Publications (9)37.74 Total impact
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Article: First-principle approach to the study of spin relaxation times of excited electrons in metals
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ABSTRACT: 6 pages, 4 figures.-- PACS nrs.: 72.15.Lh, 72.25.Ba, 72.25.Rb.-- Presented as poster communication to TNT 2007: Trends in NanoTechnology (San Sebastián, Spain, Sep 3-7, 2008). We have developed a first-principle method of calculating the spin-lattice relaxation time of excited electrons in metals based on the GW approach incorporating spin-orbit coupling. We have studied the spin-lattice relaxation time and path in Al, Cu, Au, Nb and Ta. The spin-lattice relaxation time and path in Nb appear to be much less than in Al, Cu, Au. They are particularly small in Ta, in accord with strong spin-orbit coupling. Comparing our results with experimental data on the impurity- and phonon-induced spin relaxation times and path's, we find that at the energy about of 0.9 eV, which is typical for excited electrons in spin-valve and magnetic-tunnel transistors, the inelastic scattering accompanied by creation of electron-hole pairs is the dominating mechanism of spin-lattice relaxation. Peer reviewedPhysica Status Solidi (A) Applications and Materials 05/2008; · 1.46 Impact Factor -
Article: Low-energy acoustic plasmons at metal surfaces.
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ABSTRACT: Nearly two-dimensional (2D) metallic systems formed in charge inversion layers and artificial layered materials permit the existence of low-energy collective excitations, called 2D plasmons, which are not found in a three-dimensional (3D) metal. These excitations have caused considerable interest because their low energy allows them to participate in many dynamical processes involving electrons and phonons, and because they might mediate the formation of Cooper pairs in high-transition-temperature superconductors. Metals often support electronic states that are confined to the surface, forming a nearly 2D electron-density layer. However, it was argued that these systems could not support low-energy collective excitations because they would be screened out by the underlying bulk electrons. Rather, metallic surfaces should support only conventional surface plasmons-higher-energy modes that depend only on the electron density. Surface plasmons have important applications in microscopy and sub-wavelength optics, but have no relevance to the low-energy dynamics. Here we show that, in contrast to expectations, a low-energy collective excitation mode can be found on bare metal surfaces. The mode has an acoustic (linear) dispersion, different to the dependence of a 2D plasmon, and was observed on Be(0001) using angle-resolved electron energy loss spectroscopy. First-principles calculations show that it is caused by the coexistence of a partially occupied quasi-2D surface-state band with the underlying 3D bulk electron continuum and also that the non-local character of the dielectric function prevents it from being screened out by the 3D states. The acoustic plasmon reported here has a very general character and should be present on many metal surfaces. Furthermore, its acoustic dispersion allows the confinement of light on small surface areas and in a broad frequency range, which is relevant for nano-optics and photonics applications.Nature 08/2007; 448(7149):57-9. · 36.28 Impact Factor -
Article: Vibrational properties of the Pt(111)-p(2x2)-K surface superstructure
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ABSTRACT: 9 pages, 4 figures.-- PACS nrs.: 63.20.Dj, 63.22.+m, 68.35.Ja, 68.43.Pq.-- Original Russian Text published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 8, pp. 1510–1517. The vibrational spectra of the Pt(111)-p(2x2)-K ordered surface superstructure formed on the platinum surface upon adsorption of 0.25 potassium monolayer are calculated using the interatomic interaction potentials obtained within the tight-binding approximation. The surface relaxation, the dispersion of surface phonons, the local density of surface vibrational states, and the polarization of vibrational modes of adatoms and substrate atoms are discussed. The theoretical results are in good agreement with the recently obtained experimental data. This study was supported by the Siberian and Far East Branches of the Russian Academy of Sciences, integration project no. 216. Peer reviewed -
Article: The role of an electronic surface state in the stopping power of a swift charged particle in front of a metal
