• Volume 74, Issue 2

February 2010,   pages  169-330

• Harmonic oscillator in Snyder space: The classical case and the quantum case

The harmonic oscillator in Snyder space is investigated in its classical and quantum versions. The classical trajectory is obtained and the semiclassical quantization from the phase space trajectories is discussed. An effective cut-off to high frequencies is found. The quantum version is developed and an equivalent usual harmonic oscillator is obtained through an effective mass and an effective frequency introduced in the model. This modified parameters give us a modified energy spectrum also.

• Bidirectional communication using delay coupled chaotic directly modulated semiconductor lasers

Chaotic synchronization of two directly modulated semiconductor lasers with negative delayed optoelectronic feedback is investigated and this scheme is found to be useful for efficient bidirectional communication between the lasers. A symmetric bidirectional coupling is identified as a suitable method for isochronal synchronization of such lasers. The optimum values of coupling and feedback strength that can provide maximum quality of synchronization are identified. This method is successfully employed for encoding/decoding both analog and digital messages. The importance of a symmetric coupling is demonstrated by studying the variation of decoding efficiency with respect to asymmetric coupling.

• Complete synchronization in coupled type-I neurons

For a system of type-I neurons bidirectionally coupled through a nonlinear feedback mechanism, we discuss the issue of noise-induced complete synchronization (CS). For the inputs to the neurons, we point out that the rate of change of instantaneous frequency with the instantaneous phase of the stochastic inputs to each neuron matches exactly with that for the other in the event of CS of their outputs. Our observation can be exploited in practical situations to produce completely synchronized outputs in artificial devices. For excitatory–excitatory synaptic coupling, a functional dependence for the synchronization error on coupling and noise strengths is obtained. Finally, we report a noise-induced CS between nonidentical neurons coupled bidirectionally through random nonzero couplings in an all-to-all way in a large neuronal ensemble.

• The ($G'/G$)-expansion method for a discrete nonlinear Schrödinger equation

An improved algorithm is devised for using the ($G'/G$)-expansion method to solve nonlinear differential-difference equations. With the aid of symbolic computation, we choose a discrete nonlinear Schrödinger equation to illustrate the validity and advantages of the improved algorithm. As a result, hyperbolic function solutions, trigonometric function solutions and rational solutions with parameters are obtained, from which some special solutions including the known solitary wave solution are derived by setting the parameters as appropriate values. It is shown that the improved algorithm is effective and can be used for many other nonlinear differential-difference equations in mathematical physics.

• Fundamental optimal relation of a generalized irreversible Carnot heat pump with complex heat transfer law

The fundamental optimal relation between heating load and coefficient of performance (COP) of a generalized irreversible Carnot heat pump is derived based on a new generalized heat transfer law, which includes the generalized convective heat transfer law and generalized radiative heat transfer law, $q \varpropto (\Delta T^{n})^{m}$. The generalized irreversible Carnot heat pump model incorporates several internal and external irreversibilities, such as heat resistance, bypass heat leakage, friction, turbulence and other undesirable irreversibility factors. The added irreversibilities besides heat resistance are characterized by a constant parameter and a constant coefficient. The effects of heat transfer laws and various loss terms are analysed. The heating load vs. COP characteristic of a generalized irreversible Carnot heat pump is a parabolic-like curve, which is consistent with the experimental result of thermoelectric heat pump. The obtained results include those obtained in many literatures and indicated that the analysis results of the generalized irreversible Carnot heat pump were more suitable for engineering practice than those of the endoreversible Carnot heat pump.

• Search for invisibly decaying Higgs boson at Large Hadron Collider

In several scenarios of Beyond Standard Model physics, the invisible decay mode of the Higgs boson is an interesting possibility. The search strategy for an invisible Higgs boson at the Large Hadron Collider (LHC), using weak boson fusion process, has been studied in detail, by taking into account all possible backgrounds. Realistic simulations have been used in the context of CMS experiment to devise a set of event selection criteria which eventually enhances the signal contribution compared to the background processes in characteristic distributions. In cut-based analysis, multi-jet background is found to overwhelm the signal in the finally selected sample. With an integrated luminosity of 10 fb-1, an upper limit of 36% on the branching ratio can be obtained for Higgs boson with a mass of 120 GeV/c2 for LHC energy of 14 TeV. Since the analysis essentially depends on the background estimation, detailed studies have been done to determine the background rates from real data.

• Design studies of a high-current radiofrequency quadrupole for accelerator-driven systems programme

A 3 MeV, 30 mA radiofrequency quadrupole (RFQ) accelerator has been designed for the low-energy high-intensity proton accelerator (LEHIPA) project at BARC, India. The beam and cavity dynamics studies were performed using the computer codes LIDOS, TOUTATIS, SUPERFISH and CST microwave studio. We have followed the conventional design technique with slight modifications and compared that with the equipartitioned (EP) type of design. The sensitivity of the RFQ to the variation of input beam Twiss–Courant parameters and emittance has also been studied. In this article we discuss both design strategies and the details of the 3D cavity simulation studies.

