Investigation of acceptor levels and hole scattering mechanisms in p-gallium selenide by means of transport measurements under pressure

The effect of pressure on acceptor levels and hole scattering mechanisms in p-GaSe is investigated through Hall effect and resistivity measurements under quasi-hydrostatic conditions up to 4 GPa. The pressure dependence of the hole concentration is interpreted through a carrier statistics equation with a single (nitrogen) or double (tin) acceptor whose ionization energies decrease under pressure due to the dielectric constant increase. The pressure effect on the hole mobility is also accounted for by considering the pressure dependencies of both the phonon frequencies and the hole-phonon coupling constants involved in the scattering rates.Comment: 13 pages, Latex, 4 ps figures. to appear in High Pressure Research 69 (1997

Angle-resolved photoemission study and first principles calculation of the electronic structure of GaTe

The electronic band structure of GaTe has been calculated by numerical atomic orbitals density-functional theory, in the local density approximation. In addition, the valence-band dispersion along various directions of the GaTe Brillouin zone has been determined experimentally by angle-resolved photoelectron spectroscopy. Along these directions, the calculated valence-band structure is in good concordance with the valence-band dispersion obtained by these measurements. It has been established that GaTe is a direct-gap semiconductor with the band gap located at the Z point, that is, at Brillouin zone border in the direction perpendicular to the layers. The valence-band maximum shows a marked \textit{p}-like behavior, with a pronounced anion contribution. The conduction band minimum arises from states with a comparable \textit{s}- \textit{p}-cation and \textit{p}-anion orbital contribution. Spin-orbit interaction appears to specially alter dispersion and binding energy of states of the topmost valence bands lying at $\Gamma$. By spin-orbit, it is favored hybridization of the topmost \textit{p}$_z$-valence band with deeper and flatter \textit{p$_x$}-\textit{p$_y$} bands and the valence-band minimum at $\Gamma$ is raised towards the Fermi level since it appears to be determined by the shifted up \textit{p$_x$}-\textit{p$_y$} bands.Comment: 7 text pages, 6 eps figures, submitted to PR

Tin-related double acceptors in gallium selenide single crystals

Gallium selenide single crystals doped with different amounts of tin are studied through resistivity and Hall effect measurements in the temperature range from 30 to 700 K. At low doping concentration tin is shown to behave as a double acceptor impurity in gallium selenide with ionization energies of 155 and 310 meV. At higher doping concentration tin also introduces deep donor levels, but the material remains p-type in the whole studied range of tin doping concentrations. The deep character of donors in gallium selenide is discussed by comparison of its conduction band structure to that of indium selenide under pressure. The double acceptor center is proposed to be a tin atom in interlayer position, with a local configuration that is similar to that of tin diselenide. The hole mobility exhibits an anomalous dependence on the tin content, attaining its maximum value in the ingot with 0.2% nominal tin content. This is proposed to be related to impurity pairing effects giving rise to thermal shallow acceptors with low ionization energy and low carrier scattering cross section, making the hole mobility to be controlled by phonon scattering mechanisms even for relatively high impurity [email protected] ; [email protected]

Forecasting Latin America’s Country Risk Scores by Means of a Dynamic Diffusion Model

