• Issue front cover thumbnail

      Volume 71, Issue 1

      July 2008,   pages  1-192

    • Deformed special relativity with an invariant minimum speed and its cosmological implications

      Cláudio Nassif

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      The paper aims to introduce a new symmetry principle in the space-time geometry through the elimination of the classical idea of rest and by including a universal minimum speed limit in the subatomic world. Such a limit, unattainable by particles, represents the preferred reference frame associated with a universal background field that breaks Lorentz symmetry. Thus, the structure of space-time is extended due to the presence of a vacuum energy density, which leads to a negative pressure at cosmological scales. The tiny values of the cosmological constant and the vacuum energy density shall be successfully obtained, which are in good agreement with current observational results.

    • Bianchi Type-I cosmological mesonic stiff fluid models in Lyra's geometry

      S D Katore S V Thakare K S Adhao

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      Bianchi Type-I cosmological models in Lyra's geometry are obtained when the source of gravitational field is a perfect fluid coupled with massless mesonic scalar field. Some physical and kinematical properties of the models are also discussed.

    • Anomalous absorption in H2CN and CH2CN molecules

      S Chandra S V Shinde

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      Structures of H2CN and CH2CN molecules are similar to that of H2CO molecule. The H2CO has shown anomalous absorption for its transition $1_{11} - 1_{10}$ at 4.8 GHz in a number of cool molecular clouds. Though the molecules H2CN and CH2CN have been identified in TMC-1 and Sgr B2 through some transitions in ortho as well as in para species, here we have investigated the condition under which transitions $1_{11} - 1_{10}$ and $2_{12} - 2_{11}$ of these molecules may show anomalous absorption.

      For the present investigation, we have calculated energy levels and radiative transition probabilities. However, we have used scaled values for collisional rate coefficients. We found that relative values of collisional rate coefficients can produce the required anomalous absorption in $1_{11} - 1_{10}$ and $2_{12} - 2_{11}$ transitions in the molecules.

    • Bulk viscous cosmology in early Universe

      C P Singh

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      The effect of bulk viscosity on the early evolution of Universe for a spatially homogeneous and isotropic Robertson-Walker model is considered. Einstein's field equations are solved by using `gamma-law' equation of state $p = (\gamma - 1)\rho$, where the adiabatic parameter gamma $(\gamma)$ depends on the scale factor of the model. The `gamma' function is defined in such a way that it describes a unified solution of early evolution of the Universe for inflationary and radiation-dominated phases. The fluid has only bulk viscous term and the coefficient of bulk viscosity is taken to be proportional to some power function of the energy density. The complete general solutions have been given through three cases. For flat space, power-law as well as exponential solutions are found. The problem of how the introduction of viscosity affects the appearance of singularity, is briefly discussed in particular solutions. The deceleration parameter has a freedom to vary with the scale factor of the model, which describes the accelerating expansion of the Universe.

    • Travelling wave-like solutions of the Zakharov-Kuznetsov equation with variable coefficients

      Yan-Ze Peng E V Krishnan Hui Feng

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      Travelling wave-like solutions of the Zakharov-Kuznetsov equation with variable coefficients are studied using the solutions of Raccati equation. The solitary wave-like solution, the trigonometric periodic wave solution and the rational wave solution are obtained with a constraint between coefficients. The property of the solutions is numerically investigated. It is shown that the coefficients of the equation do not change the wave amplitude, but may change the wave velocity.

    • Bifurcation methods of dynamical systems for generalized Kadomtsov-Petviashvili-Benjamin-Bona-Mahony equation

      Aiyong Chen Zhongjun Ma

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      By applying the bifurcation theory of dynamical system to the generalized KP-BBM equation, the phase portraits of the travelling wave system are obtained. It can be shown that singular straight line in the travelling wave system is the reason why smooth periodic waves converge to periodic cusp waves. Under different parametric conditions, various sufficient conditions to guarantee the existence of the above solutions are given. Some exact explicit parametric representations of the above waves are obtained.

    • Full wave analysis of non-radiative dielectric waveguide modulator for the determination of electrical equivalent circuit

      N P Pathak A Basu S K Koul

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      This paper reports the determination of electrical equivalent circuit of ON/OFF modulator in non-radiative dielectric (NRD) guide configurations at Ka-band. Schottky barrier mixer diode is used to realize this modulator and its characteristics are determined experimentally using vector network analyzer. Full wave FEM simulator HFSS is used to determine an equivalent circuit for the mounted diode and modulator in ON and OFF states. This equivalent circuit is used to qualitatively explain the experimental characteristics of modulator.

