A photonic scheme for the generation of dual linear chirp microwave waveform based on the external modulation technique and its airborne application

Due to high frequency and large time bandwidth product; photonic generation and processing of arbitrary microwave waveforms has been an interesting topic in recent time. Here, a relatively new photonic technique has been proposed for the generation of a dual linear chirp microwave waveform in Ku-band. In this method two single drive Mach–Zehnder Modulators are cascaded at minimum transmission point and in push–pull mode. Theoretical analysis and simulation are developed by giving a complete mathematical model. As the result of this methodology, a dual linear chirp microwave waveform in Ku-band with relatively large bandwidth is generated. Comparative analysis is done in the present cascading technique with dual parallel Mach–Zehnder Modulator (DPMZM) technique. Range-Doppler coupling of the radar system has been investigated with the help of an ambiguity function diagram of the generated waveform. Results have analyzed through MATLAB simulation and verified by experimental results.     

Single channel tunable wavelength demultiplexer using nonlinear one dimensional defect mode photonic crystal

Transmission characteristics of nonlinear one dimensional photonic crystal with a defect have been studied. GaAs/Si multilayer structure with a single defect has been simulated using transfer matrix method. In this study refractive indices of both layers have been taken to be dependent on intensity and wavelength simultaneously. It is found that central wavelength of defect mode change with intensity of wave. Average change in central wavelength of defect mode is 0.02 nm/(1 GW/cm²). This property can be exploited in the design of a single channel tunable wavelength division demultiplexer for optical communication.     

SDOWN: A Novel Algorithm and Comparative Performance Analysis of Underlying Infrastructure in Software Defined Heterogeneous Network

In this article, a comparative performance analysis of three (03) different algorithms operating in the control plane of the three (03) varied architectures such as Physically Distributed, Logically Distributed, and Physically Centralized Architecture has been done. The paper also elucidates the working and implementation of the three proposed architectures with suitable block diagram of system model. Besides, in these architecture models OpenFlow (OF) governed various MATLAB components have been designed such as Application Controller, Optical Transport Network Controller, SDOWN Controller, Ethernet switch-1 and 2, ROADM and Wi-Fi access point with suitable interfaces. Pseudo codes of the algorithms operating in above said MATLAB components are duly explained with flowcharts. Mathematical analysis of three different architecture in respect of latency is carried out, and results and discussions with suitable figures have also been represented. The results obtained show that out of three (03) aforesaid architectures the Physical Centralized Architecture has better performance upto 45 Km in terms of Q-factor, SNR, BER, Jitter and Latency.     

Investigation of structural and magnetic properties of Ni, NiFe and NiFe2O4 thin films

Soft magnetic thin films of Ni, NiFe and NiFe₂O₄ were prepared using reactive magnetron sputtering in various deposition conditions. Experimentally observed soft magnetic property was compared and correlated with nanocrystalline structure evolution. Ni and NiFe deposited films are textured with fcc(111) phase preferred orientation. Accordingly, grain size and lattice parameter were calculated from X-ray diffraction (111) peak line width and 2θ peak position. Addition of reactive gas oxygen in deposition process has substantial effect on crystalline structure of film. There is phase transition from the ordered NiFe (111) structure to the NiFe₂O₄ nanocrystalline phase. The resulting film has shown small X-ray diffraction intensity peak corresponding to (311) and (400) orientation, indicating small amount of existing NiFe₂O₄ phase. The mechanism has been discussed to be responsible for nanocrystallization and amorphization of NiFe₂O₄ films. Magnetic measurement (M-H) loop reveal soft magnetic nature of films with magnetic anisotropy. The coercivity (H_c) of films is in accordance with random anisotropy model, where H_c reduced with grain size. The structural transformation was supported by Fourier transforms infrared spectroscopy measurement. The films are highly smooth with surface roughness in the range of ∼0.53-0.93 nm. NiFe₂O₄ films have shown lowest surface roughness with highest electrical resistivity values. The structural, surface, magnetic and infrared spectroscopy results are observed and analyzed.     

Frustration-induced insulating chiral spin state in itinerant triangular-lattice magnets

We study the double-exchange model at half-filling with competing superexchange interactions on a triangular lattice, combining exact diagonalization and Monte?Carlo methods. We find that in between the expected itinerant ferromagnetic and 120° Yafet-Kittel phases a robust scalar-chiral, insulating spin state emerges. At finite temperatures the ferromagnet-scalar-chiral quantum critical point is characterized by anomalous bad-metal behavior in charge transport as observed in frustrated itinerant magnets R2Mo2O7.     

