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1T−TiSe2 is one of the most studied charge density wave (CDW) systems, not only because of its peculiar properties related to the CDW transition, but also due to its status as a promising candidate of exciton insulator signaled by the proposed plasmon softening at the CDW wave vector. Using high-resolution electron energy loss spectroscopy, we report a systematic study of the temperature-dependent plasmon behaviors of 1T−TiSe2. We unambiguously resolve the plasmon from phonon modes, revealing the existence of Landau damping to the plasmon at finite momentums, which does not support the plasmon softening picture for exciton condensation. Moreover, we discover that the plasmon lifetime at zero momentum responds dramatically to the band gap evolution associated with the CDW transition. The interband transitions near the Fermi energy in the normal phase are demonstrated to serve as a strong damping channel of plasmons, while such a channel in the CDW phase is suppressed due to the CDW gap opening, which results in the dramatic tunability of the plasmon in semimetals or small-gap semiconductors., This work was supported by the National Key R&D Program of China (No.2021YFA1400200 and No.2017YFA0303600), the National Natural Science Foundation of China (No.11874404andNo.11974399), and the Strategic Priority Research Program of Chinese Academy of Sciences (No.XDB33000000). V.M.S. acknowledges financial support by Grant No .PID2019105488GB-I00 funded by MCIN/AEI/10.13039/501100011033/. X.Z. was partially supported by the Youth Innovation Promotion Association of Chinese Academy of Sciences.

The surface excitation spectra of the B-terminated and Mg-terminated MgB(0001) surfaces are studied in the ab initio time-dependent density functional theory approach. The band structure of both these surfaces is fully included into evaluation of the surface response function using a linear response approach. We find that different surface electronic structures for these surface terminations result in very different surface collective electronic excitations. Due to a number of surface states and peculiarity of the MgB dielectric properties several surface plasmons, such as surface plasmon, interband surface plasmon, acoustic surface plasmon, and others have been found. We trace their evolution with momentum transfer change on both surfaces. Whereas at vanishing momentum transfers the plasmons in both systems can be understood considering the bulk MgB dielectric function, upon momentum increase the collective excitations present very different behavior. We relay such behavior to the differences in the surface electronic structure of the studied terminations., V.M.S. and E.V.C. acknowledge the partial support from the Spanish Ministry of Economy and Competitiveness MINECO, Grant Nos. FIS2016-76617-P and FIS2016-75862-P. V.M.S acknowledges project PI2017-30 of the Departamento de Educación, Política Lingüística y Cultura of the Basque government. E.V.C. acknowledges support of the Tomsk State University Academic D.I. Mendeleev Fund Program (Grant No. 8.1.01.2017) and Saint Petersburg State University (Project 15.61.202.2015).

Resumen del póster presentado al Symposium on Surface Science (3S), celebrado en St. Christoph am Arlberg (Austria) del 25 de febrero al 10 de marzo de 2018., The heterostructures consisting of the noble metal substrates and the late d-metal adsorbates and other heavy atoms with the degree of coverage from one atom to several monolayers (MLs), have been actively studied for the last two decades. The interest to these systems can be explained by the possibility of exploiting them in chemical industry, namely, in heterogeneous catalysis. It is known that the properties of two-dimensional systems can drastically differ from the properties of their bulk counterparts. Moreover, frequently the degree of coverage plays a crucial role in these effects. An example of such situation is the Pt/Au(111) heterostructure with the Pt adsorbate thickness varying from one to several atomic layers. In general, favorable catalytic activity, resulting in strong enhancement in rates of certain oxidation reactions, of thin Pt films deposited on Au has been found. All of this is making this and similar systems very attractive for experimental and theoretical research. On the other hand, heterostructures containing as a substrate a heavy metal and several atoms/monolayers of metallic adsorbate demonstrate unique properties caused by spin-orbit interaction that may be attractive for spintronics, a research field experiencing nowadays explosive development. A well-known example is the Bychkov-Rashba splitting effect consisting in lifting of a spin degeneration in two-dimensional systems due to spin-orbit interaction. Large Bychkov-Rashba splitting is characteristic for surface states of both noble and late d metals such as Ir and Pt metals. Moreover the heterostructures based on light noble metals (Cu and Ag) and heavy metal adsorbate such as Bi and Pb have a giant spin-orbit splitting of surface states noticeably larger than in pristine materials. The main reason for this giant splitting is the occurrence of a local potential gradient at the surface of such heterostructures that does not exist inside the bulk materials with inversion symmetry. In this work we investigate the formation of the surface and quantum-well states in thin Ir and Pt films deposited on the Au(111) substrate. From comparison of the electronic structure of Ir(111), Pt(111), and Au(111) surfaces one can observe a strong mismatch between positions of the band gaps in the projected bulk electronic structures of these materials. As a result, the electrons related to iridium or platinum atoms are reflected from the gold substrate by its energy gaps. Together with scattering produced by the potential barrier from the vacuum side this introduces the necessary conditions for realization of the quantum-well states. Since the electronic states in Ir and Pt in the Fermi level vicinity are mainly of the d character they present strong localization at the surface. In turn, this produces strong modification in the density of states in the surface region. Studying Ir- and Pt-derived surface and quantum-well states at different regions of the surface Brillouin zone (SBZ) we find distinct formation character. We also investigate in these heterostructures the effect of spin-orbit interaction on the electronic states localized at the surface since despite many experimental and theoretical studies devoted to these systems with adsorbate thickness of several monolayers, the effect of the spin-orbit interaction on the electronic structure of these systems was not addressed.

