C}\left(x\right)$ and $M_\textrm{S}\left(x\right)$. The saturation magnetization at 10 K (300 K) is found to monotonically decrease from $1.11~\mathrm{MA/m}$ ($1.03~\mathrm{MA/m}$) to $0.97~\mathrm{MA/m}$ ($0.87~\mathrm{MA/m}$), as $x$ increases from zero to one. The Curie temperature is determined to be 810 K and 615 K in Fe$_5$SiB$_2$ and Fe$_5$PB$_2$, respectively. The highest $T_\textrm{C}$ is observed for $x=0.1$, while it decreases monotonically for larger $x$. The Curie temperatures have also been theoretically determined to be 700 K and 660 K for Fe$_5$SiB$_2$ and Fe$_5$PB$_2$, respectively, using a combination of density functional theory and Monte Carlo simulations. The magnitude of the effective magnetocrystalline anisotropy was extracted using the law of approach to saturation, revealing an increase with increasing phosphorus concentration. Low--field magnetization vs. temperature results for $x = 0, 0.1, 0.2$ indicate that there is a transition from easy--axis to easy--plane anisotropy with decreasing temperature.

cond-mat.mtrl-sci↗
arXiv2025

Theory of Momentum-Resolved Electron Energy-Loss Spectra of Coupled Phonon and Magnon Excitations

We develop a theory of momentum-resolved electron energy-loss spectra in the scanning transmission microscope (STEM-EELS) that captures the effects of coupled phonon and magnon excitations within a unified formalism, and apply it to body-centered cubic iron at 300 K. By advancing the Time Autocorrelation of Auxiliary Wavefunctions (TACAW) method to incorporate atomistic spin-lattice dynamics (ASLD), we simulate the EELS signal, including phonon-magnon interaction effects, dynamical diffraction, and multiple scattering. Our results reveal non-additive spectral features arising from phonon-magnon coupling, hybridization, and energy shifts, and further allow estimation of the electron dose required to detect magnon scattering under optimized detector conditions.

cond-mat.mtrl-sci↗
arXiv2025

Detecting Linear Dichroism with Atomic Resolution

X-ray linear dichroism has been pivotal for probing electronic anisotropies, but its inherent limited spatial resolution precludes atomic-scale investigations of orbital polarization. Here we introduce a versatile electron linear dichroism methodology in scanning transmission electron microscopy that overcomes these constraints. By exploiting momentum-transfer-dependent electron energy-loss spectroscopy with an atomic-sized probe, we directly visualize orbital occupation at individual atomic columns in real space. Using strained La0.7Sr0.3MnO3 thin films as a model system, we resolve the Mn-3d eg orbital polarization with sub-angstrom precision. We show that compressive strain stabilizes 3z2-r2 occupation while tensile strain favors x2-y2. These results validate our approach against established X-ray measurements while achieving the ultimate single atomic-column sensitivity. We further demonstrate two optimized signal extraction protocols that adapt to experimental constraints without compromising sensitivity. This generalizable platform opens unprecedented opportunities to study symmetry-breaking phenomena at individual defects, interfaces, and in quantum materials where atomic-scale electronic anisotropy governs emergent functionality.

cond-mat.mtrl-sci↗
arXiv2025

Sub-10 nm Quantification of Spin and Orbital Magnetic Moment Across the Metamagnetic Phase Transition in FeRh Using EMCD

Electron magnetic circular dichroism (EMCD) in transmission electron microscopy (TEM) enables element-specific measurement of spin and orbital magnetic moments, analogous to X-ray magnetic circular dichroism (XMCD). While the EMCD technique offers unmatched spatial resolution, its quantitative accuracy remains under scrutiny, particularly in beam-splitter geometries with convergent probes. Here, we systematically evaluate the limits of quantitative EMCD analysis using the first-order magnetostructural transition in the functional phase-change material FeRh as a tunable magnetic reference. Unlike previous EMCD studies primarily focused on elemental ferromagnets such as Fe, we demonstrate its applicability to a correlated material exhibiting coupled structural and magnetic order. We demonstrate that the extracted orbital-to-spin moment ratio ($m_\text{L}/m_\text{S}$) remains consistent with XMCD benchmarks for TEM probes down to approximately 6 nm, thereby establishing the validity range for reliable quantification. For nm-sized probes with higher convergence angles, we observe an enhanced $m_\text{L}/m_\text{S}$, which we attribute to a combination of instrumental factors and sensitivity to nanoscale heterogeneity within the probed volume. Our results confirm that EMCD provides quantitative agreement with macroscale techniques under suitable conditions, while uniquely enabling spatially confined measurements of local magnetic moments in functional magnetic materials, and allowing the study of interfacial, defect-mediated, or phase-separated magnetism that is inaccessible to photon-based methods.

