972 research outputs found
Sub-nanometer free electrons with topological charge
The holographic mask technique is used to create freely moving electrons with
quantized angular momentum. With electron optical elements they can be focused
to vortices with diameters below the nanometer range. The understanding of
these vortex beams is important for many applications. Here we present a theory
of focused free electron vortices. The agreement with experimental data is
excellent. As an immediate application, fundamental experimental parameters
like spherical aberration and partial coherence are determined.Comment: 4 pages, 5 figure
First principles theory of chiral dichroism in electron microscopy applied to 3d ferromagnets
Recently it was demonstrated (Schattschneider et al., Nature 441 (2006),
486), that an analogue of the X-ray magnetic circular dichroism (XMCD)
experiment can be performed with the transmission electron microscope (TEM).
The new phenomenon has been named energy-loss magnetic chiral dichroism (EMCD).
In this work we present a detailed ab initio study of the chiral dichroism in
the Fe, Co and Ni transition elements. We discuss the methods used for the
simulations together with the validity and accuracy of the treatment, which
can, in principle, apply to any given crystalline specimen. The dependence of
the dichroic signal on the sample thickness, accuracy of the detector position
and the size of convergence and collection angles is calculated.Comment: 9 pages, 6 figures, submitted to Physical Review
Experimental application of sum rules for electron energy loss magnetic chiral dichroism
We present a derivation of the orbital and spin sum rules for magnetic
circular dichroic spectra measured by electron energy loss spectroscopy in a
transmission electron microscope. These sum rules are obtained from the
differential cross section calculated for symmetric positions in the
diffraction pattern. Orbital and spin magnetic moments are expressed explicitly
in terms of experimental spectra and dynamical diffraction coefficients. We
estimate the ratio of spin to orbital magnetic moments and discuss first
experimental results for the Fe L_{2,3} edge.Comment: 11 pages, 2 figure
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
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 into beams closely resembling
Hermite-Gaussian HG and HG 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 . This combination serves as a generator of electron vortex beams of high
brilliance
On the Number of Facets of Three-Dimensional Dirichlet Stereohedra III: Full Cubic Groups
We are interested in the maximum possible number of facets that Dirichlet
stereohedra for three-dimensional crystallographic groups can have. The problem
for non-cubic groups was studied in previous papers by D. Bochis and the second
author (Discrete Comput. Geom. 25:3 (2001), 419-444, and Beitr. Algebra Geom.,
47:1 (2006), 89-120). This paper deals with ''full'' cubic groups, while
''quarter'' cubic groups are left for a subsequent paper. Here, ''full'' and
''quarter'' refers to the recent classification of three-dimensional
crystallographic groups by Conway, Delgado-Friedrichs, Huson and Thurston
(math.MG/9911185, Beitr. Algebra Geom. 42.2 (2001), 475-507).
Our main result in this paper is that Dirichlet stereohedra for any of the 27
full groups cannot have more than 25 facets. We also find stereohedra with 17
facets for one of these groups.Comment: 28 pages, 12 figures. Changes from v1: apart of some editing (mostly
at the end of the introduction) and addition of references, an appendix has
been added, which analyzes the case where the base point does not have
trivial stabilize
Nonlinear interaction of charged particles with a free electron gas beyond the random-phase approximation
A nonlinear description of the interaction of charged particles penetrating a
solid has become of basic importance in the interpretation of a variety of
physical phenomena. Here we develop a many-body theoretical approach to the
quadratic decay rate, energy loss, and wake potential of charged particles
moving in an interacting free electron gas. Explicit expressions for these
quantities are obtained either within the random-phase approximation (RPA) or
with full inclusion of short-range exchange and correlation effects. The Z^3
correction to the energy loss of ions is evaluated beyond RPA, in the limit of
low velocities.Comment: 5 pages, 2 figures To appear in Phys. Rev.
Energy-loss magnetic chiral dichroism (EMCD): Magnetic chiral dichroism in the electron microscope
A new technique called energy-loss magnetic chiral dichroism (EMCD) has recently been developed [P. Schattschneider, et al. Nature 441, 486 (2006)] to measure magnetic circular dichroism in the transmission electron microscope (TEM) with a spatial resolution of 10 nm. This novel technique is the TEM counterpart of x-ray magnetic circular dichroism, which is widely used for the characterization of magnetic materials with synchrotron radiation. In this paper we describe several experimental methods that can be used to measure the EMCD signal [P. Schattschneider, et al. Nature 441, 486 (2006); C. HĂ©bert, et al. Ultramicroscopy 108(3), 277 (2008); B. Warot-Fonrose, et al. Ultramicroscopy 108(5), 393 (2008); L. Calmels, et al. Phys. Rev. B 76, 060409 (2007); P. van Aken, et al. Microsc. Microanal. 13(3), 426 (2007)] and give a review of the recent improvements of this new investigation tool. The dependence of the EMCD on several experimental conditions (such as thickness, relative orientation of beam and sample, collection and convergence angle) is investigated in the transition metals iron, cobalt, and nickel. Different scattering geometries are illustrated; their advantages and disadvantages are detailed, together with current limitations. The next realistic perspectives of this technique consist of measuring atomic specific magnetic moments, using suitable spin and orbital sum rules, [L. Calmels, et al. Phys. Rev. B 76, 060409 (2007); J. Rusz, et al. Phys. Rev. B 76, 060408 (2007)] with a resolution down to 2 to 3 n
Minkowski-type and Alexandrov-type theorems for polyhedral herissons
Classical H.Minkowski theorems on existence and uniqueness of convex
polyhedra with prescribed directions and areas of faces as well as the
well-known generalization of H.Minkowski uniqueness theorem due to
A.D.Alexandrov are extended to a class of nonconvex polyhedra which are called
polyhedral herissons and may be described as polyhedra with injective spherical
image.Comment: 19 pages, 8 figures, LaTeX 2.0
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