Resumen del trabajo presentado al Symposium on Surface Science (3S), celebrado en St. Christoph am Arlberg (Austria) del 21 al 27 de febrero de 2016.

Resumen del trabajo presentado al Symposium on Surface Science (3S), celebrado en St. Christoph am Arlberg (Austria) del 21 al 27 de febrero de 2016.

Resumen del trabajo presentado a la 18th International Conference on Solid Films and Surfaces (ICSFS), celebrada en Chemnitz (Alemania) del 28 de agosto al 2 de septiembre de 2016.

Resumen del trabajo presentado al Surface Plasmons and Plasmonics Workshop, celebrado en Santa Margherita Ligure (Italia) del 7 al 10 de junio de 2015.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al., We have investigated plasmonic excitations at the surface of Bi2Se3(0001) via high-resolution electron energy loss spectroscopy. For low parallel momentum transfer q≥, the loss spectrum shows a distinctive feature peaked at 104 meV. This mode varies weakly with q≥. The behavior of its intensity as a function of primary energy and scattering angle indicates that it is a surface plasmon. At larger momenta (q≥∼0.04 Å-1), an additional peak, attributed to the Dirac plasmon, becomes clearly defined in the loss spectrum. Momentum-resolved loss spectra provide evidence of the mutual interaction between the surface plasmon and the Dirac plasmon of Bi2Se3. The proposed theoretical model accounting for the coexistence of three-dimensional doping electrons and two-dimensional Dirac fermions accurately represents the experimental observations. The results reveal novel routes for engineering plasmonic devices based on topological insulators., We acknowledge funding from the University of Basque Country UPV/EHU (IT-756-13), the Departamento de Educación del Gobierno Vasco, the Tomsk State University Academic D. I. Mendeleev Fund Program (Grant No. 8.1.05.2015), the Spanish Ministry of Economy and Competitiveness MINECO (Grant No. FIS2013-48286-C2-1-P), Saint Petersburg State University (Project No. 11.50.202.2015), the Italian Ministry of Education, University, and Research (MIUR) through the program “FIRB”—Futuro in Ricerca 2010 RBFR10LULP “Fundamental research on Terahertz photonic devices.”

Resumen del trabajo presentado al JCNS Workshop: Neutron Scattering on Nano-Structured Soft Matter: Synthetic- and Bio-Materials, celebrado del 5 al 8 de octubre de 2015 en Tutzing, Germany.

Resumen del trabajo presentado al VII Encontro Sul- Americano de Colisões Inelásticas na Matéria, celebrado en Gramado, RS (Brasil) del 27 al 30 de octubre de 2014., El impacto de iones multicargados sobre superficies se ha convertido en un tema de interés en las últimas dos décadas. El grado de ionización del proyectil depende de su velocidad y ejerce una marcada influencia en las transiciones electrónicas originadas por impacto rasante sobre diferentes superficies metálicas, lo que afecta tanto la pérdida de energía del ión como la emisión electrónica. Por otro lado, en trabajos previos hallamos que los espectros de emisión electrónica producidos por incidencia rasante de protones sobre superficies de Be(0001), Cu(111), Ag(111), Au(111) y Mg(0001) presentan señales evidentes de la estructura de banda de la superficie, con “hombros” pronunciados debidos a la contribución de estados electrónicos superficiales parcialmente ocupados. El objetivo de este trabajo es, por lo tanto, investigar en qué grado tales estructuras son afectadas por el estado de carga del proyectil. Nuestras investigaciones previas para protones son extendidas aquí para estudiar la probabilidad de emisión electrónica producida por incidencia rasante sobre una superficie de Cu(111) de diferentes proyectiles - C, Be, Li y He - con distintos estados de carga. El proceso es descripto con la aproximación Band-Structured-Based (BSB), el cual es un método perturbativo que incluye una precisa representación de la interacción del electrón con la superficie, incorporando información de la estructura de bandas del sólido. La figura muestra la probabilidad de emisión electrónica para un ángulo de emisión q=30 considerando e distintos proyectiles que impactan sobre una superficie de Cu(111). Cuando la carga asintótica del proyectil disminuye se observa un incremento de las probabilidades de emisión y la atenuación de la estructura generada por la presencia de estados electrónicos superficiales. Este comportamiento es consecuencia de la influencia combinada de la trayectoria y del estado de carga del proyectil.