Copyright © 2013 R. Cervelló-Royo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Over the last years, worldwide financial market instability has shaken confidence in global economies. Global financial crisis and changes in sovereign debts ratings have affected the Latin American financial markets and their economies. However, Latin American s relative resilience to the more acute rise in risk seen in other regions like Europe during last years is offering investors new options for improving risk-return trade-offs. Therefore, forecasting the future of economic situation involves high levels of uncertainty. The Country Risk Score (CRS) represents a broadly used indicator to measure the current situation of a country regarding measures of economic, political, and financial risk in order to determine country risk ratings. In this contribution, we present a diffusion model to study the dynamics of the CRS in 18 Latin American countries which considers both the endogenous effect of each country policies and the contagion effect among them. The model predicts quite well the evolution of the CRS in the short term despite the economic and political instability. Furthermore, the model reproduces and forecasts a slight increasing trend, on average, in the CRS dynamics for almost all Latin American countries over the next months.This work has been partially supported by the Spanish M.C.Y.T. Grants MTM2009-08587 as well as the Universitat Politecnica de Valencia Grant PAID06-11 (ref. 2070).Cervelló Royo, RE.; Cortés, J.; Sánchez Sánchez, A.; Santonja, F.; Villanueva Micó, RJ. (2013). Forecasting Latin America’s Country Risk Scores by Means of a Dynamic Diffusion Model. Abstract and Applied Analysis. 2013:1-11. https://doi.org/10.1155/2013/264657S1112013Fernandez-Arias, E., & Montiel, P. J. (1996). The Surge in Capital Inflows to Developing Countries: An Analytical Overview. The World Bank Economic Review, 10(1), 51-77. doi:10.1093/wber/10.1.51Taylor, M. P., & Sarno, L. (1997). Capital Flows to Developing Countries: Long- and Short-Term Determinants. The World Bank Economic Review, 11(3), 451-470. doi:10.1093/wber/11.3.451Dornbusch, R., Park, Y. C., & Claessens, S. (2000). Contagion: Understanding How It Spreads. The World Bank Research Observer, 15(2), 177-197. doi:10.1093/wbro/15.2.177Forbes, K., & Rigobon, R. (2001). Measuring Contagion: Conceptual and Empirical Issues. International Financial Contagion, 43-66. doi:10.1007/978-1-4757-3314-3_3Kodres, L. E., & Pritsker, M. (2002). A Rational Expectations Model of Financial Contagion. The Journal of Finance, 57(2), 769-799. doi:10.1111/1540-6261.00441Hoti, S. (2005). Comparative analysis of risk ratings for the East European region. Mathematics and Computers in Simulation, 68(5-6), 449-462. doi:10.1016/j.matcom.2005.02.014Kaminsky, G. (2002). Emerging Market Instability: Do Sovereign Ratings Affect Country Risk and Stock Returns? The World Bank Economic Review, 16(2), 171-195. doi:10.1093/wber/16.2.171Ferri, G., Liu, L.-G., & Stiglitz, J. E. (1999). The Procyclical Role of Rating Agencies: Evidence from the East Asian Crisis. Economic Notes, 28(3), 335-355. doi:10.1111/1468-0300.00016Hoti, S., & McAleer, M. (2004). An Empirical Assessment of Country Risk Ratings and Associated Models. Journal of Economic Surveys, 18(4), 539-588. doi:10.1111/j.0950-0804.2004.00230.xMcAleer, M., da Veiga, B., & Hoti, S. (2011). Value-at-Risk for country risk ratings. Mathematics and Computers in Simulation, 81(7), 1454-1463. doi:10.1016/j.matcom.2010.06.016Dungey *, M., Fry, R., González-Hermosillo, B., & Martin, V. L. (2005). Empirical modelling of contagion: a review of methodologies. Quantitative Finance, 5(1), 9-24. doi:10.1080/14697680500142045Bayoumi, T., Fazio, G., Kumar, M., & MacDonald, R. (2007). Fatal attraction: Using distance to measure contagion in good times as well as bad. Review of Financial Economics, 16(3), 259-273. doi:10.1016/j.rfe.2007.01.001Kanungo, T., Mount, D. M., Netanyahu, N. S., Piatko, C. D., Silverman, R., & Wu, A. Y. (2002). An efficient k-means clustering algorithm: analysis and implementation. IEEE Transactions on Pattern Analysis and Machine Intelligence, 24(7), 881-892. doi:10.1109/tpami.2002.1017616Frambach, R. T. (1993). An Integrated Model of Organizational Adoption and Diffusion of Innovations. European Journal of Marketing, 27(5), 22-41. doi:10.1108/03090569310039705Mahajan, V., Muller, E., & Bass, F. M. 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Trapping of three-dimensional electrons and transition to two-dimensional transport in the three-dimensional topological insulator Bi2Se3 under high pressure