    • Dynamical symmetry breaking of lambda- and vee-type three-level systems on quantization of the field modes

      Mihir Ranjan Nath Surajit Sen Asoke Kumar Sen Gautam Gangopadhyay

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      We develop a scheme to construct the Hamiltonians of the lambda-, vee- and cascade-type three-level configurations using the generators of $SU(3)$ group. It turns out that this approach provides a well-defined selection rule to give different Hamiltonians for each configuration. The lambda- and vee-type configurations are exactly solved with different initial conditions while taking the two-mode classical and quantized fields. For the classical field, it is shown that the Rabi oscillation of the lambda model is similar to that of the vee model and the dynamics of the vee model can be recovered from lambda model and vice versa simply by inversion. We then proceed to solve the quantized version of both models by introducing a novel Euler matrix formalism. It is shown that this dynamical symmetry exhibited in the Rabi oscillation of two configurations for the semiclassical models is completely destroyed on quantization of the field modes. The symmetry can be restored within the quantized models when both field modes are in the coherent states with large average photon number which is depicted through the collapse and revival of the Rabi oscillations.

    • Analysis of small-signal intensity modulation of semiconductor lasers taking account of gain suppression

      Moustafa Ahmed Ali El-Lafi

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      This paper demonstrates theoretical characterization of intensity modulation of semiconductor lasers (SL’s). The study is based on a small-signal model to solve the laser rate equations taking into account suppression of optical gain. Analytical forms of the small-signal modulation response and modulation bandwidth are derived. Influences of the bias current, modulation index and modulation frequency as well as gain suppression on modulation characteristics are examined. Computer simulation of the model is applied to $1.55-\mu$m InGaAsP lasers. The results show that when the SL is biased far-above threshold, the increase of gain suppression increases both the modulation response and its peak frequency. The modulation bandwidth also increases but the laser damping rate decreases. Quantitative description of the relationships of both modulation bandwidth vs. relaxation frequency and maximum modulation bandwidth vs. nonlinear gain coefficient are presented.

    • Langmuir probe study in the nonresonant current drive regime of helicon discharge

      Manash Kumar Paul Dhiraj Bora

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      Characterization of the current drive regime is done for helicon wave-generated plasma in a torus, at a very high operating frequency. A radiofrequency-compensated Langmuir probe is designed and used for the measurement of plasma parameters along with the electron energy distributions in radial scans of the plasma. The electron energy distribution patterns obtained in the operational regime suggest that Landau damping cannot be responsible for the efficient helicon discharge in the present study. A typical peaked radial density profile, high plasma temperature and absence of an appreciable amount of energetic electrons for resonant wave–particle interactions, suggest that the chosen operational regime is suitable for the study of nonresonant current drive by helicon wave. Successful and significant current drive achieved in our device clearly demonstrates the capability of nonresonant current drive by helicon waves in the present operational regime.

    • Molecular conformation and structural correlations of liquid D-1-propanol through neutron diffraction

      A Sahoo S Sarkar P S R Krishna V Bhagat R N Joarder

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      An analysis of neutron diffraction data of liquid deuterated 1-propanol at room temperature to extract its molecular conformation is presented. Being a big molecule with twelve atomic sites, the analysis is tricky and needs careful consideration. The resulting molecular parameters are compared with electron diffraction (gas phase), X-ray diffraction (liquid phase) and MD simulation results. Information about the hydrogen-bonded intermolecular structure in liquid is extracted and nature of the probable molecular association suggested.

    • Effect of normal processes on thermal conductivity of germanium, silicon and diamond

      Banashree Saikai Anil Kumar

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      The effect of normal scattering processes is considered to redistribute the phonon momentum in (a) the same phonon branch – KK-S model and (b) between different phonon branches – KK-H model. Simplified thermal conductivity relations are used to estimate the thermal conductivity of germanium, silicon and diamond with natural isotopes and highly enriched isotopes. It is observed that the consideration of the normal scattering processes involving different phonon branches gives better results for the temperature dependence of the thermal conductivity of germanium, silicon and diamond with natural and highly enriched isotopes. Also, the estimation of the lattice thermal conductivity of germanium and silicon for these models with the consideration of quadratic form of frequency dependences of phonon wave vector leads to the conclusion that the splitting of longitudinal and transverse phonon modes, as suggested by Holland, is not an essential requirement to explain the entire temperature dependence of lattice thermal conductivity whereas KK-H model gives a better estimation of the thermal conductivity without the splitting of the acoustic phonon modes due to the dispersive nature of the phonon dispersion curves.