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ABSTRACT: 7 pages, 10 figures.-- Printed version published on Jul 30, 2008. We study the stopping power and friction coefficient of a slow charged particle moving parallel to noble metal (111) surfaces. In the description of the surface electronic structure, information about a wide energy gap at the surface Brillouin zone, at the Fermi level, and the partly occupied s–p_z surface state is introduced via the use of a model potential. The stopping power, S(b,υ), and friction coefficient, γ(b,υ), versus the projectile velocity υ and its distance from the surface b are investigated within linear response theory with self-consistent evaluation of the surface response function. The present calculations demonstrate the striking differences in the behavior of S(b,υ) and γ(b,υ) in comparison with those obtained from simpler models. In particular, for very low velocities, S(b,υ) and γ(b,υ) decay as b^−3 at large b, mainly due to the electron–hole excitations within the surface state, instead of the ~b^−4 behavior expected from a jellium model. For velocities close to the surface state Fermi velocity, υ_F^SS, the energy losses with characteristic ~b^−2 decay are dominated by the excitation of the acoustic surface plasmons that can exist at some surfaces with partly occupied surface states. This work has been partially funded by the University of the Basque Country UPV/EHU (Grant No. 9/UPV 00206.215-13639/2001), the Eusko Jaurlaritza, and the Spanish MCyT (Grant No. FIS2007-066711-CO2-00). Peer reviewed -
Article: Vibrations of alkali metal overlayers on metal surfaces
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ABSTRACT: 10 pages. We review the current progress in the understanding of vibrations of alkalis adsorbed on metal surfaces. The analysis of alkali vibrations was made on the basis of available theoretical and experimental results. We also include in this discussion our recent calculations of vibrations in K/Pt(111) and Li(Na)/Cu(001) systems. The dependence of alkali adlayer localized modes on atomic mass, adsorption position and coverage as well as the dependence of vertical vibration frequency on the substrate orientation is discussed. The square root of atomic mass dependence of the vertical vibration energy has been confirmed by using computational data for alkalis on the Al(111) and Cu(001) substrates. We have confirmed that in a wide range of submonolayer coverages the stretch mode energy remains nearly constant while the energy of in-plane polarized modes increases with the increase of alkali coverage. It was shown that the spectrum of both stretch and in-plane vibrations can be very sensitive to the adsorption position of alkali atoms and substrate orientation. The work was partially supported by the Basque Country University and the Siberian Branch of RAS (grant No. 206). GB is grateful to the Donostia International Physics Center for supporting his visit to DIPC. Peer reviewed -
Article: Dynamic screening and electron dynamics in non-homogeneous metal systems
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ABSTRACT: 9 pages, 8 figures.-- PACS nrs.: 73.21.Fg, 73.20.At, 74.70.Ad, 73.50.Gr.-- Printed version published on Aug 2008. Recent advances in the theoretical description of electron and hole dynamics in several metallic systems are discussed. The role of initial and final states in the decay rates together with the dynamical screening is studied for the case of three different non-homogeneous systems, Na/Cu(111), MgB2, and Mg(0001). We acknowledge partial support by the University of the Basque Country UPV/EHU (GIC07IT36607), and the Spanish MCyT (Grant No. FIS200766711C0101). Peer reviewed -
Article: Interlayer exchange coupling in digital magnetic alloys
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ABSTRACT: 11 pages, 3 figures.-- PACS nrs.: 73.40.Sx, 75.70.-i. We discuss possible mechanisms for indirect exchange between ferromagnetic δ layers of transition metal inserted into a semiconducting host, taking into account the role of carrier confinement at these layers. We show that the Ruderman-Kittel-Kasuya-Yoshida mechanism is not the ultimate explanation for an interlayer interaction and an efficient interlayer coupling can be mediated by the undoped semiconducting spacer due to virtual excitations across the energy threshold. We emphasize the important role of quasi-two-dimensional spin-polarized states inside the bulk energy gap, which are caused by the confinement and the exchange scattering of itinerant electrons by the δ layers. Quasiparticle excitations from these states to the band edge of the spacer contribute to the interlayer