A stepwise transmission/reflection multiline-based algorithm for broadband permittivity measurements of dielectric materials

Transmission/reflection (T/R) techniques for measuring dielectric material's complex permittivity are broadband but have usually problems when the length of the tested sample is a multiple of λ/2. In this paper, we apply a stepwise scheme to a multiline T/R measurement method that solve those ambiguity problems thus allowing wideband and accurate permittivity estimation

Electronic structure of one-dimensional copper oxide chains in LiCu2O2 from angle-resolved photoemission and optical spectroscopy

Angle-resolved photoemission (ARPES) and optical measurements were performed on single crystal samples of LiCu2O2, an antiferromagnetic S=1/2 spin-chain compound. The ARPES spectra show several dispersive branches associated with hybrid copper-oxygen states. The occurrence of the valence band maximum halfway between the center and the edge of the Brillouin zone, and the complex spectral line shapes are not reproduced by the existing calculations of the electronic structure. We suggest that they can be interpreted within a one-dimensional scenario of strongly correlated antiferromagnetic insulators. The combination of ARPES and optics allows us to estimate the magnitude of the charge-transfer gap (Delta=1.95 eV). Moreover, the temperature-dependent optical conductivity bears signatures of the three different magnetic phases of this material

Reference Force Field and CDW Amplitude of Mixed-Valence Halogen-Bridged Pt Complexes

The spectroscopic effects of electron-phonon coupling in mixed-valence chlorine-bridged Pt chains complexes are investigated through a parallel infrared and Raman study of three compounds with decreasing Pt-Pt distance along the chain. The e-ph interaction is analyzed in terms of the Herzberg-Teller coupling scheme. We take into account the quadratic term and define a precise reference state. The force field relevant to this state is constructed, whereas the electronic structure is analyzed in terms of a simple phenomenological model, singling out a trimeric unit along the chain. In this way we are able to account for all the available optical data of the three compounds, and to estimate the relevant microscopic parameters, such as the e-ph coupling constants and the CDW amplitude.Comment: 10 pages, compressed postscript, 6 Tables and 5 Figures also in a compressed ps.Z file. Revision is in the submission format only (postscript instead of tex

Numerically efficient methods for the solution of the electric field integral equation arising in electromagnetic scattering problems

In this thesis we present a new approach for formulating the electric field integral equations (EFIE) in the contest of MoM. This method doesn't use the potentials formalism, thus avoiding the use of the Green's functions and, consequently, completely circumventing the problems of the singular (or near-singular) integrals calculation and of the low–frequency breakdown. We introduced a physical–based classes of micro–domain basis functions (BF), namely the dipole moments (DMs). Those BFs have the physical meaning of representing the scattering from elementary spheres, which can be metallic or dielectric. The DM basis functions are associated with formulas for the radiated EM field, that are analytical and valid over a very large frequency band, comprising very low frequencies. Hence the DM formulation can be effectively deployed in a MoM–like scheme, bypassing the use of Green's functions, to solve the EFIE. The concept of micro–domain basis function is further exploited by introducing micro current distributions, which are associated with analytical formulas for the radiated field and can improve the accuracy of the method. Finally the micro BFs can be clustered into higher–order basis functions, thus reducing the number of unknowns needed to model the scatterer and enhancing the computational performance of the method. In questa tesi viene presentato un nuovo metodo per calcolare l'equazione integrale del campo elettrico (EFIE) nell'ambito del MoM. Questo metodo non usa il formalismo dei potenziali, evitando così di ricorrere alle funzioni di Green e, di conseguenza, aggirando i problemi della singolarità del kernel delle equazioni integrali e del cosidetto low-frequency breakdown. Viene prima introdotta una nuova classe di funzioni base (BF), quella dei dipoli elementari, che sono definiti su domini elettricamente molto piccoli. Tali funzioni base possono essere interpretate dal un punto di vista fisico come correnti equivalenti associate a sfere elementari, che possono essere metalliche o dielettriche. Alle funzioni base dipolari sono associati campi elettromagnetici espressi mediante formule analitiche e che sono valide su un range assai ampio di frequenze, a partire da quelle molto basse. Di conseguenza queste funzioni base possono essere impiegate efficacemente in una nuova formulazione del MoM, che non necessiti dell'uso delle funzioni di Green. L'idea delle micro-funzioni base è poi estesa ad altre distribuzioni locali di corrente, i cui campi elettromagnetici possono essere ancora calcolati analiticamente e che permettono di migliorare l'accuratezza del metodo. Infine, è mostrato come le micro-funzioni base possano essere raggruppate in funzioni base di ordine superiore, in modo da ridurre le incognite del problema MoM e migliorare l'efficienza numerica del procedimento

Tecniche ibride ed asintotiche per il calcolo dello scattering elettromagnetico da superfici di grandi dimensioni

Sviluppo di nuovi algoritmi efficienti per il calcolo dello scattering elettromagnetico da superfici piane di grandi dimensioni. In recent years, the topic of efficient and accurate solution of the electromagnetic scattering problems from electrically large bodies has drawn increasing attention. The Method of Moments(MoM)-based procedures are widely used for this task, but they place a heavy burden on the CPU time as well as memory requirements when electrically large structures are. Recently, the Characteristic Basis Function (CBF) technique and its analytical version, namely the High-Frequency Integral Equation method, have been introduced for an efficient solution of electromagnetic scattering problems from complex bodies. We present an approach based on a Singular Value Decomposition (SVD) procedure for constructing sets of universal basis functions derived from Physical Optics, for solving the electromagnetic scattering from faceted bodies . These basis functions can be constructed avoiding any MoM-type approach and can be used for any direction of propagation of the illuminating wave; moreover, their use leads to a coefficient matrix with relatively small dimensions. The method enables us to solve scattering problems in a computationally efficient and numerically rigorous manner, and yields good results both for 2D and 3D problems

A sectorial Fabry - Perot antenna for radar application

A Fabry - Perot (FP) antenna for radar systems application is proposed. The investigated design allows to fulfill all the radar applications requirements with a simple and robust layout and it is preferable to conventional linear arrays for a number of reasons. The required asymmetric far - field pattern has led to the design of a sectorial FP with a lateral shielded cavity. In order to avoid the need of multiple sources to achieve the requested antenna gain performance, a high reflective FSS (Frequency Selective Surface) has been employed as a partially reflective screen (PRS). A prototype of this antenna was realized and the comparison between measurements and simulations is reported