arXiv subjects

A. Lubk

Publications and source records attributed to A. Lubk.

18 recordsLinked to original sources

arXiv2025

Space Group Symmetry of Chiral Fe-deficient van der Waals Magnet $\text{Fe}_{\text{3-x}}\text{GeTe}_{\text{2}}$ Probed by Convergent Beam Electron Diffraction

The crystal symmetry of Fe-deficient $\text{Fe}_{\text{3-x}}\text{GeTe}_{\text{2}}$ at room temperature has been investigated by a combination of selected-area electron diffraction (SAED) and convergent-beam electron diffraction (CBED). By symmetry analysis of CBED patterns along different zone axis, the space group of $\text{Fe}_{\text{3-x}}\text{GeTe}_{\text{2}}$ at room-temperature has been identified as $P6_3mc$ (No.186), which derives from the high-symmetry parent system $\text{Fe}_{\text{3}}\text{GeTe}_{\text{2}}$ ($P6_3/mmc$) by breaking the mirror symmetry along the 6-fold rotation axis. The $P3m1$ (No.156) space group previously reported for $\text{Fe}_{\text{3-x}}\text{GeTe}_{\text{2}}$ is a subgroup of $P6_3mc$ suggesting further possible symmetry breaks in this non-stoichiometric system.

cond-mat.mes-hall↗

arXiv2025

Microscopic Observation of Non-Ergodic States in Two-Dimensional Non-Topological Bubble Lattices

Disordered 2D lattices, including hexatic and various glassy states, are observed in a wide range of 2D systems including colloidal nanoparticle assemblies and fluxon lattices. Their disordered nature determines the stability and mobility of these systems, as well as their response to the external stimuli. Here we report on the controlled creation and characterization of a disordered 2D lattice of non-topological magnetic bubbles in the non-centrosymmetric ferrimagnetic alloy Mn$_{1.4}$PtSn. By analyzing the type and frequency of fundamental lattice defects, such as dislocations, the orientational correlation, as well as the induced motion of the lattice in an external field, a non-ergodic glassy state, stabilized by directional application of an external field, is revealed.

cond-mat.mtrl-sci↗

arXiv2024

Determination of Magnetic Symmetries by Convergent Beam Electron Diffraction

Convergent-beam electron diffraction (CBED) is a well-established probe for spatial symmetries of crystalline samples, mainly exploiting the well-defined mapping between the diffraction groups (symmetry group of CBED patterns) and the point-group symmetries of the crystalline sample. In this work, we extend CBED to determine magnetic point groups. We construct all magnetic CBED groups, of which there exist 125. Then, we provide the complete mapping of the 122 magnetic point groups to corresponding magnetic CBED groups for all crystal orientations. In order to verify the group-theoretical considerations, we conduct electron-scattering simulations on antiferromagnetic crystals and provide guidelines for the experimental realization. Based on its feasibility using existing technology, as well as on its accuracy, high spatial resolution, and small required sample size, magnetic CBED promises to be become a valuable alternative method for magnetic structure determination.

cond-mat.mes-hall↗

arXiv2024

Transport of Intensity Phase Retrieval in the Presence of Intensity Variations and Unknown Boundary Conditions

The so-called Transport of Intensity Equation (TIE) phase retrieval technique is widely applied in light, x-ray and electron optics to reconstruct, e.g., refractive indices, electric and magnetic fields in solids. Here, we present a largely improved TIE reconstruction algorithm, which properly considers intensity variations as well as unknown boundary conditions in a finite difference implementation of the Transport of Intensity partial differential equation. That largely removes reconstruction artifacts encountered in state-of-the-art Poisson solvers of the TIE, and hence significantly increases the applicability of the technique.

cond-mat.mtrl-sci↗

arXiv2024

Optical and acoustic plasmons in the layered material Sr$_2$RuO$_4$

We use momentum-dependent electron energy-loss spectroscopy in transmission to study collective charge excitations in the layer metal Sr$_2$RuO$_4$. This metal has a transition from a perfect Fermi liquid below $T\approx30\,$K into a "strange" metal phase above $T\approx800\,$K. We cover a complete range between in-phase and out-of-phase oscillations. Outside the classical range of electron-hole excitations, leading to a Landau damping, we observe well-defined plasmons. The optical (acoustic) plasmon due to an in-phase (out-of-phase) charge oscillation of neighbouring layers exhibits a quadratic (linear) positive dispersion. Using a model for the Coulomb interaction of the charges in a layered system, it is possible to describe the range of optical plasmon excitations at high energies in a mean-field random phase approximation without taking correlation effects into account. In contrast, resonant inelastic X-ray scattering data show at low energies an enhancement of the acoustic plasmon velocity due to correlation effects. This difference can be explained by an energy dependent effective mass which changes from $\approx$ 3.5 at low energy to 1 at high energy near the optical plasmon energy. There are no signs of over-damped plasmons predicted by holographic theories.