[EN] This paper reports an experimental and theoretical investigation on the electronic structure of bismuth selenide (Bi2Se3) up to 9 GPa. The optical gap of Bi2Se3 increases from 0.17 eV at ambient pressure to 0.45 eV at 8 GPa. The quenching of the Burstein-Moss effect in degenerate samples and the shift of the free-carrier plasma frequency to lower energies reveal a quick decrease of the bulk three-dimensional (3D) electron concentration under pressure. On increasing pressure the behavior of Hall electron concentration and mobility depends on the sample thickness, consistently with a gradual transition from mainly 3D transport at ambient pressure to mainly two-dimensional (2D) transport at high pressure. Two-carrier transport equations confirm the trapping of high-mobility 3D electrons, an effect that can be related to a shallow-to-deep transformation of donor levels, associated with a change in the ordering of the conduction band minima. The high apparent areal density and low electron mobility of 2D electrons are not compatible with their expected properties in a Dirac cone. Measured transport parameters at high pressure are most probably affected by the presence of holes, either in an accumulation surface layer or as minority carriers in the bulk. ©2012 American Physical SocietyThis work has been done under financial support from Spanish MICINN under Grants No. MAT2008-06873-C02-02, No. MAT2007-66129, No. MAT2010-21270-C04-03/04, No. CSD2007-00045, and Prometeo No. GV2011/035. The supercomputer time has been provided by the Red Espanola de Supercomputacion (RES) and the MALTA cluster.Segura, A.; Panchal, V.; Sánchez-Royo, JF.; Marín-Borrás, V.; Muñoz-Sanjosé, V.; Rodríguez-Hernández, P.; Muñoz, A.... (2012). Trapping of three-dimensional electrons and transition to two-dimensional transport in the three-dimensional topological insulator Bi2Se3 under high pressure. Physical Review B. 85:195139-1-195139-9. https://doi.org/10.1103/PhysRevB.85.195139S195139-1195139-985Mishra, S. K., Satpathy, S., & Jepsen, O. (1997). Electronic structure and thermoelectric properties of bismuth telluride and bismuth selenide. Journal of Physics: Condensed Matter, 9(2), 461-470. doi:10.1088/0953-8984/9/2/014Hor, Y. S., Richardella, A., Roushan, P., Xia, Y., Checkelsky, J. G., Yazdani, A., … Cava, R. J. (2009). p-typeBi2Se3for topological insulator and low-temperature thermoelectric applications. Physical Review B, 79(19). doi:10.1103/physrevb.79.195208Zhang, H., Liu, C.-X., Qi, X.-L., Dai, X., Fang, Z., & Zhang, S.-C. (2009). Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface. Nature Physics, 5(6), 438-442. doi:10.1038/nphys1270Hasan, M. Z., & Kane, C. L. (2010). Colloquium: Topological insulators. Reviews of Modern Physics, 82(4), 3045-3067. doi:10.1103/revmodphys.82.3045Moore, J. E. (2010). 