Resolubility of Image-Potential Resonances

A theory of image-potential states is presented for the general case where these surface electronic states are resonant with a bulk continuum. The theory extends the multiple scattering approach of Echenique and Pendry into the strong coupling regime while retaining independence from specific forms of surface and bulk potentials. The theory predicts the existence of a well-resolved series of resonances for arbitrary coupling strengths. Surprisingly, distinct image-potential resonances are thus expected to exist on almost any metal surface, even in the limiting case of jellium

Nonlocal microscopic theory of quantum friction between parallel metallic slabs

We present a new derivation of the friction force between two metallic slabs moving with constant relative parallel velocity, based on T=0 quantum-field theory formalism. By including a fully nonlocal description of dynamically screened electron fluctuations in the slab, and avoiding the usual matching-condition procedure, we generalize previous expressions for the friction force, to which our results reduce in the local limit. Analyzing the friction force calculated in the two local models and in the nonlocal theory, we show that for physically relevant velocities local theories using the plasmon and Drude models of dielectric response are inappropriate to describe friction, which is due to excitation of low-energy electron-hole pairs, which are properly included in nonlocal theory. We also show that inclusion of dissipation in the nonlocal electronic response has negligible influence on friction

Ideal two-dimensional electron systems with a giant Rashba-type spin splitting in real materials: surfaces of bismuth tellurohalides

Spintronics is aimed at active controlling and manipulating the spin degrees of freedom in semiconductor devices. A promising way to achieve this goal is to make use of the tunable Rashba effect that relies on the spin-orbit interaction (SOI) in a two-dimensional (2D) electron system immersed in an inversion-asymmetric environment. The SOI induced spin-splitting of the 2D-electron state provides a basis for many theoretically proposed spintronic devices. However, the lack of semiconductors with large Rashba effect hinders realization of these devices in actual practice. Here we report on a giant Rashba-type spin splitting in 2D electron systems which reside at tellurium-terminated surfaces of bismuth tellurohalides. Among these semiconductors, BiTeCl stands out for its isotropic metallic surface-state band with the Gamma-point energy lying deep inside the bulk band gap. The giant spin-splitting of this band ensures a substantial spin asymmetry of the inelastic mean free path of quasiparticles with different spin orientations.Comment: 12 pages, 5 figure

Unusual dispersion of image potential states on the Be(101¯0) surface

We present a self-consistent pseudopotential calculation of image potential states on the Be(101¯0) surface. The one-electron potential inside the crystal and surface region is described by the local density approximation and by the image potential in the vacuum region (at z>zim). The calculated first image state (E1=−1.20 eV) lies in the symmetry gap and the second image state (E2=−0.31 eV) is located inside the absolute energy gap. High anisotropy of the dispersion of both image states is found. The effective masses that reflect this anisotropy are obtained m*1/me=1.55±0.1, m*2/me=1.30±0.1 along the ¯Γ¯M direction and m*1/me=0.40±0.05, m*2/me=0.55±0.05 along ¯ΓĀ. The unusual dispersion of the image states on Be(101¯0) is due to a large penetration value pn of these states (p1=0.34 and p2=0.15), and the very anisotropic character of the symmetry energy gap edges.This project has been supported by the Ministerio de Educacion y Ciencia, Spain; the Departamento de Educacion del Gobierno Vasco; and Iberdrola S.A.Peer reviewe

Two types of excited electron dynamics in zinc oxide

8 páginas, 5 figuras, 1 tabla.-- PACS number(s): 77.55.hf, 71.38.-k, 78.47.D-, 78.47.J-We present first-principle evaluations of the electron-phonon coupling strength parameter and associated characteristics of relaxation for the excited electrons in the conduction band of zinc oxide. The evaluations are based on the pseudopotential plane-wave approach to the electronic band structure, the density-functional perturbation theory for the calculations of phonons and electron-phonon interactions, and on the “Fermi golden rule” for evaluations of the electron relaxation time and the energy-loss time. The calculations demonstrate existence of two types of electron dynamics, the picosecond one for electrons near the bottom of the conduction band and the femtosecond for the higher energies. Sensibly good agreement with experimental data confirms the validity of the calculations.We acknowledge partial support from the University of the Basque Country (Grant No. GIC07IT36607), the Spanish Ministerio de Ciencia y Tecnologia (Grant No. FIS2007- 66711-C02-01) and the Ikerbasque Fellowship foundation.Peer reviewe