cond-mat.str-el↗

arXiv2024

2024 Roadmap on Magnetic Microscopy Techniques and Their Applications in Materials Science

Considering the growing interest in magnetic materials for unconventional computing, data storage, and sensor applications, there is active research not only on material synthesis but also characterisation of their properties. In addition to structural and integral magnetic characterisations, imaging of magnetization patterns, current distributions and magnetic fields at nano- and microscale is of major importance to understand the material responses and qualify them for specific applications. In this roadmap, we aim to cover a broad portfolio of techniques to perform nano- and microscale magnetic imaging using SQUIDs, spin center and Hall effect magnetometries, scanning probe microscopies, x-ray- and electron-based methods as well as magnetooptics and nanoMRI. The roadmap is aimed as a single access point of information for experts in the field as well as the young generation of students outlining prospects of the development of magnetic imaging technologies for the upcoming decade with a focus on physics, materials science, and chemistry of planar, 3D and geometrically curved objects of different material classes including 2D materials, complex oxides, semi-metals, multiferroics, skyrmions, antiferromagnets, frustrated magnets, magnetic molecules/nanoparticles, ionic conductors, superconductors, spintronic and spinorbitronic materials.

cond-mat.mtrl-sci↗

arXiv2022

Dilution of the magnetic lattice in the Kitaev candidate $α$-RuCl$_3$ by Rh$^{3+}$ doping

Magnetic dilution of a well-established Kitaev candidate system is realized in the substitutional Ru$_{1-x}$Rh$_x$Cl$_3$ series ($x = 0.02-0.6$). Optimized syntheses protocols yield uniformly-doped single crystals and polycrystalline powders that are isostructural to the parental $α$-RuCl$_3$ as per X-ray diffraction. The Rh content $x$ is accurately determined by the quantitative energy-dispersive X-ray spectroscopy technique with standards. We determine the magnetic phase diagram of Ru$_{1-x}$Rh$_x$Cl$_3$ for in-plane magnetic fields from magnetization and specific-heat measurements as a function of $x$ and stacking periodicity, and identify the suppression of the magnetic order at $x \approx 0.2$ towards a disordered phase, which does not show any clear signature of freezing into a spin glass. Comparing with previous studies on the substitution series Ru$_{1-x}$Ir$_x$Cl$_3$, we propose that chemical pressure would contribute to the suppression of magnetic order especially in Ru$_{1-x}$Ir$_x$Cl$_3$ and that the zigzag magnetic ground state appears to be relatively robust with respect to the dilution of the Kitaev--$Γ$--Heisenberg magnetic lattice. We also discovered a slight dependence of the magnetic properties on thermal cycling, which would be due to an incomplete structural transition.

cond-mat.str-el↗

arXiv2021

Field tunable three-dimensional magnetic nanotextures in cobalt-nickel nanowires

Cylindrical magnetic nanowires with large transversal magnetocrystalline anisotropy have been shown to sustain non-trivial magnetic configurations resulting from the interplay of spatial confinement, exchange, and anisotropies. Exploiting these peculiar 3D spin configurations and their solitonic inhomogeneities are prospected to improve magnetization switching in future spintronics, such as power-saving magnetic memory and logic applications. Here we employ holographic vector field electron tomography to reconstruct the remanent magnetic states in CoNi nanowires with 10 nm resolution in 3D, with a particular focus on domain walls between remanent states and ubiquitous real-structure effects stemming from irregular morphology and anisotropy variations. By tuning the applied magnetic field direction, both longitudinal and transverse multi-vortex states of different chiralities and peculiar 3D features such as shifted vortex cores are stabilized. The chiral domain wall between the longitudinal vortices of opposite chiralities exhibits a complex 3D shape characterized by a push out of the central vortex line and a gain in exchange and anisotropy energy. A similar complex 3D texture, including bent vortex lines, forms at the domain boundary between transverse-vortex states and longitudinal configurations. Micromagnetic simulations allow an understanding of the origin of the observed complex magnetic states.

cond-mat.mes-hall↗

arXiv2021

Hard X-ray photoemission spectroscopy of LaVO$_3$/SrTiO$_3$: Band alignment and electronic reconstruction