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STM Imaging of Electronic Waves on the Surface ofBi2Te3: Topologically Protected Surface States and Hexagonal Warping Effects. Physical Review Letters, 104(1). doi:10.1103/physrevlett.104.016401Roushan, P., Seo, J., Parker, C. V., Hor, Y. S., Hsieh, D., Qian, D., … Yazdani, A. (2009). Topological surface states protected from backscattering by chiral spin texture. Nature, 460(7259), 1106-1109. doi:10.1038/nature08308Butch, N. P., Kirshenbaum, K., Syers, P., Sushkov, A. B., Jenkins, G. S., Drew, H. D., & Paglione, J. (2010). Strong surface scattering in ultrahigh-mobilityBi2Se3topological insulator crystals. Physical Review B, 81(24). doi:10.1103/physrevb.81.241301Wang, Z., Lin, T., Wei, P., Liu, X., Dumas, R., Liu, K., & Shi, J. (2010). Tuning carrier type and density in Bi2Se3 by Ca-doping. Applied Physics Letters, 97(4), 042112. doi:10.1063/1.3473778Ren, Z., Taskin, A. A., Sasaki, S., Segawa, K., & Ando, Y. (2010). Large bulk resistivity and surface quantum oscillations in the topological insulatorBi2Te2Se. Physical Review B, 82(24). doi:10.1103/physrevb.82.241306Kulbachinskii, V. A., Miura, N., Nakagawa, H., Arimoto, H., Ikaida, T., Lostak, P., & Drasar, C. (1999). Conduction-band structure ofBi2−xSbxSe3mixed crystals by Shubnikov–de Haas and cyclotron resonance measurements in high magnetic fields. Physical Review B, 59(24), 15733-15739. doi:10.1103/physrevb.59.15733Analytis, J. G., McDonald, R. D., Riggs, S. C., Chu, J.-H., Boebinger, G. S., & Fisher, I. R. (2010). Two-dimensional surface state in the quantum limit of a topological insulator. Nature Physics, 6(12), 960-964. doi:10.1038/nphys1861Cho, S., Butch, N. P., Paglione, J., & Fuhrer, M. S. (2011). Insulating Behavior in Ultrathin Bismuth Selenide Field Effect Transistors. Nano Letters, 11(5), 1925-1927. doi:10.1021/nl200017fZhang, Y., He, K., Chang, C.-Z., Song, C.-L., Wang, L.-L., Chen, X., … Xue, Q.-K. (2010). Crossover of the three-dimensional topological insulator Bi2Se3 to the two-dimensional limit. Nature Physics, 6(8), 584-588. doi:10.1038/nphys1689Kong, D., Cha, J. J., Lai, K., Peng, H., Analytis, J. G., Meister, S., … Cui, Y. (2011). Rapid Surface Oxidation as a Source of Surface Degradation Factor for Bi2Se3. ACS Nano, 5(6), 4698-4703. doi:10.1021/nn200556hBenia, H. M., Lin, C., Kern, K., & Ast, C. R. (2011). Reactive Chemical Doping of theBi2Se3Topological Insulator. Physical Review Letters, 107(17). doi:10.1103/physrevlett.107.177602King, P. D. C., Hatch, R. C., Bianchi, M., Ovsyannikov, R., Lupulescu, C., Landolt, G., … Hofmann, P. (2011). Large Tunable Rashba Spin Splitting of a Two-Dimensional Electron Gas inBi2Se3. Physical Review Letters, 107(9). doi:10.1103/physrevlett.107.096802Hamlin, J. J., Jeffries, J. R., Butch, N. P., Syers, P., Zocco, D. A., Weir, S. T., … Maple, M. B. (2011). High pressure transport properties of the topological insulator Bi2Se3. 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Electronic properties and Fermi surface of Ag(111) films deposited onto H-passivated Si(111)-(1x1) surfaces