Singular effect of disorder on electronic transport in strongly coupled electron-phonon systems

We solve the disordered Holstein model in three dimensions considering the phonon variables to be classical. After mapping out the phases of the "clean" strong coupling problem, we focus on the effect of disorder at strong electron-phonon (EP) coupling. The presence of even weak disorder (i) enormously enhances the resistivity (rho) at T=0, simultaneously suppressing the density of states at the Fermi level, (ii) suppresses the temperature dependent increase of rho, and (iii) leads to a regime with drho/dT<0. We locate the origin of these anomalies in the disorder induced tendency towards polaron formation, and the associated suppression in effective carrier density and mobility. These results, explicitly at "metallic" density, are of direct relevance to disordered EP materials such as covalent semiconductors, the manganites, and to anomalous transport in the A-15 compounds.     

Modifications induced by gamma irradiation upon structural,optical and chemical properties of polyamide nylon-6,6 polymer

The effects of gamma rays were studied on the optical, structural and chemical properties of the PA-66 polymer samples. The polymer samples obtained from Goodfellow (Cambridge, UK) were irradiated with gamma rays at various doses ranging from 100 to 1250 kGy. The pristine and gamma rays irradiated samples were characterized by UV–visible (UV–VIS) spectroscopy, X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. UV–VIS shows a shift in absorption toward the visible region for irradiated samples and a decrease in band gap energy (E_g). The XRD analyses show an increase in the crystalline nature of the polymer at higher doses as a result of significant decrease in the peak width of XRD patterns. The FTIR spectra show decrease in intensity and shift of various bands with increase in gamma dose.     

Dynamical stability and phase transition of ZrC under pressure

A comprehensive first principles study of structural, elastic, electronic, and phonon properties of zirconium carbide (ZrC) is reported within the density functional theory scheme. The aim is to primarily focus on the vibrational properties of this transition metal carbide to understand the mechanism of phase transition. The ground state properties such as lattice constant, elastic constants, bulk modulus, shear modulus, electronic band structure, and phonon dispersion curves (PDC) of ZrC in rock-salt (RS) and high-pressure CsCl structures are determined. The pressure-dependent PDCs are also reported in NaCl phase. The phonon modes become softer and finally attain imaginary frequency with the increase of pressure. The lattice degree of freedom is used to explain the phase transition. Static calculations predict the RS to CsCl phase transition to occur at 308?GPa at 0?K. Dynamical calculations lower this pressure by about 40?GPa. The phonon density of states, electron–phonon interaction coefficient, and Eliashberg's function are also presented. The calculated electron–phonon coupling constant λ and superconducting transition temperature agree reasonably well with the available experimental data.     

A first principles lattice dynamics and Raman spectra of the ferroelastic rutile to CaCl2 phase transition in SnO2 at high pressure

A first principles calculation of the lattice dynamical properties of rutile SnO₂ has been performed using density functional perturbation theory at ambient and high‐pressure conditions to understand the pressure‐induced phase transition. The calculated zone centre phonon modes at ambient and high pressures have been compared with Raman scattering and infrared measurements. Full phonon dispersion curves and phonon densities of states and Raman intensities at high pressures are calculated and given for the first time in literature. The ferroelastic transition from the rutile to the CaCl₂‐type structure was confirmed. It is clearly illustrated that the first transition is associated with macroscopic shear instability which arises from the strong coupling between elastic constants and softening of Raman active B_1g mode. The observed pressure of phase transition in experimental measurements was reproduced more accurately than in previous calculations, and the difference between observed and calculated transition pressure is only of the order of 2%. The mode Grüneisen parameter is quantitatively as well as qualitatively different from the earlier reported values. Copyright © 2013 John Wiley & Sons, Ltd.     

Raman investigations of Zn1−xCoxO nanocrystals: role of starting precursors on vibrational properties

The Raman spectra of sol–gel derived Co‐doped ZnO nanoparticles (NPs) in the spectral range 100–1500 cm⁻¹ were investigated. In the sol–gel method, three different series of Co‐doped ZnO particles, i.e. Zn_1−xCo_xO (x = 0.05, 0.10, 0.15, and 0.20), were obtained using three different starting precursors, viz. cobalt chloride hexahydrate, cobalt acetate tetrahydrate, and cobalt nitrate hexahydrate, respectively. It has been observed that cobalt acetate is a better precursor in comparison to cobalt chloride and cobalt nitrate to obtain single‐phase Co‐doped ZnO NPs. As for cobalt acetate‐derived NPs, no hidden secondary phase of Co₃O₄ was observed for the lower (x = 0.05) Co concentration. The Fröhlich interaction associated with the longitudinal modes was found to be destroyed with increasing Co concentration due to structural disorder and defects induced by the dopant. In addition to ZnO and Co₃O₄ vibrational modes, a few additional modes near 550 and 715 cm⁻¹ were also observed in all cases, which could be attributed to the modes due to Co doping in ZnO. Copyright © 2011 John Wiley & Sons, Ltd.