We present a study of the electron dynamics in the layered compound 2H-NbSe 2. First-principles calculations are used to obtain the band structure employed in the evaluation of the loss function with inclusion of local-field (LF) effects. Two different symmetry directions [(100) and (010)] were explored in the hexagonal basal plane. In both cases, a low-energy charge-carrier plasmon (CCP) at ∼1 eV presenting a negative dispersion over a wide momentum transfer range is found, in agreement with recent experimental results. On the contrary, in the (001) perpendicular direction, the CCP has negative dispersion at small momenta only, presenting strong positive dispersion at larger momenta. Our calculations reveal that this behavior can be explained without invoking many-body effects, as long as band structure effects are properly included in the evaluation of the excitation spectra. In addition to this CCP mode, we find another one with an arclike oscillating dispersion along the perpendicular direction, as well as the appearance of a CCP replica at high momenta due to LF effects. © 2012 American Physical Society., This work was supported in part by the Basque Departamento de Educación, Universidades, e Investigación (Grant No. IT-366-07) and the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02-00).

Resumen del trabajo presentado al Symposium on Surface Science (3S), celebrado en Baqueira Beret, Lleida, Spain del 6 al 12 de marzo de 2011.

Resumen del trabajo presentado al Workshop on Graphene, celebrado en el Donostia International Physics Center (DIPC) del 29 de agosto al 2 de septiembre de 2011,San Sebastián, Spain., Recently, it was shown that polarization of a two-dimensional graphene sheet by an external charge produces an attractive image potential that supports, in addition to the wellknown π* bands, a double series of Rydberg-like image potential states converging onto the vacuum level. Being quantified in the direction perpendicular to the graphene sheet, in the parallel plane these states have nearly free electron character, like similar states on metal surfaces. A large extension of wave function of image-potential states into the vacuum implies that they are very sensitive to any change of shape and environment of the graphene sheet. Thus, the properties of these states can be modified by an external electric field and a shape/geometry variation. The lowest-energy members of such states have been experimentally observed in graphene sheets grown on some substrates. Some examples of such unoccupied electronic states in other carbon-based materials, like graphite, multilayer graphene, fullerenes, and nanotubes as having common origin with the graphene image-potential states will be presented.

1 página, 1 figura.-- PACS: 73.20.-r, 68.37.Ef, 68.55.-a, 73.22.Pr.-- et al.

10 páginas, 6 figuras.-- et al., Image potential states (IPSs) are electronic states localized in front of a surface in a potential well, formed by the surface projected bulk band gap on one side and the image potential barrier on the other. In the limit of a two-dimensional solid, a double Rydberg series of IPSs has been predicted, which is in contrast to a single series present in three-dimensional solids. Here, we confirm this prediction experimentally for mono- and bilayer graphene. The IPSs of epitaxial graphene on SiC are measured by scanning tunneling spectroscopy and the results are compared with ab-initio band structure calculations. Despite the presence of the substrate, both calculations and experimental measurements show that the first pair of the double series of IPSs survives and eventually evolves into a single series for graphite. Thus, IPSs provide an elegant quantum probe of the interfacial coupling in graphene systems., SB thanks the Alexander von Humboldt Foundation and IB thanks the Marie Curie action program for support. The present work was partially supported by the University of the Basque Country (grant no. GIC07IT36607), the Departamento de Educación del Gobierno Vasco and the Spanish Ministerio de Educación (grant no. FIS200766711C0101).

4 páginas, 4 figuras.-- PACS number(s): 73.21.Fg, 73.20.Mf, 79.60.Dp.-- et al., We report on low-energy dynamical surface response properties of the p(2×2)K/Be(0001) 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., We acknowledge partial support from the University of the Basque Country (Grant No. GIC07IT36607), the Departamento de Educación del Gobierno Vasco, and the Spanish Ministerio de Ciencia y Tecnología (MCyT) (Grant No. FIS200766711C0101).

5 p., 4 figuras We present ab initio time-dependent density-functional theory calculation results for low-energy collective electron excitations in MgB2. The existence of a long-lived collective excitation corresponding to coherent charge density fluctuations between the boron sigma and pi bands (sigma pi 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 (sigma sigma mode). We show the strong impact of local-field effects on dielectric functions in MgB2. 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. Partial support from the University of the Basque Country (Grant No. GIC07IT36607), the Departamento de Educacion del Gobierno Vasco, and the Spanish Ministerio de Ciencia y Tecnologia (MCyT) (Grant No. FIS200766711C0101).

