• Issue front cover thumbnail

      Volume 74, Issue 3

      March 2010,   pages  331-480 and L481-L489

    • Energy fluctuations in a biharmonically driven nonlinear system

      Navinder Singh Sourabh Lahiri A M Jayannavar

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      We study the fluctuations of work done and dissipated heat of a Brownian particle in a symmetric double well system. The system is driven by two periodic input signals that rock the potential simultaneously. Confinement in one preferred well can be achieved by modulating the relative phase between the drives. We show that in the presence of pumping the stochastic resonance signal is enhanced when analysed in terms of the average work done on the system per cycle. This is in contrast with the case when pumping is achieved by applying an external static bias, which degrades resonance. We analyse the nature of work and heat fluctuations and show that the steady state fluctuation theorem holds in this system.

    • Exergetic efficiency optimization for an irreversible heat pump working on reversed Brayton cycle

      Yuehong Bi Lingen Chen Fengrui Sun

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      This paper deals with the performance analysis and optimization for irreversible heat pumps working on reversed Brayton cycle with constant-temperature heat reservoirs by taking exergetic efficiency as the optimization objective combining exergy concept with finite-time thermodynamics (FTT). Exergetic efficiency is defined as the ratio of rate of exergy output to rate of exergy input of the system. The irreversibilities considered in the system include heat resistance losses in the hot- and cold-side heat exchangers and non-isentropic losses in the compression and expansion processes. The analytical formulas of the heating load, coefficient of performance (COP) and exergetic efficiency for the heat pumps are derived. The results are compared with those obtained for the traditional heating load and coefficient of performance objectives. The influences of the pressure ratio of the compressor, the allocation of heat exchanger inventory, the temperature ratio of two reservoirs, the effectiveness of the hot- and cold-side heat exchangers and regenerator, the efficiencies of the compressor and expander, the ratio of hot-side heat reservoir temperature to ambient temperature, the total heat exchanger inventory, and the heat capacity rate of the working fluid on the exergetic efficiency of the heat pumps are analysed by numerical calculations. The results show that the exergetic efficiency optimization is an important and effective criterion for the evaluation of an irreversible heat pump working on reversed Brayton cycle.

    • Influence of pairing in double beta decay of48Ca

      Prianka Roy Shashi K Dhiman

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      Two-neutrino $\beta \beta$ decay matrix elements and half-life of 48Ca are calculated after including neutron–proton pairing correlations in projected Hartree–Fock–Bogoliubov (PHFB) formalism. The GT matrix elements in $2\nu \beta \beta$ decay are reduced due to broader smearing of Fermi surfaces. Half-life results for $2\nu \beta \beta$ decay of 48Ca with np pairing are better than without pairing.

    • Simultaneous catalytic regime of tritium and helium-3 in D–D fusion without external breeding

      M Mahdavi T Koohrokhi

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      A catalytic regime of tritium and helium-3 in deuterium–deuterium fusion, including ion–electron collisions, mechanical expansion, bremsstrahlung radiation, inverse Compton scattering losses and reacting particles energy effect has been investigated. In this paper a new fuel configuration, DT$_{x}$ 3He$_{y}$, is formed by adding 3He to DT fuel. According to our calculations this fuel (DT$_{x=0.0112}$ 3He$_{y=0.0399}$) has greater energy gain than the fuel (DT$_{x=0.0112}$) used by Eliezer et al [Eliezer et al, Nucl. Fusion 40, 195 (2000)] and also it does not require external tritium and helium-3 breeding. Furthermore, neutron yields in D–D and D–T reactions are reduced due to the reduced quantity of initial amount of deuterium and tritium.

    • An attempt to understand the observed plateau of the magnetization in tetranuclear iron(II) complexes of thiacalixarene macrocycles

      Ali Bayri H Gokhan Bakir Mustafa Sulu Mustafa Karakaplan

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      The next-nearest-neighbour (NNN) effect in tetranuclear iron(II) complexes of thiacalixarene macrocycles using the isotropic Heisenberg model has been investigated in order to understand its effects on the observed plateau of magnetization. Although NNN effect is generally very weak in these kinds of systems, it was calculated that its response is quite significant to the external perturbations in certain temperature regions. Using the isotropic Heisenberg exchange Hamiltonian, zero-field energy spectra have been calculated for this particular tetranuclear system. The average magnetic moments with and without next-nearest-neighbour interactions were also calculated. In order to verify the calculations, the results were compared with experimental data taken from the literature, whence, it is suggested that observed magnetic behaviour can be improved by taking into account the NNN effect.

    • Variation of plasma parameters in a modified mode of plasma production in a double plasma device

      A Phukan M K Mishra B K Saikia M Chakraborty

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      A modified mode of plasma production in a double plasma device is presented and plasma parameters are controlled in this configuration. Here plasma is produced by applying a discharge voltage between the hot filaments in the source (cathode) and the target magnetic cage (anode) of the device. In this configuration, the hot electron emitting filaments are present only in the source and the magnetic cage of this is kept at a negative bias such that due to the repulsion of the cage bias, the primary electrons can go to the grounded target and produce plasma there. The plasma parameters can be controlled by varying the voltages applied to the source magnetic cage and the separation grid of the device.

