A theoretical study of the phonon-induced linewidths of the occupied electronic bands and the electron-phonon coupling (EPC) constant in graphene is presented. We propose an approximation in the spirit of the rigid ion approximation which considerably simplifies calculations of the EPC in metallic systems. We apply a tight- binding approach for the electrons and a force-constant model for the phonons with parameters obtained from first principles. . A simple procedure is presented in order to estimate the influence of the graphene-substrate interaction on the phonon dispersion and thereby also on, e.g., the linewidths. The EPC is the strongest in the energy region where pi and sigma bands overlap. We find that the electron scattering is predominantly driven by the out-of-plane acoustic ZA and optical ZO phonon modes, while in general the high-energy optical phonon modes LO and TO are of secondary importance.

The linewidths of the π and σ bands originating from the electron-phonon coupling in graphene are analyzed based on model calculations and experimental angle-resolved photoemission spectroscopy (ARPES) data. We find evidence for crucial contributions to the lifetime broadening from interband scattering π→σ and σ→π, respectively, driven by the out-of-plane ZA acoustic phonons. The essential features of the calculated σ band linewidths are in agreement with recent published ARPES data [Mazzola, Phys. Rev. B 95, 075430 (2017)2469-995010.1103/PhysRevB.95.075430] and of the π band linewidth with ARPES data presented here.

First-principles studies of the electron-phonon coupling in graphene predict a high coupling strength for the sigma band with. values of up to 0.9. Near the top of the s band,. is found to be approximate to 0.7. This value is consistent with the recently observed kinks in the s band dispersion by angle-resolved photoemission. While the photoemission intensity from the s band is strongly influenced by matrix elements due to sublattice interference, these effects differ significantly for data taken in the first and neighboring Brillouin zones. This can be exploited to disentangle the influence of matrix elements and electron-phonon coupling. A rigorous analysis of the experimentally determined complex self-energy using Kramers-Kronig transformations further supports the assignment of the observed kinks to strong electron-phonon coupling and yields a coupling constant of 0.6(1), in excellent agreement with the calculations.

The aim of this model study of the electron-phonon coupling in graphene was to find out about the relative importance of the inter- and intraband scattering and which phonon modes are the most active. This was achieved by analyzing the electron-phonon matrix element of the carbon dimer in the unit cell. We found that for the intra molecular orbital matrix elements the longitudinal optical phonon mode is the active phonon mode. The matrix element corresponding to sigma -> sigma a is greater than the matrix element for pi -> pi. The inter molecular orbital scattering pi -> sigma it a is driven by the out-of-plane acoustic phonon mode, while the out-of-plane optical mode does not contribute for symmetry reasons. We found the unexpected result that the magnitude of matrix element of the inter molecular orbital scattering pi -> sigma exceeds the intra molecular orbital scattering pi -> pi. These results indicate that the in general not considered inter-band scattering has to be taken into account when analyzing e.g. photo-hole lifetimes and the electron-phonon coupling constant lambda from photoemission data of graphene. (C) 2016 Elsevier B.V. All rights reserved.

A prediction how to experimentally distinguish excitations of extrinsic plasmarons from intrinsic plasmarons is presented. In surface systems where excitations of acoustic surface plasmons is possible it is shown that the photo-electron yield in normal photoemission should decay according to an inverse square root dependence with respect to the photon energy. A computational analysis of the system p(2x2)-K/Graphite confirms this prediction.

Plasmarons formed by the compound of photoelectrons and acoustic surface-plasmon excitations is investigated in the system p(2 x 2)-K/graphite. The physics behind this type of plasmarons (e plasmarons) differs from the physics of plasmarons recently found in graphene, where the loss feature is argued to result from the photohole-plasmon interaction (h plasmarons). Based on first principles methods we calculate the dispersion of the e-plasmaron excitation rate, which yields a broad feature below the parabolic quantum-well band with a peak about 0.4 eV below the quantum-well band at the (Gamma) over bar point.

We show that electron correlations lead to a bad metallic state in chalcogenides FeSe and FeTe despite the intermediate value of the Hubbard repulsion U and Hund’s rule coupling J . The evolution of the quasiparticle weight Z as a function of the interaction terms reveals a clear crossover at U � 2.5 eV. In the weak coupling limit Z decreases for all correlated d orbitals as a function of U and beyond the crossover coupling they become weakly dependent on U while strongly dependent on J . A marked orbital dependence of the Z’s emerges even if in general the orbital-selective Mott transition only occurs for relatively large values of U . This two-stage reduction of the quasiparticle coherence due to the combined effect of Hubbard U and the Hund’s J suggests that the iron-based superconductors can be referred to as Hund’s correlated metals.

Spectroscopic signatures of vibrational excitations on initially oxidized Be(0001) are identified by a combination of electron energy loss spectroscopy and density functional calculations. Prominent spectral features are due to vibrations in a Be-Omixing layer. Scanning tunneling microscopy indicates that initial oxidation occurs locally in the form of islands. The acoustic surface plasmon persists on the oxygen-covered surface. Its dispersion has been determined along the (Gamma) over bar (K) over bar direction and is virtually identical to the dispersion of the acoustic surface plasmon of the clean surface.

