Electronic structure of few-electron concentric double quantum rings

The ground state structure of few-electron concentric double quantum rings is investigated within the local spin density approximation. Signatures of inter-ring coupling in the addition energy spectrum are identified and discussed. We show that the electronic configurations in these structures can be greatly modulated by the inter-ring distance: At short and long distances the low-lying electron states localize in the inner and outer rings, respectively, and the energy structure is essentially that of an isolated single quantum ring. However, at intermediate distances the electron states localized in the inner and the outer ring become quasidegenerate and a rather entangled, strongly-correlated system is formed.

Vertically coupled double quantum rings at zero magnetic field

Within local-spin-density functional theory, we have investigated the ¿dissociation¿ of few-electron circular vertical semiconductor double quantum ring artificial molecules at zero magnetic field as a function of interring distance. In a first step, the molecules are constituted by two identical quantum rings. When the rings are quantum mechanically strongly coupled, the electronic states are substantially delocalized, and the addition energy spectra of the artificial molecule resemble those of a single quantum ring in the few-electron limit. When the rings are quantum mechanically weakly coupled, the electronic states in the molecule are substantially localized in one ring or the other, although the rings can be electrostatically coupled. The effect of a slight mismatch introduced in the molecules from nominally identical quantum wells, or from changes in the inner radius of the constituent rings, induces localization by offsetting the energy levels in the quantum rings. This plays a crucial role in the appearance of the addition spectra as a function of coupling strength particularly in the weak coupling limit.

Optical response of two-dimensional few-electron concentric double quantum rings: A local-spin-density-functional theory study

We have investigated the dipole charge- and spin-density response of few-electron two-dimensional concentric nanorings as a function of the intensity of a erpendicularly applied magnetic field. We show that the dipole response displays signatures associated with the localization of electron states in the inner and outer ring favored by the perpendicularly applied magnetic field. Electron localization produces a more fragmented spectrum due to the appearance of additional edge excitations in the inner and outer ring.

Beam shaping of sectoral electromagnetic horn antennas using corner reflector technique

In the present thesis, possibility of beam shaping of sectoral horns and corner reflector systems'has been studied in detail. The experimental results obtained in the above two cases are compared. As far as the flanged sectoral horns are concerned, the special advantage is that the gain is increased without impairing impedance conditions. An intense study on corner reflector antennas shows that the been broadening or focussing will be possible by adjusting parameters involved. Beam tilting by imposing asymmetries is another interesting property of the systems. A comprehensive study of these fields has been presented in Chapter II. Chapter III is exclusively for describing the experimental techniques used in the present investigation. In Chapter IV, experimental results on flanged sectoral horns and corner reflector eyetses are presented. A comparative analysis of the experimental results obtained with flanged sectoral horns and corner reflector systems is presented in the Chapter V. The similarity and close resemblance in each aspects are shown by presenting typical results from these two eysteee. Theoretical aspects of both types of antennas are considered in Chapter VI. Attempts are made for co-ordinating the theoretical aspects and drawing a final conclusion. In Chapter VII. the final conclusion that the flanged sectoral horn may be considered as a corner reflector system has been drawn. The importance of the conclusions and usefulness are pointed out. The scope for further work in these lines has been indicated.

Corrugated flange technique for beam shaping of sectoral electromagnetic horn antennas

The need for improved feed systems for large reflector antennas employed in Radio Astronomy and Satellite tracking spurred the interest in horn antenna research in the 1960's. The major requirements were to reduce spill over, cross-polarisation losses,and to enhance the aperture efficiency to the order of about 75-8O%L The search for such a feed culminated in the corrugated horn. The corrugat1e 1 horn triggered widespread interest and enthusiasm, and a large amount of work(32’34’49’5O’52’53’58’65’75’79)has already been done on this type of antennas. The properties of corrugated surfaces has been investigated in detail. It was strongly felt that the flange technique and the use of corrugated surfaces could be merged together to obtain the advantages of both. This is the idea behind the present work. Corrugations are made on the surface of flange elements. The effect of various corrugation parameters are studied. By varying the flange parameters, a good amount of data is collected and analysed to ascertain the effects of corrugated flanges. The measurements are repeated at various frequencies, in the X— and S-bands. The following parameters of the system were studied: (a) beam shaping (b) gain (c) variation of V.S.U.R. (d) possibility of obtaining circularly polarised radiation from the flanged horn. A theoretical explanation to the effects of corrugated flanges is attempted on the basis of the line-source theory. Even though this theory utilises a simplified model for the calculation of radiation patterns, fairly good agreement between the computed pattern and experimental results are observed.

