arXiv subjects

V. M. Silkin

Publications and source records attributed to V. M. Silkin.

At least 37 records · Page 2Linked to original sources

arXiv2015

Photoemission footprints of extrinsic plasmarons

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.

cond-mat.mtrl-sci↗

arXiv2014

Energy bands in graphene: Comparison between the tight-binding model and {\it ab initio} calculations

We compare the classification of the electron bands in graphene, obtained by group theory algebra in the framework of tight-binding model (TBM), with that calculated in the density-functional theory (DFT) framework. Identification in the DFT band-structure of all eight energy bands (four valence and four conduction bands) corresponding to the TBM-derived energy bands is performed and corresponding analysis is presented. The four occupied (three $σ$- and one $π$-like) and three unoccupied (two $σ$- and one $π$-like) bands given by DFT closely correspond to those predicted by TBM, both by their symmetry and their dispersion law. However, the two lowest lying at the $Γ$-point unoccupied bands (one of them of a $σ$-like type and the other of a $π$-like one), are not of TBM type. According both to their symmetry and to the electron density these bands are plane waves orthogonal to the TBM valence bands; dispersion of these states can be determined unambiguously up to the Brillouin zone borders. On the other hand, the fourth unoccupied band given by the TBM, can be identified among those given by the DFT band calculations; it is situated rather high with respect to energy. The interaction of this band with the free-electron states is so strong, that it exists only in a part of $k$-space.

cond-mat.mes-hall↗

arXiv2014

Plasmaron excitations in p(2x2)-K/Graphite

A new type of plasmarons formed by the compound of photoelectrons and acoustic surface plasmon excitations is investigated in the system p(2x2)-K/Graphite. The physics behind these types of plasmarons, e-plasmarons, is different from the ones 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 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 in the Gamma-point.

cond-mat.mtrl-sci↗

arXiv2013

Influence of the surface band structure on electron emission spectra from metal surfaces

Electron distributions produced by grazing impact of fast protons on Mg(0001), Cu(111), Ag(111) and Au(111) surfaces are investigated, focusing on the effects of the electronic band structure. The process is described within the Band-Structure-Based approximation, which is a perturbative method that includes an accurate representation of the electron-surface interaction, incorporating information of the electronic band structure of the solid. For all the studied surfaces, the presence of partially occupied surface electronic states produces noticeable structures in double differential - energy- and angle- resolved - electron emission probabilities from the valence band. These structures remain visible in electron emission spectra after adding contributions coming from core electrons, which might make it possible their experimental detection.

physics.atom-ph↗

arXiv2013

Space Group Symmetry Classification of Energy Bands in Graphene and Destruction of Dirac Points by Supercell Potential

In the previous publications (E. Kogan and V. U. Nazarov, Phys. Rev. B {\bf 85}, 115418 (2012) and E. Kogan, Graphene {\bf 2}, 74 (2013)) we presented point group symmetry classification of energy bands in graphene. In the present note we generalize classification by considering the space group symmetry. Also in the framework of group theory we describe destruction of the Dirac points in graphene by a supercell potential.

cond-mat.mes-hall↗

arXiv2013

Acoustic plasmons in extrinsic free-standing graphene

An acoustic plasmon is predicted to occur, in addition to the conventional two-dimensional (2D) plasmon, as the collective motion of a system of two types of electronic carriers coexisting in the very same 2D band of extrinsic (doped or gated) graphene. The origin of this novel mode resides in the strong anisotropy that is present in the graphene band structure near the Dirac point. This anisotropy allows for the coexistence of carriers moving with two distinct Fermi velocities along the Gamma-K direction, which leads to two modes of collective oscillation: one mode in which the two types of electrons oscillate in phase with one another [this is the conventional 2D graphene plasmon, which at long wavelengths (q->0) has the same dispersion, q^1/2, as the conventional 2D plasmon of a 2D free electron gas], and the other mode found here corresponding to a low-frequency acoustic oscillation [whose energy exhibits at long wavelengths a linear dependence on the 2D wavenumber q] in which the two types of electrons oscillate out of phase. If this prediction is confirmed experimentally, it will represent the first realization of acoustic plasmons originated in the collective motion of a system of two types of carriers coexisting within the very same band.

