28 research outputs found
Inelastic electron-vortex-beam scattering
Recent theoretical and experimental developments in the field of electron
vortex beam physics have raised questions on what exactly this novelty in the
field of electron microscopy (and other fields, such as particle physics)
really provides. An important part in the answer to those questions lies in
scattering theory. The present investigation explores various aspects of
inelastic quantum scattering theory for cylindrically symmetric beams with
orbital angular momentum. The model system of Coulomb scattering on a hydrogen
atom provides the setting to address various open questions: How is momentum
transferred? Do vortex beams selectively excite atoms, and how can one employ
vortex beams to detect magnetic transitions? The analytical approach presented
here provides answers to these questions. OAM transfer is possible, but not
through selective excitation; rather, by pre- and post-selection one can filter
out the relevant contributions to a specific signal
Rutherford scattering of electron vortices
By considering a cylindrically symmetric generalization of a plane wave, the
first Born approximation of screened Coulomb scattering unfolds two new
dimensions in the scattering problem: transverse momentum and orbital angular
momentum of the incoming beam. In this paper, the elastic Coulomb scattering
amplitude is calculated analytically for incoming Bessel beams. This reveals
novel features occurring for wide angle scattering when the incoming beam is
correctly prepared. The result successfully generalizes the well known
Rutherford formula, incorporating transverse and orbital angular momentum into
the formalism.Comment: 9 pages, 5 figure
Prospects for versatile phase manipulation in the TEM: beyond aberration correction
In this paper we explore the desirability of a transmission electron
microscope in which the phase of the electron wave can be freely controlled. We
discuss different existing methods to manipulate the phase of the electron wave
and their limitations. We show how with the help of current techniques the
electron wave can already be crafted into specific classes of waves each having
their own peculiar properties. Assuming a versatile phase modulation device is
feasible, we explore possible benefits and methods that could come into
existence borrowing from light optics where so-called spatial light modulators
provide programmable phase plates for quite some time now. We demonstrate that
a fully controllable phase plate building on Harald Rose's legacy in aberration
correction and electron optics in general would open an exciting field of
research and applications.Comment: 9 pages, 4 figures, special Ultramicroscopy issue for PICO2015
conferenc
Shaping electron beams for the generation of innovative measurements in the (S)TEM
In TEM, a typical goal consists of making a small electron probe in the
sample plane in order to obtain high spatial resolution in scanning
transmission electron microscopy. In order to do so, the phase of the electron
wave is corrected to resemble a spherical wave compensating for aberrations in
the magnetic lenses. In this contribution we discuss the advantage of changing
the phase of an electron wave in a specific way in order to obtain
fundamentally different electron probes opening up new application in the
(S)TEM. We focus on electron vortex states as a specific family of waves with
an azimuthal phase signature and discuss their properties, production and
applications. The concepts presented here are rather general and also different
classes of probes can be obtained in a similar fashion showing that electron
probes can be tuned to optimise a specific measurement or interaction
Spin effects in electron vortex states
The recent experimental realization of electron vortex beams opens up a wide research domain previously unexplored. The present paper explores the relativistic properties of these electron vortex beams, and quantifies deviations from the scalar wave theory. It is common in electron optics to use the Schrödinger equation neglecting spin. The present paper investigates the role of spin and the total angular momentum Jz and how it pertains to the vortex states. As an application, we also investigate if it is possible to use holographic reconstruction to create novel total angular momentum eigenstates in a transmission electron microscope. It is demonstrated that relativistic spin coupling effects disappear in the paraxial limit, and spin effects in holographically created electron vortex beams can only be exploited by using specialized magnetic apertures
Identification and characterization of Crumbs polarity complex proteins in Caenorhabditis elegans
Crumbs proteins are evolutionarily conserved transmembrane proteins with essential roles in promoting the formation of the apical domain in epithelial cells. The short intracellular tail of Crumbs proteins are known to interact with several proteins, including the scaffolding protein PALS1 (protein associated with LIN7, Stardust in Drosophila). PALS1 in turn binds to a second scaffolding protein PATJ (PALS1-associated tight junction protein) to form the core Crumbs/PALS1/PATJ complex. While essential roles in epithelial organization have been shown for Crumbs proteins in Drosophila and mammalian systems, the three Caenorhabditis elegans crumbs genes are dispensable for epithelial polarization and development. Here, we investigated the presence and function of PALS1 and PATJ orthologs in C. elegans. We identified MAGU-2 as the C. elegans ortholog of PALS1 and show that MAGU-2 interacts with all three Crumbs proteins and localizes to the apical membrane domain of intestinal epithelial cells in a Crumbs-dependent fashion. Similar to crumbs mutants, magu-2 deletion showed no epithelial polarity defects. We also identified MPZ-1 as a candidate ortholog of PATJ based on the physical interaction with MAGU-2 and sequence similarity with PATJ proteins. However, MPZ-1 is not broadly expressed in epithelial tissues and, therefore, not likely a core component of the C. elegans Crumbs complex. Finally, we show overexpression of the Crumbs proteins EAT-20 or CRB-3 can lead to apical membrane expansion in the intestine. Our results shed light on the composition of the C. elegans Crumbs complex and indicate that the role of Crumbs proteins in promoting apical domain formation is conserved