    • Onset of itinerant ferromagnetism associated with semiconductor-metal transition in Ti$_{x}$Nb$_{1-x}$CoSn half Heusler solid solution compounds

      M A Kouacou A A Koua J T Zoueu K Konan J Pierre

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      In this paper, the magnetic and transport properties of the Ti$_{x}$Nb$_{1−x}$CoSn solid solution compounds with half Heusler cubic MgAgAs-type structure have been studied. This work shows the onset of ferromagnetism associated with a semiconductor to metal transition. The transition occurs directly from ferromagnetic metal to semiconducting state as it is the case in the TiCo$_{x}$Ni$_{1−x}$Sn series studied previously. A weak quantity of Ti in NbCoSn is sufficient to allow the appearance of ferromagnetic order and metallic state. The variations of the Curie temperature as a function of saturation and effective paramagnetic moments are related to the itinerant ferromagnetism model. A comparison is made with the TiCoSn$_{x}$Sb$_{1−x}$ series (also studied previously), where the transition from TiCoSn ferromagnetic metal to non-magnetic semiconductor TiCoSb occurs through an intermediate metallic Pauli-like state.

    • Investigations on the local structure and 𝑔 factors for the interstitial Ti3+ in TiO2

      L H Wei S Y Wu Z H Zhang X F Wang Y X Hu

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      The EPR 𝑔 factors $g_{i}$ $(i = x, y, z)$ for the interstitial Ti3+ in rutile are theoretically studied from the perturbation formulas of these parameters for a 3d1 ion in rhombically compressed octahedra. The ligand octahedron in the impurity center is found to be less compressed than that on the host interstitial site due to the Jahn–Teller effect. The local compression parameter $(\approx 0.026)$ and the rhombic distortion angle $\delta \phi'$ $(\approx 0.7^{\circ})$ around the impurity Ti3+ are smaller than the host values ($\approx 0.091$ and 3.5°). The theoretical 𝑔 factors based on the above local structural parameters are in good agreement with the experimental data. In addition, the 𝑔 factors for a tetragonal interstitial Ti3+ center are also reasonably interpreted.

    • Level density and shape changes in excited sd shell nuclei

      S Santosh Kumar

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      In the present calculation we have used the Monte Carlo method of generating collective spin and total energy of the nucleus for various configurations of the system with $N_{0}$ single particle states available for n number of particles. The different configurations (arrangements of occupied single particle states) leading to a particular energy 𝐸 and spin 𝐽 are then collected to get the density of states for the given energy 𝐸 and spin 𝐽. We find that if we use the cranked Nilsson model single particle states for the rotational frequency $\Omega$ = 0.0$\hbar$ω, 0.05$\hbar$ω and 0.1$\hbar$ω there is a shift in the maximum density of states $W_{\text{max}}$ with a tendency for the system to become more oblate or prolate depending on the shift in the maximum density of states as the angular momentum decreases or increases. The change in nuclear level density with collectivity, i.e. with the use of cranked Nilsson model single particle levels has been noticed.

    • Pressure-induced phase transition and stability of EuO and EuS with NaCl structure

      Atul Gour Sadhna Singh R K Singh M Singh

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      We have predicted the phase transition pressures and corresponding relative volume changes of EuO and EuS having NaCl-type structure under high pressure using three-body interaction potential (TBIP) approach. In addition, the conditions for relative stability in terms of modified Born criterion has been checked. Our calculated results of phase transitions, volume collapses and elastic behaviour of these compounds are found to be close to the experimental results. This shows that the inclusion of three-body interaction effects makes the present model suitable for high pressure studies.

    • Growth of CdS nanoparticles by chemical method and its characterization

      R Bhattacharya S Saha

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      In the present work a simple chemical reduction method is followed to grow CdS nanoparticles at room temperature. The grown sample is ultrasonicated in acetone. The dispersed sample is characterized using electron diffraction technique. Simultaneously optical absorption of this sample is studied in the range of 400–700 nm. The photoluminescence spectrum of the sample is also studied. Results show the formation of nanoparticles. Hence an increase in band gap compared to bulk CdS and the as-prepared CdS nanoparticles have surface sulphur vacancies.

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