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[en] Treating Coulomb scattering of two free electrons in a stationary approach, we explore the momentum and spin entanglement created by the interaction. We show that a particular discretisation provides an estimate of the von Neumann entropy of the one-electron reduced density matrix from the experimentally accessible Shannon entropy. For spinless distinguishable electrons the entropy is sizeable at low energies, indicating strong momentum entanglement, and drops to almost zero at energies of the order of 10 keV when the azimuthal degree of freedom is integrated out, i.e. practically no entanglement and almost pure one-electron states. If spin is taken into account, the entropy for electrons with antiparallel spins should be larger than in the parallel-spin case, since it embodies both momentum and spin entanglement. Surprisingly, this difference, as well as the deviation from the spin-less case, is extremely small for the complete scattering state. Strong spin entanglement can however be obtained by post-selecting states at scattering angle π/2.
[en] Highlights: • Secondary electron imaging of doped patterns inverts its contrast along the energy scale. • Secondary electron contrast is proportional to the dopant density. • At units of eV the p-type doped patterns can be visible by means of mirror imaging. • Photoemission electron images have the contrast proportional to the dopant density. • Energy filtered PEEM images visualize even low dopant densities. - Abstract: Methods available for the mapping of dopants in silicon-based semiconductor structures with p-type as well as n-type doped patterns using low and very-low-energy electrons are reviewed together with the results of demonstration experiments.
[en] Highlights: • Plasma environment effects on the energies and transition properties for the 3C and 3D lines of Ne-like Fe16+ ion are provided. • Two kinds of ion-sphere (IS) potentials are adopted to describe the plasma screening. • The present work should be of help in the modelling and diagnostic of fusion plasma. - Abstract: Investigations on the energies and radiative properties of the 2p53d 1P1 → 2p61S0 (3C line) and the 2p53d 3D1 → 2p61S0 (3D line) transitions of a highly charged ion embedded in the strong-coupling plasma are made, taking the Fe16+ ion as an example. Two kinds of ion-sphere (IS) potentials are adopted to describe the plasma screening. The multiconfiguration Dirac–Fock (MCDF) method is employed as well to describe the relativistic and electronic correlation effects in the above atomic system. The screening effects on the transition energies, oscillator strengths, and 3C/3D oscillator-strength ratios are estimated. Systematic trends are observed for all the properties under study with respect to increased temperature and electron density. Our results show that both the 3C and 3D energies decrease, yet the oscillator strength of the 3C (3D) line decreases (increases), as the strength of plasma increases (temperature increases and electron density decreases). The present results are compared with the available values in the literature.
[en] Highlights: • A theoretical technique is outlined which allows for a more comprehensive understanding of ARPES measurements in strongly correlated electron systems. • The so-called equation of motion is used to relate specific features in the electronic spectral function to collective charge, spin or pairing fluctuations. • By means of these theoretical approaches, antiferromagnetic spin fluctuations can be identified as driving force for the pseudogap phenomenon in cuprates. - Abstract: State-of-the-art spectroscopic techniques allow for a comprehensive understanding of one-electron excitations in various physically interesting and/or technologically relevant materials. While for weakly-correlated systems the corresponding one-particle spectral function A(ω, k) contains essentially all information about their physical properties the situation is much more complicated in the presence of strong electronic correlations. In fact, in the latter case different theoretical treatments often lead to very different explanations of the origin of specific features in the spectrum. A typical example is the pseudogap in the cuprates, i.e., the momentum-selective suppression of spectral weight at the Fermi level, which has been related to spin, charge or (d-wave) pairing fluctuations by different authors. This ambiguity about the underlying physical mechanism at work can be overcome by considering two-particle correlation functions as they are able to describe the collective modes of the system and can be also related to certain ground state properties. In this work, we will present different theoretical approaches for analyzing the spectrum of correlated systems by exploiting the information contained in these two-particle correlation functions. For the specific case of the pseudogap these procedures have allowed us (1) to identify antiferromagnetic spin fluctuations as microscopic origin of this spectral feature which (2) can be related to the formation of a resonating valence bond ground state in the system.
[en] Highlights: • Noise characteristics and lifetime measurements of a graphene-coated point-cathode emitter under the upper High Vacuum range are presented. • The energy spread broadening at high emission current density due to mutual coulomb repulsion between the electron-electron interactions was studied. • These results provide data from which low energy beam applications can be developed for the cold field graphene coated nickel electron source. - Abstract: Graphene coated nickel electron sources have been recently reported to have significant advantages over state-of-the-art conventional tungsten cold field emission sources for electron microscopy/lithography applications. In this article, noise characteristics and lifetime measurements of a graphene-coated point-cathode emitter are presented in a vacuum chamber with a base pressure of 2×10−9 Torr. The results show that the emitter is able to operate at a high current density (1 μA/sr) for 600 h without decay. The energy spread broadening at high emission current density due to mutual coulomb repulsion between the electron-electron interactions (the Boersch effect) was experimentally estimated, and found to increase with greater extraction voltages and decreasing tip sizes. These results provide data from which low energy beam applications can be developed for the cold field graphene coated nickel electron source.