Formation of elliptically polarized beam from h-plane sectoral horns using corrugated flanges

The flange technique, suggested by Reynolds72 is simple technique to improve antenna characteristics. Using flange technique we can trim the antenna characteristic by suitably adjusting the flange parameters75. Later corrugated flanges87 are used for beam shaping. The important parameters of the corrugated flanges are (a) flange angle, (b) flange width, (c) flange position, (d) conductivity of the flange, (e) amplitude excitation of the flange elements, (f) period of corrugation etc. Compared to a compound horn the flange technique offers great convenience in trimming antenna characteristics. Horns are commonly used as a feed in radar and satellite communications. A large number of work had been done to improve the characteristics of horn antennas. It is an established fact that grooved walls on the inner surface of a horn can improve the antenna characteristics44. Corrugated comb surface can be used for the circular polarization98, tilt of polarization99 etc. This suggests the possibility to combine these two phenomena and to obtain a resultant beam. This thesis presents the result of an investigation to study the possibility of controlling different antenna characteristics like polarization, beam shaping, matching etc, using corrugated flange techniques.

Coherent patterns and self-induced diffraction of electrons on a thin nonlinear layer

Electron scattering on a thin layer where the potential depends self-consistently on the wave function has been studied. When the amplitude of the incident wave exceeds a certain threshold, a soliton-shaped brightening (darkening) appears on the layer causing diffraction of the wave. Thus the spontaneously formed transverse pattern can be viewed as a self-induced nonlinear quantum screen. Attractive or repulsive nonlinearities result in different phase shifts of the wave function on the screen, which give rise to quite different diffraction patterns. Among others, the nonlinearity can cause self-focusing of the incident wave into a beam, splitting in two "beams," single or double traces with suppressed reflection or transmission, etc.

Pauli distorted double folded potential

The enhancement in the production of even-Z nuclei observed in nuclear fission has also been observed in fragments produced from heavy ion collsions. Beams of 40Ar, 40Cl, and 40Ca at 25 MeV/nucleon were impinged on 58Fe and 58Ni targets. The resulting fragments were detected using the MSU 4pi detector array, which had additional silicon detectors for better isotopic resolution. Comparison of the ratios of yields for each element showed enhancement of even-Z fragment production. The enhancement was more pronounced for reactions with a greater difference in the N/Z of the compound system. However, this effect was less for systems that were more neutron rich. The average N/Z for fragments also displayed an odd-even effect with a lower average N/Z for the even-Z fragments. This is related to the greater availability of neutron-poor isotopes for even-Z nuclei

Performance analysis of double digit decimal multiplier on various FPGA logic families

Decimal multiplication is an integral part of financial, commercial, and internet-based computations. This paper presents a novel double digit decimal multiplication (DDDM) technique that performs 2 digit multiplications simultaneously in one clock cycle. This design offers low latency and high throughput. When multiplying two n-digit operands to produce a 2n-digit product, the design has a latency of (n / 2) 1 cycles. The paper presents area and delay comparisons for 7-digit, 16-digit, 34-digit double digit decimal multipliers on different families of Xilinx, Altera, Actel and Quick Logic FPGAs. The multipliers presented can be extended to support decimal floating-point multiplication for IEEE P754 standard

High performance, low latency double digit decimal multiplier on ASIC and FPGA

Decimal multiplication is an integral part of financial, commercial, and internet-based computations. This paper presents a novel double digit decimal multiplication (DDDM) technique that offers low latency and high throughput. This design performs two digit multiplications simultaneously in one clock cycle. Double digit fixed point decimal multipliers for 7digit, 16 digit and 34 digit are simulated using Leonardo Spectrum from Mentor Graphics Corporation using ASIC Library. The paper also presents area and delay comparisons for these fixed point multipliers on Xilinx, Altera, Actel and Quick logic FPGAs. This multiplier design can be extended to support decimal floating point multiplication for IEEE 754- 2008 standard.

Flexure-shear Analysis of Concrete Beam Reinforced with GFRP bars

This paper gives the details of flexure-shear analysis of concrete beams reinforced with GFRP rebars. The influence of vertical reinforcement ratio, longitudinal reinforcement ratio and compressive strength of concrete on shear strength of GFRP reinforced concrete beam is studied. The critical value of shear span to depth ratio (a/d) at which the mode of failure changes from flexure to shear is studied. The fail-ure load of the beam is predicted for various values of a/d ratio. The prediction show that the longitudinally FRP reinforced concrete beams having no stirrups fail in shear for a/d ratio less than 9.0. It is expected that the predicted data is useful for structural engineers to design the FRP reinforced concrete members.

Formation of carbon-encapsulated metallic nano-particles from metal acetylides by electron beam irradiation

Transition metal acetylides, MC2 (M=Fe, Co and Ni), exhibit ferromagnetic behavior of which TC is characteristic of their size and structure. CoC2 synthesized in anhydrous condition exhibited cubic structure with disordered C2− 2 orientation. Once being exposed to water (or air), the particles behave ferromagnetically due to the lengthening of the Co–Co distance by the coordination of water molecules to Co2+ cations. Heating of these particles induces segregation of metallic cores with carbon mantles. Electron beam or 193 nm laser beam can produce nanoparticles with metallic cores covered with carbon mantles

