705 research outputs found
Surface Core Hole Electron Energy-Loss Fine Structure in MgO: Experiment and Theory
No full textCore holes are an important contributing factor to the core-loss fine structure in electron energy-loss spectroscopy (EELS). While there has been much work on bulk materials, less is known about core hole screening in more complex dielectric environments, such as free surfaces or interfaces between two different materials, even though the latter is frequently encountered in high spatial resolution EELS analysis. In this work, experimental O K- and Mg L3,2-EELS edges from the free surfaces of a MgO cube are measured using scanning transmission electron microscopy (STEM). The free surface O K-spectrum shows extra intensity at the edge onset compared with the “bulk” spectrum. Core hole screening near a free surface is modeled using electrodynamic theory. It is shown that much of the extra intensity is due to reduced core hole screening, while genuine surface states make a smaller contribution to the fine structure. The low energy Mg L3,2-edge, however, does not show any significant change at a free surface. This is because the measurement is less surface sensitive due to strong delocalization
Musika eta Mintzairaren Sorrera
No full textEn el presente artículo el autor se refiere a la relación evolutiva entre música y lenguaje y a la naturaleza de ambas habilidades, exponiendo distintas teorías del pasado y actuales en torno a la cuestión. El articulista defiende con B.G. Levman que tanto el lenguaje como la música nacieron de una misma capacidad primitiva y considera aquel lenguaje fundamentalmente como música. Da cuenta de los pros y los contras de las hipótesis que adelanta. En este sentido, expone las conductas de diversos pueblos y razas y diferentes animales, componiendo un tratamiento particularmente completo del tema con análisis efectuados desde diversas disciplinas: etnomusicología, etología, antropología, psicología, acústica.Artikulu honetan autoreak musika eta mintzairaren harreman ebolutiboaz nahiz bi gaitasunon izaeraz dihardu, horren gainean diren eta izan diren teoria ezberdinak azalduz. B.G. Levman artikulugileak berak bai mintzaira eta bai musika antzin-gaitasun beretik sortu zirela defenditzen du, eta orduko mintzaira hura funtsean musikazkotzat jotzen du. Agertzen dituen hipotesien aldekotasun eta kontrakotasunak ematen ditu. Horretarako mundu zabaleko hainbat herri eta animalia arrazen jokamoldeak ekartzen ditu paperera, eta etnomusikologia, etologia, antropologia, psikologia, akustikaren ikuspegitik aztertzen, gaiaren tratamendu bereziki borobila eta osatua eginez.Dans cet article, l'auteur parle de la relation évolutive entre musique et langage et la nature de ces deux talents, en exposant différentes théories du passé et du présent concernant cette question. L'auteur et B.G. Levman défendent la théorie qui soutient que le langage et la musique naquirent d'une même capacité primitive et considèrent ce langage fondamentalement comme de la musique. Il explique le pour et le contre de l'hypothèse qu'il avance. Dans ce sens, il expose les conduites de divers peuples et races et de différents animaux, en traitant le sujet d'une façon particulièrement complète, avec des analyses sur diverses disciplines: ethno-musicologie, éthologie, anthropologie, psychologie, acoustic.In the present article the author makes reference to the evolutionary relationship between music and language and to the nature of both skills, exposing different theories in the past and present in connection with this issue. The author maintains, together with B.G.Levman, that both language and music were born out of the same primitive capacity and considers that language is, fundamentally, music. He also reports on the pros and cons of the hypothesis he advances. In this sense, he describes the conduct of various peoples and races and of various animals, constructing a particularly complete treatment of the topic with analysis carried out from various disciplines: ethnomusicology, ethology, anthropology and psychology
Coherent electron Compton scattering and the non-diagonal electron momentum density of solids
No full textExperimental techniques that probe the electronic structure of crystalline solids are vital for exploring novel condensed matter phenomena. In coherent Compton scattering the Compton signal due to interference of an incident and Bragg diffracted beam is measured. This gives the projected, non-diagonal electron momentum density of the solid, a quantity that is sensitive to both the amplitude and phase of the electron wavefunction. Here coherent electron Compton scattering is demonstrated using electron energy loss spectroscopy in the transmission electron microscope. The technique has several advantages over coherent X-ray Compton scattering, such as a superior spatial resolution and the use of smaller specimens to generate Bragg beams of sufficient intensity. The conditions for a directly interpretable coherent electron Compton signal are established. Results are presented for the projected, non-diagonal electron momentum density for silicon under 004 and Bragg beam set ups
Quantum theory of magnon excitation by high energy electron beams
No full textThe role of magnon inelastic scattering in high energy electron diffraction of spin unpolarised electron beams, including vortex beams, is investigated theoretically for a Heisenberg ferromagnet. The interaction is between the atomic magnetic dipoles in the specimen and orbital angular momentum (OAM) of the electron beam. Magnon inelastic scattering by vortex beams is allowed despite many atoms along the magnon spin wave experiencing mixed OAM states. The scattering cross-section is however independent of the vortex beam winding number. In the case of planes waves in ferromagnetic iron, the magnon diffuse scattered intensity is significantly smaller than phonons in the energy loss range currently accessible by state-of-the-art monochromated electron energy loss spectroscopy (EELS). Nevertheless, it is shown that the long-range magnetic field of the atomic dipoles has a similar role to dipole scattering in phonon excitation. This means that magnons could, in principle, be detected using aloof beam EELS, where long acquisition times can be realised without any specimen beam damage, an important pre-requisite for detecting the weak magnon signal
