108 research outputs found
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.Comment: 11 page
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.Comment: 9 pages, 20 figure
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
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.Comment: 5 pages, 4 figure
Fe1-xNix alloy nanoparticles encapsulated inside carbon nanotubes: Controlled synthesis, structure and magnetic properties
In the present work, different synthesis procedures have been demonstrated to fill carbon nanotubes (CNTs) with Fe1-xNix alloy nanoparticles (x = 0.33, 0.5). CNTs act as templates for the encapsulation of magnetic nanoparticles, and provide a protective shield against oxidation as well as prevent nanoparticles agglomeration. By variation of the reaction parameters, the purity of the samples, degree of filling, the composition and size of filling nanoparticles have been tailored and therefore the magnetic properties. The samples were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Bright-field (BF) TEM tomography, X-ray powder diffraction, superconducting quantum interference device (SQUID) and thermogravimetric analysis (TGA). The Fe1-x Nix-filled CNTs show a huge enhancement in the coercive fields compared to the corresponding bulk materials, which make them excellent candidates for several applications such as magnetic storage devices
Maximal Anderson Localization and Suppression of Surface Plasmons in Two-Dimensional Random Au Networks
Two-dimensional random metal networks possess unique electrical and optical
properties, such as almost total optical transparency and low sheet resistance,
which are closely related to their disordered structure. Here we present a
detailed experimental and theoretical investigation of their plasmonic
properties, revealing Anderson (disorder-driven) localized surface plasmon
(LSP) resonances of very large quality factors and spatial localization close
to the theoretical maximum, which couple to electromagnetic waves. Moreover,
they disappear above a geometry-dependent threshold at ca. 1.7 eV in the
investigated Au networks, explaining their large transparencies in the optical
spectrum
Electronic structure of epitaxial perovskite films in the two-dimensional limit: Role of the surface termination
An often-overlooked property of transition metal oxide thin films is their microscopic surface structure and its effect on the electronic properties in the ultrathin limit. Contrary to the expected conservation of the perovskite stacking order in the (001) direction, heteroepitaxially grown SrIrO3 films on TiO2-terminated SrTiO3 are found to exhibit a terminating SrO surface layer. The proposed mechanism for the self-organized conversion involves the adsorption of excess oxygen ions at the apical sites of the IrO2-terminated surface and the subsequent decomposition of the IrO6 octahedra into gaseous molecular IrO3 and the remaining SrO-terminated surface. Whereas the ab initio calculated electronic structure of SrO-terminated SrIrO3 in the monolayer limit exhibits a striking similarity to bulk Sr2IrO4, the broken octahedral symmetry at the IrO2-terminated surface would mix the otherwise crystal field split e(g) and t(2g) states, resulting in distinctly different low-energy electronic states. Published under license by AIP Publishing
Direct Observation of Plasmon Band Formation and Delocalization in Quasi-Infinite Nanoparticle Chains
Chains of metallic nanoparticles sustain strongly confined surface plasmons
with relatively low dielectric losses. To exploit these properties in
applications,such as waveguides, the fabrication of long chains of low disorder
and a thorough understanding of the plasmon-mode properties, such as dispersion
relations, are indispensable. Here, we use a wrinkled template for directed
self-assembly to assemble chains of gold nanoparticles. With this up-scalable
method, chain lengths from two particles (140 nm) to 20 particles (1500 nm) and
beyond can be fabricated. Electron energy-loss spectroscopy supported by
boundary element simulations, finite-difference time-domain, and a simplified
dipole coupling model reveal the evolution of a band of plasmonic waveguide
modes from degenerated single-particle modes in detail. In striking difference
from plasmonic rod-like structures, the plasmon band is confined in excitation
energy, which allows light manipulations below the diffraction limit. The
non-degenerated surface plasmon modes show suppressed radiative losses for
efficient energy propagation over a distance of 1500 nm
Dilution of the magnetic lattice in the Kitaev candidate -RuCl by Rh doping
Magnetic dilution of a well-established Kitaev candidate system is realized
in the substitutional RuRhCl series (). Optimized
syntheses protocols yield uniformly-doped single crystals and polycrystalline
powders that are isostructural to the parental -RuCl as per X-ray
diffraction. The Rh content is accurately determined by the quantitative
energy-dispersive X-ray spectroscopy technique with standards. We determine the
magnetic phase diagram of RuRhCl for in-plane magnetic fields
from magnetization and specific-heat measurements as a function of and
stacking periodicity, and identify the suppression of the magnetic order at 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 RuIrCl, we propose that chemical pressure
would contribute to the suppression of magnetic order especially in
RuIrCl and that the zigzag magnetic ground state appears to be
relatively robust with respect to the dilution of the
Kitaev----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
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