Silver films were deposited at room temperature onto H-passivated Si(111) surfaces. Their electronic properties have been analyzed by angle-resolved photoelectron spectroscopy. Submonolayer films were semiconducting and the onset of metallization was found at a Ag coverage of $\sim$0.6 monolayers. Two surface states were observed at $\bar{\Gamma}$-point in the metallic films, with binding energies of 0.1 and 0.35 eV. By measurements of photoelectron angular distribution at the Fermi level in these films, a cross-sectional cut of the Fermi surface was obtained. The Fermi vector determined along different symmetry directions and the photoelectron lifetime of states at the Fermi level are quite close to those expected for Ag single crystal. In spite of this concordance, the Fermi surface reflects a sixfold symmetry rather than the threefold symmetry of Ag single crystal. This behavior was attributed to the fact that these Ag films are composed by two domains rotated 60$^o$.Comment: 9 pages, 8 figures, submitted to Physical Review

Study of the Secondary Electron Yield in Dielectrics Using Equivalent Circuital Models

© 2018 IEEE. Personal use of this material is permitted. Permissíon from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertisíng or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.[EN] Secondary electron emission has an important role on the triggering of the multipactor effect; therefore, its study and characterization are essential in radio-frequency waveguide applications. In this paper, we propose a theoretical model, based on equivalent circuit models, to properly understand charging and discharging processes that occur in dielectric samples under electron irradiation for secondary electron emission characterization. Experimental results obtained for Pt, Si, GaS, and Teflon samples are presented to verify the accuracy of the proposed model. Good agreement between theory and experiments has been found.The authors would like to thank the European High Power Space Materials Laboratory for its contribution-a laboratory funded by the European Regional Development Fund-a way of making Europe. Many thanks to the University of Valencia (Spain) for supporting this research activity with the internal program "Assistance for temporary stays of invited researchers within the framework of the Subprogramme Attraction of Talent 2015".Bañón, D.; Socuellamos, JM.; Mata-Sanz, R.; Mercadé-Morales, L.; Gimeno Martínez, B.; Boria Esbert, VE.; Raboso García-Baquero, D.... (2018). Study of the Secondary Electron Yield in Dielectrics Using Equivalent Circuital Models. IEEE Transactions on Plasma Science. 46(4):859-867. https://doi.org/10.1109/TPS.2018.2809602S85986746

Quantum-well states in ultrathin Ag(111) films deposited onto H-passivated Si(111)-(1x1) surfaces

Ag(111) films were deposited at room temperature onto H-passivated Si(111)-(1x1) substrates, and subsequently annealed at 300 C. An abrupt non-reactive Ag/Si interface is formed, and very uniform non-strained Ag(111) films of 6-12 monolayers have been grown. Angle resolved photoemission spectroscopy has been used to study the valence band electronic properties of these films. Well-defined Ag sp quantum-well states (QWS) have been observed at discrete energies between 0.5-2eV below the Fermi level, and their dispersions have been measured along the GammaK, GammaMM'and GammaL symmetry directions. QWS show a parabolic bidimensional dispersion, with in-plane effective mass of 0.38-0.50mo, along the GammaK and GammaMM' directions, whereas no dispersion has been found along the GammaL direction, indicating the low-dimensional electronic character of these states. The binding energy dependence of the QWS as a function of Ag film thickness has been analyzed in the framework of the phase accumulation model. According to this model, a reflectivity of 70% has been estimated for the Ag-sp states at the Ag/H/Si(111)-(1x1) interface.Comment: 6 pages, 6 figures, submitted to Phys. Rev.

Two-domains bulklike Fermi surface of Ag films deposited onto Si(111)-(7x7)

Thick metallic silver films have been deposited onto Si(111)-(7x7) substrates at room temperature. Their electronic properties have been studied by using angle resolved photoelectron spectroscopy (ARPES). In addition to the electronic band dispersion along the high-symmetry directions, the Fermi surface topology of the grown films has been investigated. Using ARPES, the spectral weight distribution at the Fermi level throughout large portions of the reciprocal space has been determined at particular perpendicular electron-momentum values. Systematically, the contours of the Fermi surface of these films reflected a sixfold symmetry instead of the threefold symmetry of Ag single crystal. This loss of symmetry has been attributed to the fact that these films appear to be composed by two sets of domains rotated 60$^o$ from each other. Extra, photoemission features at the Fermi level were also detected, which have been attributed to the presence of surface states and \textit{sp}-quantum states. The dimensionality of the Fermi surface of these films has been analyzed studying the dependence of the Fermi surface contours with the incident photon energy. The behavior of these contours measured at particular points along the Ag $\Gamma$L high-symmetry direction puts forward the three-dimensional character of the electronic structure of the films investigated.Comment: 10 pages, 12 figures, submitted to Physical Review