The heterostructure consisting of the Mott insulator LaVO$_3$ and the band insulator SrTiO$_3$ is considered a promising candidate for future photovoltaic applications. Not only does the (direct) excitation gap of LaVO$_3$ match well the solar spectrum, but its correlated nature and predicted built-in potential, owing to the non-polar/polar interface when integrated with SrTiO$_3$, also offer remarkable advantages over conventional solar cells. However, experimental data beyond the observation of a thickness-dependent metal-insulator transition is scarce and a profound, microscopic understanding of the electronic properties is still lacking. By means of soft and hard X-ray photoemission spectroscopy as well as resistivity and Hall effect measurements we study the electrical properties, band bending, and band alignment of LaVO$_3$/SrTiO$_3$ heterostructures. We find a critical LaVO$_3$ thickness of five unit cells, confinement of the conducting electrons to exclusively Ti 3$d$ states at the interface, and a potential gradient in the film. From these findings we conclude on electronic reconstruction as the driving mechanism for the formation of the metallic interface in LaVO$_3$/SrTiO$_3$.

cond-mat.mtrl-sci↗

arXiv2021

Layered van der Waals topological metals of TaTMTe4 (TM = Ir, Rh, Ru) family

Layered van~der~Waals materials of the family TaTMTe$_4$ (TM=Ir, Rh, Ru) are showing very interesting electronic properties. Here we report the synthesis, crystal growth and structural characterization of TaIrTe$_4$, TaRhTe$_4$, TaIr$_{1-x}$Rh$_{x}$Te$_4$ ($x = 0.06$; 0.14; 0.78; 0.92) and Ta$_{1+x}$Ru$_{1-x}$Te$_4$ single crystals. For Ta$_{1+x}$Ru$_{1-x}$Te$_4$ off-stoichiometry is shown. X-ray powder diffraction confirms that TaRhTe4 is isostructural to TaIrTe4. We show that all these compounds are metallic with diamagnetic behavior. Ta$_{1.26(2)}$Ru$_{0.75(2)}$Te$_{4.000(8)}$ exhibits an upturn in the resistivity at low temperatures which is strongly field dependent. Below $T \approx 4$K we observed signatures of the superconductivity in the TaIr$_{1-x}$Rh$_{x}$Te$_4$ compounds for $x = 0.92$. Magnetotransport measurements on all samples show weak magnetoresistance (MR) field dependence that is typically quadratic-in-field. However, for TaIr$_{1-x}$Rh$_{x}$Te$_4$ with $x\approx 0.78$, the MR has a linear term dominating in low fields that indicates the presence of Dirac cones in the vicinity of the Fermi energy. For TaRhTe$_4$ series the MR is almost isotropic. We have performed electronic structure calculations for isostructural TaIrTe$_4$ and TaRhTe$_4$ together with the projected total density of states. The main difference is appearance of the Rh-band close to the Fermi level.

cond-mat.str-el↗

arXiv2021

Structural Study of a Self-Assembled Gold Mesocrystal Grain by Coherent X-ray Diffraction Imaging

Mesocrystals are nanostructured materials consisting of individual nanocrystals having a preferred crystallographic orientation. On mesoscopic length scales, the properties of mesocrystals are strongly affected by structural heterogeneity. Here, we report the detailed structural characterization of a faceted mesocrystal grain self-assembled from 60 nm sized gold nanocubes. Using coherent X-ray diffraction imaging, we determined the structure of the mesocrystal with the resolution sufficient to resolve each gold nanoparticle. The reconstructed electron density of the gold mesocrystal reveals its intrinsic structural heterogeneity, including local deviations of lattice parameters, and the presence of internal defects. The strain distribution shows that the average superlattice obtained by angular X-ray cross-correlation analysis and the real, multidomain structure of a mesocrystal are very close to each other, with a deviation less than 10 percent. These results will provide an important impact to understanding of the fundamental principles of structuring and self-assembly including ensuing properties of mesocrystals.

cond-mat.mtrl-sci↗

arXiv2020

Polymorphic PtBi2: Growth, structure and superconducting properties

PtBi$_2$ is a polymorphic system with interesting electronic properties. Here we report optimized crystal growth and structural characterization of pyrite-type and trigonal modification of PtBi$_2$. Selected area electron diffraction, X-ray powder diffraction and further Rietveld refinement confirms that trigonal PtBi$_2$ crystallizes in non-centrosymmetric $P31m$ space group, pyrite-type PtBi$_2$ in $Pa\bar{3}$ space group. Series of Pt$_{1-x}$Rh$_x$Bi$_2$ samples was obtained for $x=0, 0.03, 0.35$ in the trigonal PtBi$_2$ structure. These Pt$_{1-x}$Rh$_x$Bi$_2$ compounds become superconducting where critical temperature increases from $T_c=600$ mK for $x=0$ up to $T_c=2.7$ K for $x=0.35$. Furthermore we calculate the electronic band structure, using the structure parameters obtained. The calculated density of states (DOS) shows a minimum for the stochiometric compound at the Fermi level. These findings warrant further research by broader array of experimental techniques, as well as the effect of the substitution on the non-trivial band structure.

cond-mat.mtrl-sci↗

arXiv2020

The incommensurate magnet iron monophosphide FeP: Crystal growth and characterization