We present a self-consistent numerical approach to solve the Gutzwiller variational problem for general multiband models with arbitrary on-site interaction. The proposed method generalizes and improves the procedure derived by Deng et al. [Phys. Rev. B 79, 075114 (2009)], overcoming the restriction to density-density interaction without increasing the complexity of the computational algorithm. Our approach drastically reduces the problem of the high-dimensional Gutzwiller minimization by mapping it to a minimization only in the variational density matrix, in the spirit of the Levy and Lieb formulation of density functional theory (DFT). For fixed density the Gutzwiller renormalization matrix is determined as a fixpoint of a proper functional, whose evaluation requires only ground-state calculations of matrices defined in the Gutzwiller variational space. Furthermore, the proposed method is able to account for the symmetries of the variational function in a controlled way, reducing the number of variational parameters. After a detailed description of the method we present calculations for multiband Hubbard models with full (rotationally invariant) Hund's rule on-site interaction. Our analysis shows that the numerical algorithm is very efficient, stable, and easy to implement. For these reasons this method is particularly suitable for first-principles studies (e. g., in combination with DFT) of many complex real materials, where the full intra-atomic interaction is important to obtain correct results.

The influence of K, deposited on a carbon surface, on the oxidation of carbon in O2 was investigated. Reaction rate measurements, carried through in a UHV-system by use of AES, showed that potassium increases the reaction rate by up to ∼ 104 times. A theoretical model, based on the assumption that O2 dissociation is the rate limiting step, has been developed. The model shows that a charge transfer mechanism can explain the observed rate increase. Results from TPD/TPR-measurements indicate that the sticking probablility for O2 on a graphite surface with predeposited K is approximately independent on K coverage for coverages down to 0.5 × 1014 cm−2, corresponding to an effective radius of K of ∼ 7.3 Å.

We consider the finite temperature metal-insulator transition in the half filled paramagnetic Hubbard model on the infinite dimensional Bethe lattice. A new method for calculating the Dynamical Mean Field Theory fixpoint surface in the phase diagram is presented and shown to be free from the convergence problems of standard forward recursion. The fixpoint equation is then analyzed using dynamical systems methods. On the fixpoint surface the eigenspectra of its Jacobian is used to characterize the hysteresis boundaries of the first order transition line and its second order critical end point. The critical point is shown to be a cusp catastrophe in the parameter space, opening a pitchfork bifurcation along the first order transition line, while the hysteresis boundaries are shown to be saddle-node bifurcations of two merging fixpoints. Using Landau theory the properties of the critical end point is determined and related to the critical eigenmode of the Jacobian. Our findings provide new insights into basic properties of this intensively studied transition.

A detailed derivation of analytic solutions is presented for overlap, kinetic, nuclear attraction and electron repulsion integrals involving Cartesian Gaussian-type orbitals. It is demonstrated how s-type orbitals can be used to evaluate integrals with higher angular momentum via the properties of Hermite polynomials and differentiation with respect to non-integration variables.

We report on low-energy dynamical surface response properties of the p2 2K / Be0001 system calculated within a first-principles approach. It is shown that a partly occupied adsorbate-induced quantum-well band dramatically affects the dynamical properties of the beryllium surface. We demonstrate clear evidence that the observed anomalous features in photoemission spectra of this and a similar system can be explained by acoustic surface plasmon excitations. DOI: 10.1103/PhysRevB.81.113406

Recent studies of the surface dynamics of Al(001) and Cu(111) based on density functional perturbation theory have substantiated the existence of subsurface optical phonon resonances of all three polarizations, thus confirming early predictions of the embedded-atom method. The hybridization of the shear-vertical optical resonance with the longitudinal acoustic phonon branch accounts for the ubiquitous anomalous acoustic resonance as an intrinsic feature of metal surfaces. The DFPT calculation of the phonon-induced surface charge density oscillations shows that helium atom scattering spectroscopy (HAS) can indeed probe subsurface resonances. This opens new perspectives to HAS for the measurement of subsurface phonon dispersion curves in thin films, as proved by recent HAS studies on Pb and Fe ultrathin films on copper. After discussing these recent advances, this paper briefly reviews other important trends of surface dynamics expressed in recent years.

We study the validity of two frequently used approximations in calculations of electron-phonon coupling at surfaces. The rigid-ion approximation is a standard approximation used for the bulk metals. On the basis of density functional theory calculations, we find that for Be this approximation is as valid for surface atoms as for bulk atoms. In addition, the slab method for calculations of a phonon induced surface state lifetime is examined. The convergence of the electron-phonon matrix element with respect to the thickness of the slab is studied for several systems. When the number of slab layers is increased, the net effect of decreasing overlap and increasing number of final states depends strongly on the decay length of the surface state wavefunction and the band structure.

A simplified calculation scheme is proposed for the analysis of the phonon-induced lifetime broadening of surface-electronic states. The aim has been to include the details of the electron and phonon structures. In contrast to the present analysis based on phonon Debye models, where all is hidden in the parameters Debye frequency ${\ensuremath{\omega}}_{D}$ and electron-phonon coupling constant $\ensuremath{\lambda}$, our procedure gives a single multiplicative fitting parameter ${V}_{d}^{2}$ for the spectral Eliashberg function, where ${V}_{d}$ represents the effective deformation potential. We apply this procedure to the image and surface states of Cu(111) and to the quantum-well state and the recently found gap state of $p(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\text{Cs}/\text{Cu}(111)$. We demonstrate that the crucial contribution to the electron-phonon coupling by the overlayer localized low-frequency phonon modes is well captured by the scheme. The presented scheme should be useful for combined experimental and theoretical studies of overlayer systems for which still no full first-principles calculation of the electron-phonon coupling has been presented due to the complexity of the system.

We present a simplified procedure for the analysis of the phonon-induced lifetimes of surface states. The model includes information about the electron and phonon structure and is thus more reliable than procedures based on phonon Debye models. We apply the model to calculate the lifetime broadening of Cu(111) and Al(001) surface states. The obtained Eliashberg functions and lifetimes are in reasonable agreement with previous detailed studies.