A novel vortex generator and mode converter for electrons

A mode converter for electron vortex beams is described. Numerical simulations, confirmed by experiment, show that the converter transforms a vortex beam with topological charge $m=\pm 1$ into beams closely resembling Hermite-Gaussian HG$_{10}$ and HG$_{01}$ modes. The converter can be used as a mode discriminator or filter for electron vortex beams. Combining the converter with a phase plate turns a plane wave into modes with topological charge $m=\pm 1$. This combination serves as a generator of electron vortex beams of high brilliance

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

Various Compressed Sensing Set-Ups Evaluated Against Shannon Sampling Under Constraint of Constant Illumination

Under the constraint of constant illumination, an information criterion is formulated for the Fisher information that compressed sensing measurements in optical and transmission electron microscopy contain about the underlying parameters. Since this approach requires prior knowledge of the signal's support in the sparse basis, we develop a heuristic quantity, the detective quantum efficiency (DQE), that tracks this information criterion well without this knowledge. It is shown that for the investigated choice of sensing matrices, and in the absence of read-out noise, i.e. with only Poisson noise present, compressed sensing does not raise the amount of Fisher information in the recordings above that of Shannon sampling. Furthermore, enabled by the DQE's analytical tractability, the experimental designs are optimized by finding out the optimal fraction of on-pixels as a function of dose and read-out noise. Finally, we introduce a regularization and demonstrate, through simulations and experiment, that it yields reconstructions attaining minimum mean squared error at experimental settings predicted by the DQE as optimal.Comment: 18 pages, 13 figures. New Monte Carlo simulations in Figure 13 showing the behavior of the single-pixel camera under various magnitudes of read-out nois

Synthesis and Characterization of Photoreactive TiO2/Carbon Nanosheet Composites

We report the atomic layer deposition of titanium dioxide on carbon nanosheet templates and investigate the effects of post-deposition annealing in a helium environment using different characterization techniques. The crystallization of the titanium dioxide coating upon annealing is observed using in-situ X-ray diffraction. The (micro)-structural characterization of the films is carried out by scanning electron microscopy and advanced transmission electron microscopy techniques. Our study shows that the annealing of the atomic layer deposition processed and carbon nanosheets templated titanium dioxide layers in helium environment results in the formation of a porous, nanocrystalline and photocatalytically active titanium dioxide-carbon nanosheet composite film. Such composites are suitable for photocatalysis and dye-sensitized solar cells applications

Fault activity in the epicentral area of the 1580 Dover Strait (Pas-de-Calais) earthquake (northwestern Europe)

On 1580 April 6 one of the most destructive earthquakes of northwestern Europe took place in the Dover Strait (Pas de Calais). The epicentre of this seismic event, the magnitude of which is estimated to have been about 6.0, has been located in the offshore continuation of the North Artois shear zone, a major Variscan tectonic structure that traverses the Dover Strait. The location of this and two other moderate magnitude historical earthquakes in the Dover Strait suggests that the North Artois shear zone or some of its fault segments may be presently active. In order to investigate the possible fault activity in the epicentral area of the AD 1580 earthquake, we have gathered a large set of bathymetric and seismic-reflection data covering the almost-entire width of the Dover Strait. These data have revealed a broad structural zone comprising several subparallel WNW–ESE trending faults and folds, some of them significantly offsetting the Cretaceous bedrock. The geophysical investigation has also shown some indication of possible Quaternary fault activity. However, this activity only appears to have affected the lowermost layers of the sediment infilling Middle Pleistocene palaeobasins. This indicates that, if these faults have been active since Middle Pleistocene, their slip rates must have been very low. Hence, the AD 1580 earthquake appears to be a very infrequent event in the Dover Strait, representing a good example of the moderate magnitude earthquakes that sometimes occur in plate interiors on faults with unknown historical seismicity

Controlled growth of hexagonal gold nanostructures during thermally induced self-assembling on Ge(001) surface

Nano-sized gold has become an important material in various fields of science and technology, where control over the size and crystallography is desired to tailor the functionality. Gold crystallizes in the face-centered cubic (fcc) phase, and its hexagonal closed packed (hcp) structure is a very unusual and rare phase. Stable Au hcp phase has been reported to form in nanoparticles at the tips of some Ge nanowires. It has also recently been synthesized in the form of thin graphene-supported sheets which are unstable under electron beam irradiation. Here, we show that stable hcp Au 3D nanostructures with well-defined crystallographic orientation and size can be systematically created in a process of thermally induced self-assembly of thin Au layer on Ge(001) monocrystal. The Au hcp crystallite is present in each Au nanostructure and has been characterized by different electron microscopy techniques. We report that a careful heat treatment above the eutectic melting temperature and a controlled cooling is required to form the hcp phase of Au on a Ge single crystal. This new method gives scientific prospects to obtain stable Au hcp phase for future applications in a rather simple manner as well as redefine the phase diagram of Gold with Germanium

Coupling charge and topological reconstructions at polar oxide interfaces

In oxide heterostructures, different materials are integrated into a single artificial crystal, resulting in a breaking of inversion-symmetry across the heterointerfaces. A notable example is the interface between polar and non-polar materials, where valence discontinuities lead to otherwise inaccessible charge and spin states. This approach paved the way to the discovery of numerous unconventional properties absent in the bulk constituents. However, control of the geometric structure of the electronic wavefunctions in correlated oxides remains an open challenge. Here, we create heterostructures consisting of ultrathin SrRuO$_3$, an itinerant ferromagnet hosting momentum-space sources of Berry curvature, and LaAlO$_3$, a polar wide-bandgap insulator. Transmission electron microscopy reveals an atomically sharp LaO/RuO$_2$/SrO interface configuration, leading to excess charge being pinned near the LaAlO$_3$/SrRuO$_3$ interface. We demonstrate through magneto-optical characterization, theoretical calculations and transport measurements that the real-space charge reconstruction modifies the momentum-space Berry curvature in SrRuO$_3$, driving a reorganization of the topological charges in the band structure. Our results illustrate how the topological and magnetic features of oxides can be manipulated by engineering charge discontinuities at oxide interfaces.Comment: 5 pages main text (4 figures), 29 pages of supplementary informatio