coupling even for a wide gap semiconducting spacer. Our analysis shows that the related exchange integral can change its sign at some "critical" spacer thickness, i.e., a ferromagnetic coupling mechanism is active at short distance between δ layers and an antiferromagnetic coupling mechanism is active at large distance. Taking into account the effects of crystal symmetry, we also obtain the expression for the interband coupling energy in the case of both direct- and indirect-gap spacers. We show that the carrier confinement gives rise to a renormalization of the intensity of excitations through the band gap. The interband coupling decays exponentially with the distance between the δ layers and is strongly determined by the electron structure of the host. The estimates of the interlayer interaction parameters across Si, Ge, and GaAs spacers are presented. The combination of two mechanisms (confinement-mediated exchange and interband exchange) mainly determines the behavior of the interlayer coupling in the digital magnetic alloys with undoped spacer. The work was partially supported by the University of the Basque Country (Grant No. 9/UPV 00206.215-13639/2001), by Spanish Ministerio de Ciencia y Tecnología (Grant No. FIS 2004-06490-C03-01), and by RFBR (Grant No. 07-02-00114-a). Peer reviewed -
Article: Electronic potential of a chemisorption interface
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ABSTRACT: 7 pages, 4 figures.-- PACS nrs.: 73.20.-r. Chemisorption of atoms and molecules controls many interfacial phenomena such as charge transport and catalysis. The question of how the intrinsic properties of the interacting materials define the electronic structure of their interface remains one of the most important, yet intractable problems in surface physics. Through two-photon photoemission spectroscopy we determine a common binding energy of ~1.8–2.0 eV with respect to the vacuum for the unoccupied resonance of the ns valence electron of alkali atoms (Li-Cs) chemisorbed at low coverage (less than 0.1 monolayer) on noble metal [Cu(111) and Ag(111)] surfaces. We present a theoretical model based on the semiempirical potentials of the adsorbates and the substrates, their principal mode of interaction through the Coulomb interaction, and the ab initio adsorption structures. Our analysis reveals that atomic size and ionization potential independent interfacial electronic structure is a consequence of the Coulomb interaction among the ns electron, the alkali-atom ionic core, and the induced image charge in the substrate. We expect the same interactions to define the effective electronic potentials for a broad range of molecule/metal interfaces. This work was supported by the U.S. Department of Energy Grant No. DE-FG02-03ER15434. Some calculations were performed in the Environmental Molecular Sciences Laboratory, a user facility sponsored by the DOE Office of Biological and Environmental Research. N.P. thanks the Alexander von Humboldt Foundation for support. H.P. thanks Ikerbasque for the support of his stay at DIPC. The San Sebastián group acknowledges support by the Spanish MEC grant, the European FP6-NoE grant "Nanoquanta", the UPV/EHU and the projects "NANOMATERIALES" and "NANOTRON" of the Basque Government and the Diputación de Guipúzcoa. Peer reviewed -
Article: Band structure effects on the Be(0001) acoustic surface plasmon energy dispersion
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ABSTRACT: 22 pp.-- PACS nrs.: 71.15.Mb, 71.45.Gm, 73.20.At, 73.20.Mf.-- Pre-print version available at: http://arxiv.org/abs/0806.2364. 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 map the electron-hole excitation spectrum of the quasi two-dimensional Shockley surface state band that is present on the Be(0001) surface. V.M.S., J.M.P, E.V.C, and P.M.E. acknowledge partial support from the University of the Basque Country (9/UPV 00206.215-13639/2001), the Basque Unibertsitate eta Ikerketa Saila, and the Spanish Ministerio de Educación y Ciencia (MEC) (FIS 2004-06490-C03-01 and CSD2006-53). B.D. and K.P. acknowledge the National Science Foundation; L.V., L.S., and M.R. Compagnia di San Paolo; P.H. the Danish Natural Science Research Council; D.F. the Programa Ramón y Cajal and Comunidad de Madrid. Peer reviewed
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Institutions
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2008
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Russian Academy of Sciences
Moscow, Moscow, Russia -
Donostia International Physics Center
San Sebastián, Basque Country, Spain
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