455 research outputs found
Comment on "Quantized Orbital Angular Momentum Transfer and Magnetic Dichroism in the Interaction of Electron Vortices with Matter"
It was claimed (Lloyd et al., PRL 108 (2012) 074802) that energy loss
magnetic chiral dichroism (EMCD) with electron vortex beams is feasible, and
has even advantages over the standard setup with Bragg diffracted waves. In
this Comment, we show that Lloyd et al. ignored an important constraint on the
proposed selection rule for the transfer of angular momentum in the
interaction, namely that it is only valid for an atom located in the very
center of the vortex. As an experimental consequence, the EMCD signal will only
be strong for extremely small nanoparticles of 1 to 2 nm diameter.Comment: Submitted to Physical Review Letters 11 July 2012. Accepted for
publication 3 April 2013. "Copyright (2013) by the American Physical
Society." http://prl.aps.org
HAADF-STEM block-scanning strategy for local measurement of strain at the nanoscale
Lattice strain measurement of nanoscale semiconductor devices is crucial for
the semiconductor industry as strain substantially improves the electrical
performance of transistors. High resolution scanning transmission electron
microscopy (HR-STEM) imaging is an excellent tool that provides spatial
resolution at the atomic scale and strain information by applying Geometric
Phase Analysis or image fitting procedures. However, HR-STEM images regularly
suffer from scanning distortions and sample drift during image acquisition. In
this paper, we propose a new scanning strategy that drastically reduces
artefacts due to drift and scanning distortion, along with extending the field
of view. The method allows flexible tuning of the spatial resolution and
decouples the choice of field of view from the need for local atomic
resolution. It consists of the acquisition of a series of independent small
subimages containing an atomic resolution image of the local lattice. All
subimages are then analysed individually for strain by fitting a nonlinear
model to the lattice images. The obtained experimental strain maps are
quantitatively benchmarked against the Bessel diffraction technique. We
demonstrate that the proposed scanning strategy approaches the performance of
the diffraction technique while having the advantage that it does not require
specialized diffraction cameras
Mapping spin-polarised transitions with atomic resolution
The coupling between Angstrom-sized electron probes and spin polarised
electronic transitions shows that the inelastically scattered probe is in a
mixed state containing electron vortices with non-zero orbital angular
momentum. These electrons create an asymmetric intensity distribution in energy
filtered diffraction patterns, giving access to maps of the magnetic moments
with atomic resolution. A feasibility experiment shows evidence of the
predicted effect. Potential applications are column-by-column maps of magnetic
ordering, and the creation of Angstrom-sized free electrons with orbital
angular momentum by inelastic scattering in a thin ferromagnetic foil
Is magnetic chiral dichroism feasible with electron vortices?
We discuss the feasibility of detecting magnetic transitions with focused
electron vortex probes, suggested by selection rules for the magnetic quantum
number. We theoretically estimate the dichroic signal strength in the L
edge of the ferromagnetic d metals. It is shown that under realistic
conditions, the dichroic signal is undetectable for nanoparticles larger than
~1 nm. This is confirmed by a key experiment with nanometer sized vortices
Asymmetry and non-dispersivity in the Aharonov-Bohm effect
Decades ago, Aharonov and Bohm showed that electrons are affected by
electromagnetic potentials in the absence of forces due to fields. Zeilinger's
theorem describes this absence of classical force in quantum terms as the
"dispersionless" nature of the Aharonov-Bohm effect. Shelankov predicted the
presence of a quantum "force" for the same Aharonov-Bohm physical system as
elucidated by Berry. Here, we report an experiment designed to test Shelankov's
prediction and we provide a theoretical analysis that is intended to elucidate
the relation between Shelankov's prediction and Zeilinger's theorem. The
experiment consists of the Aharonov--Bohm physical system; free electrons pass
a magnetized nanorod and far--field electron diffraction is observed. The
diffraction pattern is asymmetric confirming one of Shelankov's predictions and
giving indirect experimental evidence for the presence of a quantum "force".
Our theoretical analysis shows that Zeilinger's theorem and Shelankov's result
are both special cases of one theorem.Comment: 16 pages, 5 figure
Bandgap measurement of high refractive index materials by off-axis EELS
In the present work, Cs aberration corrected and monochromated scanning
transmission electron microscopy electron energy loss spectroscopy STEM-EELS
has been used to explore experimental set-ups that allows bandgaps of high
refractive index materials to be determined. Semi-convergence and -collection
angles in the micro-radian range were combined with off-axis or dark field EELS
to avoid relativistic losses and guided light modes in the low loss range to
contribute to the acquired EEL spectra. Off-axis EELS further suppressed the
zero loss peak and the tail of the zero loss peak. The bandgap of several
GaAs-based materials were successfully determined by direct inspection and
without any background subtraction of the EEL spectra. The presented set-up
does not require that the acceleration voltage is set to below the Cerenkov
limit and can be applied over the entire acceleration voltage range of modern
TEMs and for a wide range of specimen thicknesses.Comment: 16 pages, 8 figure
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