cond-mat.mes-hall↗

arXiv2013

Ab initio study of low-energy electronic collective excitations in bulk Pb: anisotropy and acoustic modes

A theoretical study of the dynamical dielectric response of bulk lead at low energies is presented. The calculations are performed with full inclusion of the electron band structure calculated by means of a first-principles pseudopotential approach. The effect of including the spin-orbit splittings in the band structure is analyzed, together with dynamical exchange-correlation and local-field effects. Excitations are studied in detail, showing the presence at small momenta of acoustic-like dispersing excitations. The character of these modes with sound-like dispersion depends on the direction of the momentum transfer q. In general results show strong anisotropy effects on the dielectric response of bulk Pb. Comparison with available optical experimental data shows good agreement.

cond-mat.mtrl-sci↗

arXiv2013

Quantum effects on the loss function of Pb(111) thin films: an ab initio study

A theoretical study of the surface energy-loss function of freestanding Pb(111) thin films is presented, starting from the single monolayer case. The calculations are carried applying the linear response theory, with inclusion of the electron band structure by means of a first-principles pseudopotential approach using a supercell scheme. Quantum-size effects on the plasmon modes of the thinnest films are found in qualitative agreement with previous work based on the jellium model. For thicker films, results show a dispersionless mode at all thicknesses, in agreeement with electron energy-loss measurements. For sizeable values of the momentum, the raising of the surface plasmon with increasing thickness is retrieved.

cond-mat.mtrl-sci↗

arXiv2012

Scattering resonances in graphene

We address the two-dimensional band-structure of graphene above the vacuum level in the context of discrete states immersed in the three-dimensional continuum. Scattering resonances are discovered that originate from the coupling of the in-plane and perpendicular motions, as elucidated by the analysis of an exactly solvable model. Some of the resonances turn into true bound states at high-symmetry $\kv$ vectors. {\it Ab initio} scattering theory verifies the existence of the resonances in realistic graphene and shows that they lead to a total reflection of the incident electron below and total transmission above the resonance energy.

cond-mat.mes-hall↗

arXiv2011

Tuning MgB_2(0001) surface states

Surface state localization and hybridization on B, Mg, and Li terminated MgB_2(0001) surfaces are studied within density functional theory (DFT). For the B terminated surface we find a low energy B σ_1 surface state, a spz surface state, and B σ_2 and σ_3 quantum well states, which are 90% localized in the topmost B layer. Our results demonstrate that by charging the B atomic layer, either by changing the surface termination or through electro-chemical doping, the B σ surface states are shifted down in energy, filled, and delocalize through hybridization as they cross the bulk MgB_2 bulk bands. On the other hand, the spz surface state should be shifted up in energy, emptied, and gain an increasingly metallic s character by adding a Mg, Mg^{+1}, or Li terminating atomic layer. These results clearly show both the robust nature of MgB_2(0001) surface states, and how their localization and energy range may be tuned by surface termination and charging, with implications for the superconducting and plasmonic behaviour of MgB_2.

cond-mat.mtrl-sci↗

arXiv2010

First principles quasiparticle damping rates in bulk lead

First principles calculations of the damping rates (inverse inelastic lifetimes) of low energy quasiparticles in bulk Pb are presented. Damping rates are obtained both for excited electrons and holes with energies up to 8 eV on a set of k vectors throughout the Brillouin zone (BZ). Strong localization effects in the calculated lifetimes are found. Averaged over the BZ inelastic lifetimes versus quasiparticle energy are reported as well. In addition, the effect of the spin-orbit induced splitting in the band structure on the calculated lifetimes in Pb is investigated.

cond-mat.mtrl-sci↗

arXiv2010

Potential Energy Landscape for hot electrons in periodically nanostructured graphene

