Vibrational anharmonicity of small gold and silver clusters using the VSCF method

We study the vibrational spectra of small neutral gold (Au2–Au10) and silver (Ag2–Au5) clusters using the vibrational self-consistent field method (VSCF) in order to account for anharmonicity. We report harmonic, VSCF, and correlation-corrected VSCF calculations obtained using a vibrational configuration interaction approach (VSCF/VCI). Our implementation of the method is based on an efficient calculation of the potential energy surfaces (PES), using periodic density functional theory (DFT) with a plane-wave pseudopotential basis. In some cases, we use an efficient technique (fast-VSCF) assisted by the Voter–Chen potential in order to get an efficient reduction of the number of pair-couplings between modes. This allows us to efficiently reduce the computing time of 2D-PES without degrading the accuracy. We found that anharmonicity of the gold clusters is very small with maximum rms deviations of about 1 cm−1, although for some particular modes anharmonicity reaches values slightly larger than 2 cm−1. Silver clusters show slightly larger anharmonicity. In both cases, large differences between calculated and experimental vibrational frequencies (when available) stem more likely from the quality of the electronic structure method used than from vibrational anharmonicity. We show that noble gas embedding often affects the vibrational properties of these clusters more than anharmonicity, and discuss our results in the context of experimental studies

The nature and role of the gold-krypton interactions in small neutral gold clusters

© 2015 American Chemical Society. We investigate the nature and role of krypton embedding in small neutral gold clusters. For some of these clusters, we observe a particular site-dependent character of the Kr binding that does not completely follow the criterion of binding at low-coordinated sites, widely accepted for interaction of a noble gas with closed-shell metal systems such as metal surfaces. We aim at understanding the effect of low dimensionality and open-shell electronic structure of the odd-numbered clusters on the noble gas-metal cluster interaction. First, we investigate the role of attractive and repulsive forces, and the frontier molecular orbitals. Second, we investigate the Au-Kr interaction in terms of reactivity and bonding character. We use a reactivity index derived from Fukui formalism, and criteria provided by the electron localization function (ELF), in order to classify the type of bonding. We carry out this study on the minimum energy structures of neutral gold clusters, as obtained using pseudo potential plane-wave density functional theory (DFT). A model is proposed that includes the effect of attractive electrostatic, van der Waals and repulsive forces, together with effects originating from orbital overlap. This satisfactorily explains minimum configurations of the noble gas-gold cluster systems, the site preference of the noble gas atoms, and changes in electronic properties

An alternative methodology to assess the quality of empirical potentials for small gold clusters

We present a methodology based on local comparisons of potential energy surfaces (PES) in order to assess the quality of empirical potentials. We compare five typical empirical potentials using a criterion that shows which of these potentials resembles better a PES obtained with a high-level electronic structure method. The methodology relies on a many-body expansion in terms of normal coordinates of both the empirical and high-level theory PES. Then we investigate in a systematical way, how the features of the reference high-level theory PES are reproduced by each empirical potential in the vicinity of a given minimum energy structure. We use plane-wave density functional theory (DFT) as a reference, in particular the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional and an ultrasoft Vanderbilt pseudo potential. This study is carried out on neutral gold clusters with up to five atoms

Analysis of a Waveguide-Fed Metasurface Antenna

The metasurface concept has emerged as an advantageous reconfigurable antenna architecture for beam forming and wavefront shaping, with applications that include satellite and terrestrial communications, radar, imaging, and wireless power transfer. The metasurface antenna consists of an array of metamaterial elements distributed over an electrically large structure, each subwavelength in dimension and with subwavelength separation between elements. In the antenna configuration we consider here, the metasurface is excited by the fields from an attached waveguide. Each metamaterial element can be modeled as a polarizable dipole that couples the waveguide mode to radiation modes. Distinct from the phased array and electronically scanned antenna (ESA) architectures, a dynamic metasurface antenna does not require active phase shifters and amplifiers, but rather achieves reconfigurability by shifting the resonance frequency of each individual metamaterial element. Here we derive the basic properties of a one-dimensional waveguide-fed metasurface antenna in the approximation that the metamaterial elements do not perturb the waveguide mode and are non-interacting. We derive analytical approximations for the array factors of the 1D antenna, including the effective polarizabilities needed for amplitude-only, phase-only, and binary constraints. Using full-wave numerical simulations, we confirm the analysis, modeling waveguides with slots or complementary metamaterial elements patterned into one of the surfaces.Comment: Original manuscript as submitted to Physical Review Applied (2017). 14 pages, 14 figure