cond-mat.mes-hall↗
arXiv2025

Magnon spectroscopy in the electron microscope

The miniaturisation of transistors is approaching its limits due to challenges in heat management and information transfer speed. To overcome these obstacles, emerging technologies such as spintronics are being developed, which leverage the electron's spin in addition to its charge. Local phenomena at interfaces or structural defects will greatly influence the efficiency of spin-based devices, making the ability to study and control spin wave propagation at the nano- and atomic scales a key challenge. The development of high-spatial-resolution tools to probe spin waves, also called magnons, at relevant lengthscales is thus essential to understand how their properties are affected by such local features. Here, we show the first experimental detection of bulk magnons at the nanoscale using scanning transmission electron microscopy. By employing high-resolution electron energy loss spectroscopy with hybrid-pixel direct electron detectors optimized for low acceleration voltages, we successfully overcome the challenges posed by weak signals and identify magnon excitations in a thin NiO nanocrystal. Advanced inelastic electron scattering simulations corroborate our findings. These results open new avenues for detecting magnons, exploring their dispersions and their modifications arising from nanoscale structural or chemical defects. This marks an important milestone in magnonics and presents exciting opportunities for the future development of spintronic devices.

cond-mat.mtrl-sci↗
arXiv2024

Dynamical theory of angle-resolved electron energy loss and gain spectroscopies of phonons and magnons in transmission electron microscopy including multiple scattering effects

We present a method for computing angle-resolved electron-energy-loss and gain spectroscopies for phonon and magnon excitations in transmission electron microscopy. Fractional scattering intensities are derived from the temperature-dependent time auto-correlation of the electron beam wave function. This method captures both single and multiple scattering processes, as well as dynamical diffraction effects. Our method remains computationally efficient, and it is easy to parallelize.

cond-mat.mtrl-sci↗
arXiv2024

Systematic Absences of Optical Phonon Modes in Phonon Dispersion Measured by Electron Microscopy

Phonon dispersion is widely used to elucidate the vibrational properties of materials. As an emerging technique, momentum-resolved vibrational spectroscopy in scanning transmission electron microscopy (STEM) offers an unparalleled approach to explore q-dependent phonon behavior at local structures. In this study, we systematically investigate the phonon dispersion of monolayer graphene across several Brillouin zones (BZs) using momentum-resolved vibrational spectroscopy and find that the optical phonon signals vanish at the Γ points with indices (hk0) satisfying h+2k=3n (n denoted integers). Theoretical analysis reveals that the observed phenomena arise from the complete destructive interference of the scattered waves from different basis atoms. This observation, corroborated by the study of diamond, should be a general characteristic of materials composed of symmetrically equivalent pairs of the same elements. Moreover, our results emphasize the importance of multiple scattering in interpreting the vibrational signals in bulk materials. We demonstrate that the systematic absences and dynamic effects, which have not been much appreciated before, offer new insights into the experimental assessment of local vibrational properties of materials.

cond-mat.mtrl-sci↗
arXiv2024

Direct observation of nanometer-scale orbital angular momentum accumulation

Conversion of charge to orbital angular momentum through the orbital Hall effect (OHE) holds transformative potential for the development of orbital-based electronics, however, it is challenging to directly observe the electrically generated orbital accumulation. Here, we detect the OHE by directly quantifying the orbital accumulation along the edges of a titanium thin film using a scanning transmission electron microscope. We measure the Ti L-edge using electron energy-loss spectroscopy with nanometer resolution and find a sizable orbital accumulation at the sample's outer perimeters, consistent with all signatures expected for the OHE, and determine an orbital diffusion length $\ell_o \approx 7.3$ nm. Our data points to a surprising dependence of the orbital diffusion length on the nano-structural morphology.

cond-mat.mes-hall↗
arXiv2024

Boundary-induced phase in epitaxial iron layers

We report the discovery of a boundary-induced body-centered tetragonal (bct) iron phase in thin films deposited on MgAl$_{2}$O$_{4}$ ($001$) substrates. We present evidence for this phase using detailed x-ray analysis and ab-initio density functional theory calculations. A lower magnetic moment and a rotation of the easy magnetisation direction are observed, as compared to body-centered cubic (bcc) iron. Our findings expand the range of known crystal and magnetic phases of iron, providing valuable insights for the development of heterostructure devices using ultra-thin iron layers.

cond-mat.mtrl-sci↗
arXiv2023

Real-Space Visualization of Frequency-Dependent Anisotropy of Atomic Vibrations

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.

cond-mat.mtrl-sci↗
arXiv2023

Unveiling the impact of temperature on magnon diffuse scattering detection in the transmission electron microscope

Magnon diffuse scattering (MDS) signals could 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 challenging due to their overlap with stronger thermal diffuse scattering (TDS) signals. In bcc Fe at 300 K, MDS signals greater than or comparable to TDS signals occur under the central Bragg disk, into a currently inaccesible energy-loss region. Therefore, to detect MDS in STEM, it is necessary to find conditions in which TDS and MDS signals can be separated. 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. Including the effects of atomic vibrations through a complex atomic electrostatic potential causes the linear growth of the MDS signal to change to a non-linear 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 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 acquisition times.

cond-mat.mtrl-sci↗
arXiv2023

Lessons from the harmonic oscillator -- a reconciliation of the Frequency-Resolved Frozen Phonon Multislice Method with other theoretical approaches

We compare the Frequency-Resolved Frozen Phonon Multislice (FRFPMS) method, introduced in 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 $1/ω^2$-scaling of FRFPMS spectra observed in 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 post-processing step.

cond-mat.mtrl-sci↗
arXiv2023

Control of the phonon band gap with isotopes in hexagonal boron nitride

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 $^{11}$BN the phonon band gap is about 7 meV wide, in $^{10}$BN 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.

cond-mat.mtrl-sci↗