[en] Highlights: • The field emitted current from a paraboloid of radius of curnature R• The incident current density used was calculated from the exact wavefunctions of the paraboloid. • A 3-dimensional WKB method was used for the transmission coefficient. • The Fowler-Nordheim plots obtained exhibit strong non-linearities. • The importance of the non-linearities is discussed. - Abstract: When the radius of curvature R of an electron emitter is below 5 nm existing theories of field emission from curved surfaces fail because they assume a constant supply of electrons which is a characteristic only of planar surface emitters. In a previous work of ours we proved that the eigenfuctions of a rotationally symmetric parabolic emitter-the common shape of present nanoscopic emitters- are Whittaker functions and exhibit strong localization. In this paper we calculate the transmitted current density of such an emitter as a function of angle of emission θ and R. The angular dependence of the emitted current density is not very different from that of the classical model of a hemisphere on a post. However, when use is made of the parabolic image potential it is observed that the emission falls significantly. Finally, strongly non-linear Fowler-Nordheim plots are obtained. The significance of this is discussed.
[en] Highlights: • The average grain size of La-Zr-ZnO films changes due to Zr ion doping. • Defective oxygen on the surface of La-Zr-ZnO films decreased. • La-Zr-ZnO films has two excitation peaks. • Rectification characteristics of La-Zr-ZnO/n-Si(111) schottky contact was enhanced. - Abstract: The La-Zr-ZnO/n-Si(111) schottky heterojunction was prepared by sol-gel method. The results showed that the average grain size of La-Zr-ZnO films decreases by increasing Zr ion doping concentration. Based on XPS analysis, it is found that O1s have been composed to two peaks, and the surface defect oxygen of La-Zr-ZnO films decreases. By the PL testing, La-Zr-ZnO films have two excitation peaks at 377.78 nm and 389.17 nm, respectively. As for La-Zr-ZnO/n-Si (111) schottky heterojunction, barrier height increased and ideality factor decreased with the increasing of La ion and Zr ion doping, indicating that the rectifying characteristics of ZnO/n-Si (111) schottky contacts was enhanced due to ion co-doping. It can be explained that oxygen vacancies of La-Zr-ZnO films decreases due to the ion doping and weakens Fermi level pinning.
[en] Highlights: • The correct description of the ground state dynamics of s-trans-1,3-butadiene is studied. • The first band of the photoelectron spectrum of s-trans-1,3-butadiene is computed by using the MRCI method and nuclear quantum dynamic approach. • The excellent agreement with the corresponding experimental spectrum is obtained. • The effect of additional mode S8 on the ground state dynamics is investigated. - Abstract: We used high levels of ab initio electronic structure methods and nuclear quantum dynamical approach for studying the ground state dynamics of the s-trans-1,3-butadiene cation. To benchmark the quality of the electronic structure calculations, potential energy surfaces for the energetically lowest lying doublet X2Bg, A2Au and B2Ag states along seven vibrational modes with considering mode S8 (asymmetric stretching CC bond) were calculated at the MRCI/CAS(10,7) and CAS(8,8) levels of theories. Considering contribution of S8 mode in the nuclear quantum dynamical investigation leads to a more realistic description of the ground state dynamics of s-trans-1,3-butadiene, which was not addressed before. Our calculation shows that the population transfer takes place in a time scale 50-60 fs. We also calculated the photoelectron spectrum of the molecular system under study. The excellent agreement of spectrum with the experimental one leads us the conclusion that the potential energy surfaces of three lowest cationic X2Bg, A2Au and B2Ag states and time-dependent population analysis of X2Bg were accurately determined.
[en] Highlights: • The secondary electron yield (SEY) for arbitrary energies depends sensitively on the inelastic mean free path (IMFP) values at low energies (below 100 eV). • The energy dependence of the IMFP at low energies calculated on the basis of the Mermin dielectric function is found to be the more realistic. • The optimum IMFP values were shown to be not critically affected by the choice of the inner potential Ui. • The presented approach allows reverse engineering the IMFP at low energies. - Abstract: Since the emission of secondary electrons for any incident energy always involves the formation and emission of a cascade of slow electrons (χ2 fit of the simulated SEY values with experimental results are considered to be the most reliable. The described algorithm was employed for the investigation of Be, Al, Si, Ti, V, Fe, Ni, Cu, Ge, Nb, Mo, Pd, Ag, Ta, W, Pt, Au. For most materials these optimum IMFP values are found to be close to IMFP values based on the Mermin dielectric function.
[en] More than a century after the discovery of the electron, there are still fundamental, yet unresolved, questions concerning the generation-ejection mechanism of the ubiquitous Secondary Electrons (SEs) from a solid surface. Broadness of the field of application for these SEs makes it desirable to be able to control this phenomenon, which requires the understanding of the elementary physical mechanism leading to their generation and emission. This paper reports on the dissection of such a tangled process operated by the help of spectroscopic tools of increasing finesse; measuring differential cross sections with an increasing degree of differentiation. These results demonstrate that single ionising scattering events, assisted by collective excitations, constitute the fundamental ingredient leading to SE-generation and -emission. To this end, the interaction of Low Energy Electrons (LEEs) with various Carbon allotropes has been investigated by means of Total Electron Yield (TEY) and (e,2e)-coincidence spectroscopy measurements. Carbon allotropes are chosen as targets since they are important in technological applications where both minimisation and maximisation of the SE-yield is a relevant issue. This is the first time that such complete set of benchmarks on the SE-yield from well characterised surfaces has been gathered, interpreted and is made available to the scientific community. This comprehensive investigation has led to the disentanglement of the elementary processes relevant for the understanding of the SE-generation probability, that fully take into account both energy and momentum conservation in the collision and the band structure of the solid as well as many-body effects.