Piezoresistive cantilevers with an integrated bimorph actuator

Scanning Probe Microscopy (SPM) has become of fundamental importance for research in area of micro and nano-technology. The continuous progress in these fields requires ultra sensitive measurements at high speed. The imaging speed limitation of the conventional Tapping Mode SPM is due to the actuation time constant of piezotube feedback loop that keeps the tapping amplitude constant. In order to avoid this limit a deflection sensor and an actuator have to be integrated into the cantilever. In this work has been demonstrated the possibility of realisation of piezoresistive cantilever with an embedded actuator. Piezoresistive detection provides a good alternative to the usual optical laser beam deflection technique. In frames of this thesis has been investigated and modelled the piezoresistive effect in bulk silicon (3D case) for both n- and p-type silicon. Moving towards ultra-sensitive measurements it is necessary to realize ultra-thin piezoresistors, which are well localized to the surface, where the stress magnitude is maximal. New physical effects such as quantum confinement which arise due to the scaling of the piezoresistor thickness was taken into account in order to model the piezoresistive effect and its modification in case of ultra-thin piezoresistor (2D case). The two-dimension character of the electron gas in n-type piezoresistors lead up to decreasing of the piezoresistive coefficients with increasing the degree of electron localisation. Moreover for p-type piezoresistors the predicted values of the piezoresistive coefficients are higher in case of localised holes. Additionally, to the integration of the piezoresistive sensor, actuator integrated into the cantilever is considered as fundamental for realisation of fast SPM imaging. Actuation of the beam is achieved thermally by relying on differences in the coefficients of thermal expansion between aluminum and silicon. In addition the aluminum layer forms the heating micro-resistor, which is able to accept heating impulses with frequency up to one megahertz. Such direct oscillating thermally driven bimorph actuator was studied also with respect to the bimorph actuator efficiency. Higher eigenmodes of the cantilever are used in order to increase the operating frequencies. As a result the scanning speed has been increased due to the decreasing of the actuation time constant. The fundamental limits to force sensitivity that are imposed by piezoresistive deflection sensing technique have been discussed. For imaging in ambient conditions the force sensitivity is limited by the thermo-mechanical cantilever noise. Additional noise sources, connected with the piezoresistive detection are negligible.

Elektrochemo- und mikromechanisches Verhalten elektronisch leitfähiger Polymere

Intrinsisch leitfähige Polymere sind durch eine Reihe materialspezifischer Eigenschaften gekennzeichnet. In Abhängigkeit des angelegten Potenzials und der chemischen Umgebung zeigen sie elektrochromes Verhalten, Veränderungen der Masse, des Volumens und der elektronischen Leitfähigkeit. Basierend auf diesen Eigenschaften eignen sich halbleitende organische Polymere als funktionales Material für Anwendungen in der Mikro- und Nanotechnologie, insbesondere für miniaturisierte chemische Sensoren und Aktoren. Im Gegensatz zu konventionellen Piezo-Aktoren operieren diese Aktoren z. B. bei Spannungen unterhalb 1 V. Diese Arbeit befasst sich mit den elektrochemomechanischen Eigenschaften der ausgewählten Polymere Polyanilin und Polypyrrol, d. h. mit den potenzialkontrollierten Veränderungen des Volumens, der Struktur und der mechanischen Eigenschaften. Bei diesem Prozess werden positive Ladungen innerhalb der Polymerphase generiert. Um die für den Ladungsausgleich benötigten Gegenionen bereitzustellen, werden alle Messungen in Anwesenheit eines wässrigen Elektrolyten durchgeführt. Der Ladungstransport und die Volumenänderungen werden mit den Methoden der zyklischen Voltammetrie, der elektrochemischen Quarzmikrowaage und der Rastersondenmikroskopie untersucht. Signifikante Ergebnisse können für dünne homogene Polymerschichten erhalten werden, wobei Schichtdicken oberhalb 150 nm aufgrund der insbesondere bei Polyanilin einsetzenden Bildung von Fadenstrukturen (Fibrillen) vermieden werden. Von besonderem Interesse im Rahmen dieser Arbeit ist die Kombination der funktionalen Polymere mit Strukturen auf Siliziumbasis, insbesondere mit mikrostrukturierten Cantilevern. Die zuvor erhaltenen Ergebnisse bilden die Grundlage für das Design und die Dimensionierung der Mikroaktoren. Diese bestehen aus Siliziumcantilevern, die eine Elektrodenschicht aus Gold oder Platin tragen. Auf der Elektrode wird mittels Elektrodeposition eine homogene Schicht Polymer mit Schichtdicken bis zu 150 nm aufgebracht. Die Aktorcharakteristik, die Biegung des Cantilevers aufgrund des angelegten Potenzials, wird mit dem aus der Rastersondenmikroskopie bekannten Lichtzeigerverfahren gemessen. Das Aktorsystem wird hinsichtlich des angelegten Potenzials, des Elektrolyten und der Redox-Kinetik charakterisiert. Die verschiedenen Beiträge zum Aktorverhalten werden in situ während des Schichtwachstums untersucht. Das beobachtete Verhalten kann als Superposition verschiedener Effekte beschrieben werden. Darunter sind die Elektrodenaufladung (Elektrokapillarität), die Veränderungen der Elektrodenoberfläche durch dünne Oxidschichten und die Elektrochemomechanik des Polymers.