• Structural stability and theoretical strength of Cu crystal under equal biaxial loading

Cu has been used extensively to replace Al as interconnects in ULSI and MEMS devices. However, because of the difference in the thermal expansion coefficients between the Cu film and the Si substrate, large biaxial stresses will be generated in the Cu film. Thus, the Cu film becomes unstable and even changes its morphologies which affects the device manufacturing yield and ultimate reliability. The structural stability and theoretical strength of Cu crystal under equal biaxial loading have been investigated by combining the MAEAM with Milstein-modified Born stability criteria. The results indicate that, under sufficient tension, there exists a stress-free BCC phase which is unstable and slips spontaneously to a stress-free metastable BCT phase by consuming internal energy. The stable region ranges from −15.131 GPa to 2.803 GPa in the theoretical strength or from −5.801% to 4.972% in the strain respectively.

• Variation in viscosity and ion conductivity of a polymer–salt complex exposed to gamma irradiation

We study changes in microstructure and resulting changes in the properties of PEO($1 − x$)–NH4 ClO4 (𝑥) samples where $x = 0.18$, when irradiated with gamma doses varying up to 50 kGy. Viscosities of aqueous solutions of the irradiated samples give an idea of the change in molecular weight and show correlation with ion conductivity. On the whole, there is a chain scission on irradiation, though there is evidence of some cross-linking at higher doses. The ion conductivity shows a strong increase for an irradiation of 35 kGy. DSC studies indicate a decrease in crystallinity with gamma dose.

• Temperature-dependent infrared reflectivity studies of multiferroic TbMnO3: Evidence for spin-phonon coupling

We have measured near normal incidence far-infrared (FIR) reflectivity spectra of a single crystal of TbMnO3 from 10 K to 300 K in the spectral range of 50 cm-1 –700 cm-1. Fifteen transverse optic (TO) and longitudinal optic (LO) modes are identified in the imaginary part of the dielectric function $\varepsilon_{2} (\omega)$ and energy loss function Im($−1/\varepsilon(\omega))$, respectively. Some of the observed phonon modes show anomalous softening below the magnetic transition temperature $T_{N}$ ($\sim 46$ K). We attribute this anomalous softening to the spin-phonon coupling caused by phonon modulation of the superexchange integral between the Mn3+ spins. The effective charge of oxygen ($Z_{O}$) calculated using the measured LO–TO splitting increases below $T_{N}$.

• Continuous quantum phase transitions in the one-dimensional spin-1/2 axial next-nearest-neighbour Ising model in two orthogonal magnetic fields

We have investigated the one-dimensional spin-1/2 axial next-nearest-neighbour Ising (ANNNI) model in two orthogonal magnetic fields at zero temperature. There are four different possible ground state configurations for the ANNNI model in a longitudinal field, in the thermodynamic limit. The inclusion of a transverse field introduces quantum fluctuations which destroy the existing spin order along certain critical lines. The effects of the fluctuations in three of the four ordered regions were investigated using the finite-size scaling technique. The phase boundaries of the ANNNI model in two orthogonal magnetic fields were thus determined numerically. For certain limits of the Hamiltonian we compared the obtained results with the existing literature and our results were in good agreement with the results in the existing literature.

• Comparative performance of some popular artificial neural network algorithms on benchmark and function approximation problems

We report an inter-comparison of some popular algorithms within the artificial neural network domain (viz., local search algorithms, global search algorithms, higher-order algorithms and the hybrid algorithms) by applying them to the standard benchmarking problems like the IRIS data, XOR/N-bit parity and two-spiral problems. Apart from giving a brief description of these algorithms, the results obtained for the above benchmark problems are presented in the paper. The results suggest that while Levenberg–Marquardt algorithm yields the lowest RMS error for the N-bit parity and the two-spiral problems, higher-order neuron algorithm gives the best results for the IRIS data problem. The best results for the XOR problem are obtained with the neuro-fuzzy algo- rithm. The above algorithms were also applied for solving several regression problems such as $\cos(x)$ and a few special functions like the gamma function, the complimentary error function and the upper tail cumulative $\chi^{2}$-distribution function. The results of these regression problems indicate that, among all the ANN algorithms used in the present study, Levenberg–Marquardt algorithm yields the best results. Keeping in view the highly non-linear behaviour and the wide dynamic range of these functions, it is suggested that these functions can also be considered as standard benchmark problems for function approximation using artificial neural networks.

• Experimental analysis of current conduction through thermally grown SiO2 on thick epitaxial 4H-SiC employing Poole–Frenkel mechanism

Electrical properties of SiO2 grown on the Si-face of the epitaxial 4H-SiC substrate by wet thermal oxidation technique have been experimentally investigated in metal oxide–silicon carbide (MOSiC) structure with varying oxide thicknesses employing Poole–Frenkel (P–F) conduction mechanism. The quality of SiO2 with increasing thickness in MOSiC structure has been analysed on the basis of variation in multiple oxide traps due to effective P–F conduction range. Validity of Poole–Frenkel conduction is established quantitatively employing electric field and the oxide thickness using forward $I–V$ characteristics across MOSiC structures. From P–F conduction plot (ln($J/E$) vs. $E^{1/2}$), it is revealed that Poole–Frenkel conduction retains its validation after a fixed electric field range. The experimental methodology adopted is useful for the characterization of oxide films grown on 4H-SiC substrate.

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