Coherence in Energy Loss Spectra of Plasmons

Theoretical approaches to coherent excitation of two plasmons in a metal do not well agree with one another and with experimental results from electron energy loss spectrometry (EELS). We measured EELS spectra of polycrystalline aluminum films for various specimen thickness. By means of a new deconvolution method for multiple scattering, we obtained values between 0.6% and 3.3% for the probability F2 of the coherent double plasmon event, relative to the single plamon event. A review of earlier experimental as well as theoretical investigations is given. Our results together with a discussion of possible sources of error confirm our earlier findings that F2 is much smaller than previously thought, and is thickness dependent. We found the available predictions of the effect unsatisfactory; a full theoretical treatment of the problem is still missing

Interband Transitions in Electron Energy Loss Spectrometry

Electron energy loss spectrometry (EELS) allows one to experimentally obtain the dielectric permittivity :(ω, q) as a function of frequency ω and wave vector if. From information on inter band transitions in the probed medium can be drawn. In EELS, inter band transitions are screened by the movement of the loosely bound valence or conduction electrons. The screening effect may enhance or attenuate the strength of transitions, and tends to shift the frequencies of resonant oscillations. Another aspect of screening is the occurrence of longitudinal modes in the spectrum. So, great care has to be taken in interpreting loss spectra. Examples are discussed for image mode spectra and for diffraction mode spectra which latter render investigation of non-vertical transitions, and hence tests of band structure calculations possible

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

Compton Scattering In Electron Energy Loss Spectrometry

It is well known that the distribution of electron momenta (electron density in momentum representation) of gases can be probed by Compton scattering of either photons (γ-rays or X-rays) or electrons. Recently it has been shown that Compton scattering of electrons is suited to the study of the electron momentum densities of solids on a microscopic scale. This technique, known as ECOSS, Electron Compton Scattering from Solids can be done in the electron microscope by electron energy loss spectrometry (EELS). After a discussion of inherent approximations and the introduction of the reciprocal form factor a method is proposed in order to cope with the main difficulty, namely multiple scattering. Important applications of ECOSS are the study of anisotropy of momentum densities; correlation effects of conduction electrons in metals; and charge transfer in alloys

Influence of Bragg Scattering on Plasmon Spectra of Aluminum

Plasmon spectrometry is an important method to obtain information on many-body effects in the solid state. The plasmon halfwidth and the dispersion coefficient are well investigated for a number of materials, and compare well with quantum mechanical predictions. The excitation strength of the coherent double plasmon has been investigated to a lesser extent. Experimental results are at variance with one another and with theory. This is partly due to the plural scattering which masks the coherent double plasmon. Accurate analysis of plasmon spectra requires not only to remove the inelastic plural processes but also to take into account the coupling between Bragg and plasmon scattering at high scattering angles. It is shown that the excitation strength of the coherent double plasmon in forward direction falls below the detection limit when this correction is applied

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

Determination of the position maximum for electron Compton scattering in electron microscopy

We study electron Compton scattering with an electron microscope by means of a Castaing-Henry filter. In the electron-spectroscopic-diffraction mode the positions of the Compton maxima in the diffraction plane are determined. We find a nearly constant shift of this position with respect to the value given by E=q2/2. The intensity of Compton-scattered electrons does not peak at the scattering angle predicted by the binary collision mode. The energy dispersion of the Compton profile is well described by E=q2/2

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