We present self-consistent pseudopotential calculations of surface and image-potential states on MgB2(0001) for both B-terminated (B−t) and Mg-terminated (Mg−t) surfaces. We find a variety of very clear surface and subsurface states as well as resonance image-potential states n=1,2 on both surfaces. The surface layer density of states (DOS) at EF is increased by 55% at the B−t and by 90% at the Mg−t surface compared to DOS in the corresponding bulk layers., Partial support by the Basque Country University, Basque Hezkuntza Saila, and Iberdrola is acknowledged.

En la presente tesis se ha realizado el estudio de primeros principios (esto es, sinhacer uso de parámetros ajustables) de la estructura electrónica y la dinámica deexcitaciones electrónicas en plomo, tanto en volumen como en superficie y en formade películas de espesor nanométrico. Al presentar el plomo un número atómico alto(82), deben tenerse en cuenta los efectos relativistas. Con este fin, el doctorando haimplementado el acoplo espín-órbita en los códigos computacionales que hanrepresentado la principal herramienta de trabajo.En volumen, se han encontrado fuertes efectos relativistas asi como de lalocalización de los electrones, tanto en la respuesta dieléctrica (excitacioneselectrónicas colectivas) como en el tiempo de vida de electrones excitados. Lacomparación de nuestros resultados con medidas experimentales ha ayudado aprofundizar en dichos efectos.En el estudio de las películas a escala nanométrica se han hallado fuertes efectoscuánticos debido al confinamiento de los estados electrónicos. Dichos efectos semanifiestan tanto en el estado fundamental (en acuerdo con estudiosexperimentales), como en la respuesta dieléctrica a través de la aparición y dinámicade plasmones de diversas características. Los efectos relativistas, a pesar de no serimportantes en la estructura electrónica de las películas, son los responsables de ladesaparación del plasmón de baja energía en nuestros resultados.

Doctoral Thesis submitted to the University of the Basque Country to obtain the degree of Doctor in Physics by Juan Pablo Echeverry Enciso., [ES]: Un estudio de la respuesta electrónica y las excitaciones colectivas en sistemas con una compleja he interesante dinámica electrónica cerca al nivel de Fermi donde coexisten estados electrónicos de diferente naturaleza, como son estado de superficie, estados de pozo cuántico o estados de intercapa. Se han hecho cálculos de la respuesta de superficie, la densidad electrónica inducida en superficies con el propósito de analizar las fluctuaciones de la densidad de carga inducida por perturbaciones externas, la dispersión de los plasmones presentes en estos sistemas con particular interés en las excitaciones de baja energía debidas a transiciones intra he interbanda en monocapas de Ag, un sustrato de Be cubierto por una monocapa de K. Así mismo se ha hecho un estudio sistemático de las propiedades dieléctricas de los sistemas basados en la intercalación de grafito (Graphite intercalated compounds -GICs-) con el propósito de establecer una correlación entre la llamada banda de intercapa ("interlayer" band) y la presencia de plasmones acústico, los cuales presentan una dispersión lineal con energía cero cuando el momentum transferido es cero. Los cálculos de la tesis se han llevado a cabo con el formalismo de la teoría del funcional de la densidad independiente y dependiente del tiempo (DFT, TD-DFT) haciendo uso de la aproximación conocida como aproximación de fase aleatoria (RPA). Los tres sistemas que se han elegido son LiC6, SrC6 y CaC6 de los cuales el primero de estos no presenta fase superconductora a diferencia de los dos últimos.

[ES]: En esta tesis se ha realizado un estudio de la complejidad inducida por la presión en elementos y aleaciones simples a través de cálculos basados en primeros principios. Aunque muchos metales sencillos tienen un comportamiento que se puede describir perfectamente con el modelo de electrones libres a presión ambiente, bajo presión desarrollan propiedades complejas. En esta tesis hemos estudiado los elementos litio y calcio, además de la aleación Be2Li y los hidruros de paladio. En todos ellos hemos estudiado las excitaciones electrónicas colectivas como los plasmones, la superconductividad y los efectos no armónicos. Las conclusiones más importantes del trabajo realizado son que bajo presión pueden emerger plasmones de muy baja energía, que muchos elementos no superconductores pueden convertirse en superconductores de alta temperatura y que aparecen fases, como la cúbica simple del calcio, que solamente son dinámicamente estables debido a los efectos no armónicos. En conclusión, que la presión abre una nueva dimensión para inducir propiedades muy interesantes en muchos materiales a priori sencillos., I would like to acknowledge the DIPC for the three month contract that allowed me to start in the business. Then, I have been financially supported by the Department of Education, Universities and Research of the Basque Government. Thank you very much to them as well.