Experimental Synthetic Aperture Radar with Dynamic Metasurfaces

We investigate the use of a dynamic metasurface as the transmitting antenna for a synthetic aperture radar (SAR) imaging system. The dynamic metasurface consists of a one-dimensional microstrip waveguide with complementary electric resonator (cELC) elements patterned into the upper conductor. Integrated into each of the cELCs are two diodes that can be used to shift each cELC resonance out of band with an applied voltage. The aperture is designed to operate at K band frequencies (17.5 to 20.3 GHz), with a bandwidth of 2.8 GHz. We experimentally demonstrate imaging with a fabricated metasurface aperture using existing SAR modalities, showing image quality comparable to traditional antennas. The agility of this aperture allows it to operate in spotlight and stripmap SAR modes, as well as in a third modality inspired by computational imaging strategies. We describe its operation in detail, demonstrate high-quality imaging in both 2D and 3D, and examine various trade-offs governing the integration of dynamic metasurfaces in future SAR imaging platforms

From bullets to ballots: peace agreement, transit to civil life and political career of the M-19 democratic alliance (1989-1998)

ilustraciones, gráficasLa problemática central, desde una perspectiva teórica, que se aborda en esta investigación es la del paso de organizaciones armadas a partidos políticos, fenómeno que tuvo un número importante de ejemplos a nivel internacional especialmente en el marco del final de la Guerra Fría. Para el caso colombiano esta perspectiva se analiza desde la experiencia de la Alianza Democrática M-19 (AD-M-19), partido político que se consolida luego de los acuerdos de paz de la guerrilla con el gobierno Barco en 1990 y que presentará una crisis que lo llevará a su desaparición en 1998. Esta historia pone entonces en evidencia el auge y crisis de una organización política en la particular década de los años noventa, época de cambios del sistema político y de partidos de Colombia. La gran pregunta que rodea entonces el desarrollo de este texto es el porqué del declive de una organización política de carácter alternativo al bipartidismo que tuvo tanto auge y apoyo popular en sus inicios. Tanto razones internas de la organización, como externas del contexto político del país, llevan a responder el cuestionamiento central de esta investigación. Se trata de un trabajo basado en una importante revisión de archivos y seguimiento de prensa, además de comportar un fuerte componente de historia oral a través de más de cincuenta entrevistas realizadas a protagonistas de esta historia. (Texto tomado de la fuente)The central problem, from a theoretical perspective, that is addressed in this research is the transition from armed organizations to political parties, a phenomenon that had a significant number of examples at the international level, especially in the framework of the end of the Cold War. For the Colombian case, this perspective is analyzed from the experience of the Democratic Alliance M-19 (AD-M-19), a political party that was consolidated after the peace agreements between the guerrillas and the Barco government in 1990 and that will present a crisis that will lead to its disappearance in 1998. This history then highlights the rise and crisis of a political organization in the particular decade of the 1990s, a time of changes in the political system in Colombia. The main question that surrounds the development of this research is the reason for the decline of a political organization of an alternative nature to bipartisanship that had such a boom and popular support in its beginnings. Both internal reasons of the organization, and external reasons of the political context of the country, lead to answer the central question of this research. It is a work based on an important review of archives and press monitoring, in addition to involving a strong component of oral history through more than fifty interviews with the protagonists of this story.DoctoradoDoctor en HistoriaTrabajo de historia analítico descriptivo basado en fuentes primarias con el apoyo de un marco teórico de la ciencia política

Aplicación del control difuso en un statcom

En este artículo se muestra la aplicación del Control Difuso en la operación de un STATCOM de seis pulsos conectado a un sistema de potencia. El sistema de control es puesto a prueba cuando el STATCOM es sometido a perturbaciones externas representadas por un fallo trifásico a tierra ocurrido en la carga y cambios en el valor del voltaje de referencia. Se comprobó el método propuesto mediante la simulación en el paquete PSCAD/EMTDC