Quantitative measurement of orbital angular momentum in electron microscopy

Electron vortex beams have been predicted to enable atomic scale magnetic information measurement, via transfer of orbital angular momentum. Research so far has focussed on developing production techniques and applications of these beams. However, methods to measure the outgoing orbital angular momentum distribution are also a crucial requirement towards this goal. Here, we use a method to obtain the orbital angular momentum decomposition of an electron beam, using a multi-pinhole interferometer. We demonstrate both its ability to accurately measure orbital angular momentum distribution, and its experimental limitations when used in a transmission electron microscope.Comment: 6 pages, 5 figure

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

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

Parallel functional and stoichiometric trait shifts in South American and African forest communities with elevation

The Amazon and Congo basins are the two largest continuous blocks of tropical forest with a central role for global biogeochemical cycles and ecology. However, both biomes differ in structure and species richness and composition. Understanding future directions of the response of both biomes to environmental change is paramount. We used one elevational gradient on both continents to investigate functional and stoichiometric trait shifts of tropical forest in South America and Africa. We measured community-weighted functional canopy traits and canopy and topsoil delta N-15 signatures. We found that the functional forest composition response along both transects was parallel, with a shift towards more nitrogen-conservative species at higher elevations. Moreover, canopy and topsoil delta N-15 signals decreased with increasing altitude, suggesting a more conservative N cycle at higher elevations. This cross-continental study provides empirical indications that both South American and African tropical forest show a parallel response with altitude, driven by nitrogen availability along the elevational gradients, which in turn induces a shift in the functional forest composition. More standardized research, and more research on other elevational gradients is needed to confirm our observations

Predation rates and prey selectivity in two predacious estuarine nematodes

Enoploides longispiculosus and Adoncholaimus fuscus are representatives of nematode genera prominent in sediments of the North Sea and adjacent estuaries. Both are predatory nematodes, although predation is facultative in the latter. The present study investigates functional responses and prey selectivity in both species through the use of controlled laboratory experiments. Both predators had strongly prey density-dependent predation rates. A maximal predation rate of 4 monhysterid prey nematodes per predator per 24 h was found inE. longispiculosus at prey densities of 200 ind. per petri dish and higher; no such maximal predation rate was found for A. fuscus, indicating that this species was prey-limited at all prey densities tested. Predation rates were strongly affected by temperature, with a Q10 close to 2 between 10 and 20 °C. Incubation in the light resulted in a similar decrease in predation rate compared to dark incubations, as did a temperature decrease from 20 to 10 °C. E. longispiculosus exhibited a clear preference for some nematode prey over others. An encounter probability model indicated that preferences could not be explained by encounter rates. Strike rates were low (E. longispiculosus, and exceptionally low (A. fuscus, indicating that many encounters did not result in attack, or that a portion of the attacks did not result in prey capture. The observed predation rates cannot be supported by prey nematode standing stock and production at the 2 sampling sites used in this study, where E. longispiculosus dominates the nematode community in abundance and, especially, biomass. A. fuscus may mainly derive food from feeding modes other than predation; E. longispiculosus may be prey-limited in its natural habitat. Since this nematode also feeds on other metazoans, it may also impact temporary meiofauna. The high predation rates and prey selectivity of predacious nematodes may be important structuring factors to meiofaunal communities

Selective attraction of marine bacterivorous nematodes to their bacterial food

This paper explores the role of selective attraction to food in determining the spatial (micro)distribution of closely related nematode species. The attractiveness of 3 different bacterial strains to 4 species of Monhysteridae, Diplolaimelloides meyli, Diplolaimella dievengatensis, Monhystera sp. and Geomonhystera disjuncta, was studied in a multiple choice design. In our study area, the 4 nematode species considered are associated with Spartina anglica detritus decay and have partially overlapping microhabitat preferences. As they all belong to the same feeding guild, they are potential competitors for food. Each of the 4 nematode species was attracted to the bacterial strain B1, but important interspecific differences were noted in the nematodes' response to live or heat-killed bacteria, to bacteria at different tell densities or of different age, and to the filtered supernatant of B1 culture. While the responses of D. meyli to the Gram-positive bacteria Halobacillus trueperi and to the Gramnegative Escherichia coli were similar, D. dievengatensis and Monhystera sp, were preferentially attracted to H. trueperi and E. coli, respectively. This opposite preference influenced both the numbers and their relative abundances of D. dievengatensis and Monhystera sp, inside bacterial patches in experiments with a mixed 2-species nematode inoculum. Bacterial cell density strongly influenced the nematode response, with D. meyli invariably preferring the highest cell densities offered, while D. dievengatensis and Monhystera sp. had a peak response at lower cell densities. Though chemotaxis is suggested as an underlying mechanism, the nature of the nematodes' response remains unproved. The present results strongly support the importance of food patchiness in determining the heterogeneous distribution of nematodes, and extend the concept in such a way as to allow for small differences in microhabitat choice between closely related species. They also support the view that nematodes are specialist feeders, though they probably select spots where suitable food is plentiful rather than individual food particles. Finally, the present study offers a baseline for an understanding and further study of patterns of succession among nematode species associated with decaying Spartina anglica detritus in terms of highly specific relationships with different strains, growth stages, and densities of bacteria involved in the mineralization of Spartina anglica-derived organic matter

Theory and applications of free-electron vortex states

Both classical and quantum waves can form vortices: with helical phase fronts and azimuthal current densities. These features determine the intrinsic orbital angular momentum carried by localized vortex states. In the past 25 years, optical vortex beams have become an inherent part of modern optics, with many remarkable achievements and applications. In the past decade, it has been realized and demonstrated that such vortex beams or wavepackets can also appear in free electron waves, in particular, in electron microscopy. Interest in free-electron vortex states quickly spread over different areas of physics: from basic aspects of quantum mechanics, via applications for fine probing of matter (including individual atoms), to high-energy particle collision and radiation processes. Here we provide a comprehensive review of theoretical and experimental studies in this emerging field of research. We describe the main properties of electron vortex states, experimental achievements and possible applications within transmission electron microscopy, as well as the possible role of vortex electrons in relativistic and high-energy processes. We aim to provide a balanced description including a pedagogical introduction, solid theoretical basis, and a wide range of practical details. Special attention is paid to translate theoretical insights into suggestions for future experiments, in electron microscopy and beyond, in any situation where free electrons occur.Comment: 87 pages, 34 figure