[en] We investigate the interplay between the electron-electron and the electron-phonon interaction in the Hubbard-Holstein model. We implement the flow-equation method to investigate within this model the effect of correlation on the electron-phonon effective coupling and, conversely, the effect of phonons in the effective electron-electron interaction. Using this technique we obtain analytical momentum-dependent expressions for the effective couplings and we study their behavior for different physical regimes. In agreement with other works on this subject, we find that the electron-electron attraction mediated by phonons in the presence of Hubbard repulsion is peaked at low transferred momenta. The role of the characteristic energies involved is also analyzed

[en] It has been shown that a symmetry lowering from a state with higher symmetry to a state with lower symmetry (D_(2m)d → C_(2m)) occurs in 1-D polymers with polymethine fragments as elementary units when electron-vibration interaction is taken into account. The investigations are carried out using an extended Su-Schrieffer-Heeger method, where the electron-electron interaction is taken into account

[en] The goal to describe plutonium phases from 'first principles' calculation methods is complicated by the problem of 5f-electrons localization. While for early actinides (Th, U, Np) standard density functional theory (DFT) description with itinerant 5f-electrons works well for late actinides (Am, Cm) DFT calculations with completely localized (pseudocore) 5f-electrons give satisfactory results. However, plutonium presents a border case of partial localization and both limits (itinerant and completely localized) are not valid. We present a review of the methods used to solve this problem and discuss what could be the reasons for their successes and failures

[en] The apparent contradiction between the recently observed weak charge disproportion and the traditional Mn³⁺/Mn⁴⁺ picture of the charge-orbital orders in half-doped manganites is resolved by a novel Wannier states analysis of the LDA+U electronic structure. Strong electron itinerancy in this charge-transfer system significantly delocalizes the occupied low-energy 'Mn³⁺' Wannier states such that charge leaks into the 'Mn⁴⁺'-sites. Furthermore, the leading mechanisms of the charge order are quantified via our first-principles derivation of the low-energy effective Hamiltonian. The electron-electron interaction is found to play a role as important as the electron-lattice interaction.

[en] We present a very simple model for numerically describing the steady state dynamics of a system interacting with continua of states representing a bath. Our model can be applied to equilibrium and nonequilibrium problems. For a one-state system coupled to two free electron reservoirs, our results match the Landauer formula for current traveling through a molecule. More significantly, we can also predict the nonequilibrium steady state population on a molecule between two out-of-equilibrium contacts. While the method presented here is for one-electron Hamiltonians, we outline how this model may be extended to include electron-electron interactions and correlations, an approach which suggests a connection between the conduction problem and the electronic structure problem.

[en] Graphical abstract: The electron-phonon interactions destroy the electron pairs formed by Coulomb interactions, and at the same time, form the energy gap by which the electron pairs become stable. - Abstract: In order to discuss how the nondissipative delocalized diamagnetic currents in the microscopic sized materials are closely related to the conventional superconductivity in the macroscopic sized materials, the unified theory, by which various sized superconductivity can be explained, is suggested. It has been believed for a long time that the electron-phonon interactions play an essential role in the attractive electron-electron interactions, as described in the Bardeen-Cooper-Schrieffer (BCS) theory in the conventional superconductivity. However, it is suggested in this paper that the electron-phonon interactions do not play an essential role in the attractive electron-electron interactions but play an essential role in the forming of energy gap by which the electron pairs formed by the attractive Coulomb interactions in the conventional superconducting states become more stable than those in the normal metallic states at low temperatures.

[en] This paper shows the experimental outcomes of a study of crystallinity dependent diffraction effects on both elastically-scattered and Auger electrons produced from highly-oriented pyrolytic graphite (HOPG) using a commercial scanning Auger electron microscope (SAM). This system is equipped with a coaxial cylindrical mirror analyser (CMA). Scanning electron images (SEM), backscattered electron images (BSE), Auger maps and the Auger spectra, using a range of relatively low incident electron energies, are obtained and analysed in detail. It is shown that the regional contrast shown in the SEM and BSE images and in the Auger maps, in the electron energy range used, is formed predominantly by forward focusing along the close-packed <21-bar1-bar2>-type atomic chains of the graphite lattice. The intensity of the exiting electrons received by the CMA can be interpreted as a convolution of the diffraction of both incident and exiting electrons, e.g. that of the exiting elastically scattered electrons can vary by up to 50% on changing the incident direction of the primary beam and the orientation of the specimen. Increasing the energy of incident beam enhances forward focusing and increases the contribution from diffraction along the secondary close-packed atomic chains, e.g. the <33-bar04>-type atomic chains present. The carbon (KLL) Auger electrons produced are found to behave in a way similar to the primary electrons in regard to the diffraction process but with less marked diffraction evident

[en] Recently it was pointed out that electron capture occurring in fast ion-atom collisions can proceed via a mechanism which earlier was not considered. In the present paper we study this mechanism in more detail. Similarly as in radiative capture, where the electron transfer occurs due to the interaction with the radiation field and proceeds via emission of a photon, within this mechanism the electron capture is caused by the interaction with another atomic electron leading mainly to the emission of the latter. In contrast to the electron-electron Thomas capture, this electron-electron (E-E) mechanism is basically a first-order one having similarities to the kinematic and radiative capture channels. It also possesses important differences with the latter two. Leading to transfer ionization, this first-order capture mechanism results in the electron emission mainly in the direction opposite to the motion of the projectile ion. The same, although less pronounced, feature is also characteristic for the momenta of the target recoil ions produced via this mechanism. It is also shown that the action of the E-E mechanism is clearly seen in recent experimental data on the transfer ionization in fast proton-helium collisions.

[en] We study theoretically the one-photon two-electron emission from the Cu(1 1 1) surface. Tuning the energy to either the surface state or to the d-band energy regions this sample provides a paradigm example of the two-particle emission from a (quasi) two-dimensional and a three-dimensional system, respectively. The independent electron-gas behavior of the surface-state electrons results in a characteristic angular pattern. We inspect the dependence of this angular shape on the strength of the electron-electron interaction. Furthermore, it is shown that the two-particle energy correlation behaves differently for bulk and surface states

[en] We present a new time-of-flight spectrometer for energy and angle-resolved measurements of electrons emitted in coincidence from a surface. By using a projection method, we can cover nearly 2π of the solid angle above the sample resulting in a very high coincidence efficiency. The use of this new spectrometer for the double photoemission process from surfaces will be demonstrated