We report an optimized chemical vapor transport method, which allows growing FeP single crystals up to 500 mg in mass and 80 $mm^{3}$ in volume. The high quality of the crystals obtained by this method was confirmed by means of EDX, high-resolution TEM, low-temperature single crystal XRD and neutron diffraction experiments. We investigated the transport and magnetic properties of the single crystals and calculated the electronic band structure of FeP. We show both theoretically and experimentally, that the ground state of FeP is metallic. The examination of the magnetic data reveals antiferromagnetic order below T$_{N}$ =119 K while transport remains metallic in both the paramagnetic and the antiferromagnetic phase. The analysis of the neutron diffraction data shows an incommensurate magnetic structure with the propagation vector Q=(0, 0, $\pmδ$), where $δ$ $\sim$ 0.2. For the full understanding of the magnetic state, further experiments are needed. The successful growth of large high-quality single crystals opens the opportunity for further investigations of itinerant magnets with incommensurate spin structures using a wide range of experimental tools.

cond-mat.mtrl-sci↗

arXiv2018

Spectral field mapping in plasmonic nanostructures with nanometer resolution

Plasmonic nanostructures and devices are rapidly transforming light manipulation technology by allowing to modify and enhance optical fields on sub-wavelength scales. Advances in this field rely heavily on the development of new characterization methods for the fundamental nanoscale interactions. However, the direct and quantitative mapping of transient electric and magnetic fields characterizing the plasmonic coupling has been proven elusive to date. Here we demonstrate how to directly measure the inelastic momentum transfer of surface plasmon modes via the energy-loss filtered deflection of a focused electron beam in a transmission electron microscope. By scanning the beam over the sample we obtain a spatially and spectrally resolved deflection map and we further show how this deflection is related quantitatively to the spectral component of the induced electric and magnetic fields pertaining to the mode. In some regards this technique is an extension to the established differential phase contrast into the dynamic regime.

physics.optics↗

arXiv2016

Symmetry-constrained electron vortex propagation

Electron vortex beams hold great promise for development in transmission electron microscopy, but have yet to be widely adopted. This is partly due to the complex set of interactions that occur between a beam carrying orbital angular momentum (OAM) and a sample. Herein, the system is simplified to focus on the interaction between geometrical symmetries, OAM and topology. We present multiple simulations, alongside experimental data to study the behaviour of a variety of electron vortex beams after interacting with apertures of different symmetries, and investigate the effect on their OAM and vortex structure, both in the far-field and under free-space propagation.

physics.optics↗

arXiv2014

Nanometer-scale Tomographic Reconstruction of 3D Electrostatic Potentials in GaAs/AlGaAs Core-Shell Nanowires

We report on the development of Electron Holographic Tomography towards a versatile potential measurement technique, overcoming several limitations, such as a limited tilt range, previously hampering a reproducible and accurate electrostatic potential reconstruction in three dimensions. Most notably, tomographic reconstruction is performed on optimally sampled polar grids taking into account symmetry and other spatial constraints of the nanostructure. Furthermore, holographic tilt series acquisition and alignment have been automated and adapted to three dimensions. We demonstrate 6 nm spatial and 0.2 V signal resolution by reconstructing various, previously hidden, potential details of a GaAs/AlGaAs core-shell nanowire. The improved tomographic reconstruction opens pathways towards the detection of minute potentials in nanostructures and an increase in speed and accuracy in related techniques such as X-ray tomography.

cond-mat.mtrl-sci↗

arXiv2013

Exploiting lens aberrations to create electron vortex beams

A model for a new electron vortex beam production method is proposed and experimentally demonstrated. The technique calls on the controlled manipulation of the degrees of freedom of the lens aberrations to achieve a helical phase front. These degrees of freedom are accessible by using the corrector lenses of a transmission electron microscope. The vortex beam is produced through a particular alignment of these lenses into a specifically designed astigmatic state and applying an annular aperture in the condensor plane. Experimental results are found to be in good agreement with simulations.

physics.optics↗

arXiv2009

First-principles study of ferroelectric domain walls in multiferroic bismuth ferrite

We present a first-principles density functional study of the structural, electronic and magnetic properties of the ferroelectric domain walls in multiferroic BiFeO3. We find that domain walls in which the rotations of the oxygen octahedra do not change their phase when the polarization reorients are the most favorable, and of these the 109 degree domain wall centered around the BiO plane has the lowest energy. The 109 degree and 180 degree walls have a significant change in the component of their polarization perpendicular to the wall; the corresponding step in the electrostatic potential is consistent with a recent report of electrical conductivity at the domain walls. Finally, we show that changes in the Fe-O-Fe bond angles at the domain walls cause changes in the canting of the Fe magnetic moments which can enhance the local magnetization at the domain walls.

cond-mat.mtrl-sci↗