We explore the spatial variations of the unoccupied electronic states of graphene epitaxially grown on Ru(0001) and observed three unexpected features: the first graphene image state is split in energy, unlike all other image states, the split state does not follow the local work function modulation, and a new interfacial state at +3 eV appears on some areas of the surface. These results show the system behaves as a self-organized periodic array of quantum dots.

cond-mat.mes-hall↗

arXiv2009

Ab initio calculation of low-energy collective charge-density excitations in MgB2

We present {\it ab-initio} time-dependent density-functional theory calculation results for low-energy collective electron excitations in $\textrm{MgB}_2$. The existence of a long-lived collective excitation corresponding to coherent charge density fluctuations between the boron $σ$- and $π$- bands ($σπ$ mode) is demonstrated. This mode has a sine-like oscillating dispersion for energies below 0.5 eV. At even lower energy we find another collective mode ($σσ$ mode). We show the strong impact of local-field effects on dielectric functions in MgB$_2$. These effects account for the long q-range behavior of the modes. We discuss the physics that these collective excitations bring to the energy region typical for lattice vibrations.

cond-mat.supr-con↗

arXiv2009

Study of the induced potential produced by ultrashort pulses on metal surfaces

The influence of the induced potential on photoelectron emission from metal surfaces is studied for grazing incidence of ultrashort laser pulses. To describe this process we introduce a distorted wave method, the Surface Jellium Volkov approach, which includes the perturbation on the emitted electron produced by both the laser and the induced fields. The method is applied to an Al(111) surfaces, contrasting the results with the numerical solutions to the time-dependent Schrodinger equation (TDSE). We found that SJV approach reproduces well the main features of emission spectra, accounting properly for effects originated by the induced potential.

cond-mat.other↗

arXiv2008

Energy loss of charged particles moving parallel to a magnesium surface

We present it ab initio calculations of the electronic energy loss of charged particles moving outside a magnesium surface, from a realistic description of the one-electron band structure and a full treatment of the dynamical electronic response of valence electrons. Our results indicate that the finite width of the plasmon resonance, which is mainly due to the presence of band-structure effects, strongly modifies the asymptotic behaviour of the energy loss at large distances from the surface. This effect is relevant for the understanding of the interaction between charged particles and the internal surface of microcapillaries.

cond-mat.mtrl-sci↗

arXiv2008

Band structure effects on the Be(0001) acoustic-surface-plasmon energy dispersion

We report first-principles calculations of acoustic surface plasmons on the (0001) surface of Be, as obtained in the random-phase approximation of many-body theory. The energy dispersion of these collective excitations has been obtained along two symmetry directions. Our results show a considerable anisotropy of acoustic surface plasmons, and underline the capability of experimental measurements of these plasmons to {\it map} the electron-hole excitation spectrum of the quasi two-dimensional Shockley surface state band that is present on the Be(0001) surface.

cond-mat.mtrl-sci↗

arXiv2006

Theory of surface plasmons and surface-plasmon polaritons

Collective electronic excitations at metal surfaces are well known to play a key role in a wide spectrum of science, ranging from physics and materials science to biology. Here we focus on a theoretical description of the many-body dynamical electronic response of solids, which underlines the existence of various collective electronic excitations at metal surfaces, such as the conventional surface plasmon, multipole plasmons, and the recently predicted acoustic surface plasmon. We also review existing calculations, experimental measurements, and applications.

cond-mat.mtrl-sci↗

arXiv2006

Nesting Induced Peierls-type Instability for Compressed Li-CI16

Alkalies are considered to be simple metals at ambient conditions. However, recently reported theoretical and experimental results have shown an unexpected and intriguing correlation between complex structures and an enhanced superconducting transition temperature in lithium under pressure. In this article we analyze the pressure induced Fermi surface deformation in bcc lithium, and its relation to the observed cI16 structure. According to our calculations, the Fermi surface becomes increasingly anisotropic with pressure and develops an extended nesting along the bcc [121] direction. This nesting induces a phonon instability of both transverse modes at N, so that a Peierls-type mechanism is proposed to explain the stability of Li-cI16.

cond-mat.mtrl-sci↗