A semi-classical theory of magnetic inelastic scattering in transmission electron energy loss spectroscopy
No full textThe feasibility of detecting magnetic excitations using monochromated electron energy loss spectroscopy in the transmission electron microscope is examined. Inelastic scattering cross-sections are derived using a semi-classical electrodynamic model, and applied to AC magnetic susceptibility measurements and magnon characterization. Consideration is given to electron probes with a magnetic moment, such as vortex beams, where additional inelastic scattering can take place due to the change in magnetic potential energy of the incident electron in a non-uniform magnetic field. This so-called ‘Stern-Gerlach’ energy loss can be used to enhance the strength of the scattering by increasing the orbital angular momentum of the vortex beam, and enables separation of magnetic from non-magnetic (i.e. dielectric) energy losses, thus providing a promising experimental route for detecting magnons. AC magnetic susceptibility measurements are however not feasible using Stern-Gerlach energy losses for a vortex beam
On the electron vortex beam wavefunction within a crystal
No full textElectron vortex beams are distorted by scattering within a crystal, so that the wavefunction can effectively be decomposed into many vortex components. Using a Bloch wave approach equations are derived for vortex beam decomposition at any given depth and with respect to any frame of reference. In the kinematic limit (small specimen thickness) scattering largely takes place at the neighbouring atom columns with a local phase change of π/2 rad. When viewed along the beam propagation direction only one vortex component is present at the specimen entrance surface (i.e. the ‘free space’ vortex in vacuum), but at larger depths the probe is in a mixed state due to Bragg scattering. Simulations show that there is no direct correlation between vortex components and the 〈Lz〉 pendellösung, i.e. at a given depth probes with relatively constant 〈Lz〉 can be in a more mixed state compared to those with more rapidly varying 〈Lz〉. This suggests that minimising oscillations in the 〈Lz〉 pendellösung by probe channelling is not the only criterion for generating a strong electron energy loss magnetic circular dichroism (EMCD) signal
Dynamic scattering of electron vortex beams – A Bloch wave analysis
No full textTwo important applications of electron vortex beams are in electron magnetic chiral dichroism (EMCD) measurements and nanoparticle manipulation. In both cases orbital angular momentum (〈Lz〉) transfer between the vortex beam and the specimen due to dynamic scattering is critical. In general the 〈Lz〉 pendellösung consists of short and long wavelength oscillations. The former is due to interference between the tightly bound 1s and more dispersive non-1s Bloch states, while the latter is due to interference between the non-1s states. For EMCD experiments with ±ħ angular momentum beams, momentum transfer can be minimised by selecting the appropriate aperture size, so that the probe wavefunction approximately matches that of the 2p-type Bloch states. For manipulating nanoparticles with large angular momentum beams small apertures are required to excite the 1s state and thereby enhance the short wavelength oscillations in 〈Lz〉. This enables efficient momentum transfer to the specimen, provided the nanoparticle dimension corresponds to a minimum in the 〈Lz〉 pendellösung
An inelastic multislice simulation method incorporating plasmon energy losses
No full textQuantitative electron microscopy requires accurate simulation methods that take into account both elastic and inelastic scattering of the high energy electrons within the specimen. Here a method to combine plasmon excitations, the dominant energy loss mechanism in a solid, with conventional frozen phonon, multislice simulations is presented. The Monte Carlo based method estimates the plasmon scattering path length and scattering angle using random numbers and modifies the transmission and propagator functions in the multislice calculation accordingly. Comparison of energy filtered, convergent beam electron diffraction patterns in [110]-Si show good agreement between simulation and experiment. Simulations also show that plasmon excitation decreases the high angle annular dark field signal from atom columns, due to the plasmon scattering angle suppressing electron beam channeling along the atom columns. The effect on resolution and peak-to-background ratio of the atom columns is however small
Background subtraction in electron Compton spectroscopy
No full textCompton scattering in electron energy loss spectroscopy (EELS) is used to quantify the momentum distribution of occupied electronic states in a solid. The Compton signal is a broad feature with a width of several hundred eV. Furthermore, the weak intensity results in a low peak-to-background ratio. Removing the background under the Compton profile is therefore particularly challenging, especially if there is an overlap with EELS core loss edges. Here an empirical background subtraction routine is proposed that uses input data from a bright-field EELS spectrum that does not have a Compton signal. The routine allows for multiple elastic-inelastic scattering within the EELS collection angles. Background subtraction is demonstrated on a Compton profile in silicon that overlaps with the Si L-edge. Systematic errors in the method are also discussed
Understanding quaternary compound Cu2ZnSnSe4 synthesis by microscopic scale analyses at an identical location
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