Ultrafast dynamics and decoherence of quasiparticles in surface bands: Preasymptotic decay and dephasing of quasiparticle states

We develop a many-body description of ultrafast electron dynamics in surface bands appropriate for studying relaxation of hot electrons and holes excited in the processes of one- and two-photon photoemission and inverse photoemission from surfaces. The description is based on the formalism for calculation of quasiparticle survival probabilities combined with self-consistent treatment of the electronic response of the system. We show that the calculation of survival amplitudes which carry information on the quasiparticle decay and decoherence can be conveniently mapped onto the problem of renormalization of quasiparticles by the interactions with bosonized excitations constituting the system heatbath. Applying this approach to the benchmark Cu(111) surface we are able to assess the regimes of preasymptotic non-Markovian quasiparticle dynamics in surface bands and locate transitions to the regime of exponential decay governed by the modified Fermi golden rule-type of transition rates. The general validity of these findings enables us to establish borderlines between different regimes of ultrafast electronic relaxation and on that basis to introduce a simple interpolation scheme for modeling of quasiparticle decay in the course of spectroscopic measurements

Spin-helical Dirac states in graphene induced by polar-substrate surfaces with giant spin-orbit interaction: a new platform for spintronics

Spintronics, or spin electronics, is aimed at efficient control and manipulation of spin degrees of freedom in electron systems. To comply with demands of nowaday spintronics, the studies of electron systems hosting giant spin-orbit-split electron states have become one of the most important problems providing us with a basis for desirable spintronics devices. In construction of such devices, it is also tempting to involve graphene, which has attracted great attention because of its unique and remarkable electronic properties and was recognized as a viable replacement for silicon in electronics. In this case, a challenging goal is to lift spin degeneracy of graphene Dirac states. Here, we propose a novel pathway to achieve this goal by means of coupling of graphene and polar-substrate surface states with giant Rashba-type spin-splitting. We theoretically demonstrate it by constructing the graphene@BiTeCl system, which appears to possess spin-helical graphene Dirac states caused by the strong interaction of Dirac and Rashba electrons. We anticipate that our findings will stimulate rapid growth in theoretical and experimental investigations of graphene Dirac states with real spin-momentum locking, which can revolutionize the graphene spintronics and become a reliable base for prospective spintronics applications.We acknowledge partial support from the Basque Country Government, Departamento de Educación, Universidades e Investigación (Grant No. IT-756-13), the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02-01), and the Ministry of Education and Science of Russian Federation (Grant No. 2.8575.2013).Peer Reviewe

Hole-phonon relaxation and photocatalytic properties of titanium dioxide and zinc oxide: first-principles approach

This is an open access article distributed under the Creative Commons Attribution License.First-principles calculations for the temporal characteristics of hole-phonon relaxation in the valence band of titanium dioxide and zinc oxide have been performed. A first-principles method for the calculations of the quasistationary distribution function of holes has been developed. The results show that the quasistationary distribution of the holes in TiO2 extends to an energy level approximately 1 eV below the top of the valence band. This conclusion in turn helps to elucidate the origin of the spectral dependence of the photocatalytic activity of TiO2. Analysis of the analogous data for ZnO shows that in this material spectral dependence of photocatalytic activity in the oxidative reactions is unlikely.The authors acknowledge financial support from the Spanish MICINN (Grant no. FIS2010-19609-C02-01), the Departamento de Educacíon del Gobierno Vasco, the University of the Basque Country (Grant no. GIC07-IT-366-07), and the Presidium of the Ural Branch of Russian Academy of Sciences (Grant no. 12-U-3-1001).Peer Reviewe

Energy shift and wave function overlap of metal-organic interface states

The properties of Shockley-type interface states between π-conjugated organic molecular layers and metal surfaces are investigated by time-resolved two-photon photoemission experiments and density functional theory. For perylene- and naphthalene-tetracarboxylic acid dianhydride (PTCDA and NTCDA) adsorbed on Ag(111), a common mechanism of formation of the interface state from the partly occupied surface state of the bare Ag(111) is revealed. The energy position is found to be strongly dependent on the distance of the molecular carbon rings from the metal and their surface density. Bending of the carboxyl groups enhances the molecular overlap of the interface state.Peer Reviewe