    • Dielectric relaxation in glassy Se75In$_{25−x}$Pb$_{x}$ alloys

      J Sharma S Kumar

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      In this paper we report the effect of Pb incorporation in the dielectric properties of a-Se75In25 glassy alloy. The temperature and frequency dependence of the dielectric constants and the dielectric losses in glassy Se75In$_{25−x}$Pb$_{x}$ ($x$ = 0, 5, 10 and 15) alloys in the frequency range (1 kHz–5 MHz) and temperature range (300–340 K) have been measured. A detailed analysis shows that the dielectric losses are dipolar in nature and can be understood in terms of hopping of charge carriers over a potential barrier as suggested by Elliott for the case of chalcogenide glasses. It has been found that both dielectric constant and the dielectric loss are highly dependent on frequency and temperature and also found to increase with increasing concentration of Pb in binary a-Se 75 In 25 glassy system. The results have been interpreted in terms of increase in the density of defect states by the incorporation of Pb as a metallic additive in the aforesaid glassy system.

    • Lattice location of dopant atoms: An 𝑁-body model calculation

      N K Deepak

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      The channelling and scattering yields of 1 MeV 𝛼-particles in the $\langle 1 0 0 \rangle$, $\langle 1 1 0 \rangle and $\langle 1 1 1 \rangle$ directions of silicon implanted with bismuth and ytterbium have been simulated using 𝑁-body model. The close encounter yield from dopant atoms in silicon is determined from the flux density, using the Bontemps and Fontenille method. All previous works reported in literature so far have been done with computer programmes using a statistical analytical expression or by a binary collision model or a continuum model. These results at the best gave only the transverse displacement of the lattice site from the concerned channelling direction. Here we applied the superior 𝑁-body model to study the yield from bismuth in silicon. The finding that bismuth atom occupies a position close to the silicon substitutional site is new. The transverse displacement of the suggested lattice site from the channelling direction is consistent with the experimental results. The above model is also applied to determine the location of ytterbium in silicon. The present values show good agreement with the experimental results.

    • Intensity-dependent change in polarization state of light in normal incidence on an isotropic nonlinear Kerr medium

      Hari Prakash Devendra K Singh

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      It is shown that all optical polarization states of light except plane and circular polarization states undergo an intensity-dependent change in normal incidence of light in an isotropic nonlinear Kerr medium. This effect should be detectable and we propose an experiment for detecting nonlinear susceptibility involved in that part of nonlinear polarization, which depends on the polarization state of light also.

    • Monitoring structural defects and crystallinity of carbon nanotubes in thin films

      S S Mahajan M D Bambole S P Gokhale A B Gaikwad

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      We report the influence of catalyst formulation and reaction temperature on the formation of carbon nanotube (CNT) thin films by the chemical vapour deposition (CVD) method. Thin films of CNTs were grown on Fe–Mo/Al2O3-coated silicon wafer by thermal decomposition of methane at different temperatures ranging from 800 to 1000°C. The electron microscopic investigations, SEM as well as HRTEM, of the as-grown CNT thin films revealed the growth of uniform multi-walled CNTs in abundance. The intensity ratio of D-band to G-band and FWHM of G-band through Raman measurements clearly indicated the dependency of structural defects and crystallinity of CNTs in thin films on the catalyst formulation and CVD growth temperature. The results suggest that thin films of multi-walled CNTs with negligible amount of defects in the nanotube structure and very high crystallinity can be obtained by thermal CVD process at $925^{\circ}$C.

    • Dosimetric measurements and Monte Carlo simulation for achieving uniform surface dose in pulsed electron beam irradiation facility

      V C Petwal J N Rao Jishnu Dwivedi V K Senecha K V Subbaiah

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      A prototype pulsed electron beam irradiation facility for radiation processing of food and medical products is being commissioned at our centre in Indore, India. Analysis of surface dose and uniformity for a pulsed beam facility is of crucial importance because it is influenced by various operating parameters such as beam current, pulse repetition rate (PRR), scanning current profile and frequency, scanning width and product conveying speed. A large number of experiments are required to determine the harmonized setting of these operating parameters for achieving uniform dose. Since there is no readily available tool to set these parameters, use of Monte Carlo methods and computational tools can prove to be the most viable and time saving technique to support the assessment of the dose distribution. In the present study, Monte Carlo code, MCNP, is used to simulate the transport of 10 MeV electron beam through various mediums coming into the beam path and generate an equivalent dose profile in a polystyrene phantom for stationary state. These results have been verified with experimentally measured dose profile, showing that results are in good agreement within 4%. The Monte Carlo simulation further has been used to optimize the overlapping between the successive pulses of a scan to achieve $\pm 5%$ dose uniformity along the scanning direction. A mathematical model, which uses the stationary state data, is developed to include the effect of conveyor speed. The algorithm of the model is discussed and the results are compared with the experimentally measured values, which show that the agreement is better than 15%. Finally, harmonized setting for operating parameters of the accelerator are derived to deliver uniform surface dose in the range of 1–13 kGy/pass.

    • Degree distribution of a new model for evolving networks

      Xuan Zhang Qinggui Zhao

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      We propose and study an evolving network model with both preferential and random attachments of new links, incorporating the addition of new nodes, new links, and the removal of links. We first show that the degree evolution of a node follows a nonhomogeneous Markov chain. Based on the concept of Markov chain, we provide the exact solution of the degree distribution of this model and show that the model can generate scale-free evolving network.

    • Observation of Meyer–Neldel rule in thermally activated Poole–Frenkel photoconduction in a-Se70Te$_{30−x}$Cd$_{x}$ thin films

      V S Kushwaha N Mehta A Kumar

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      In the present paper, we report the compensation effect on photoconductivity of thin films of Se70Te$_{30−x}$Cd$_{x}$ ($x = 2, 4, 6$) alloys for high field conduction. Steady state photoconductivity measurements have been made in thin films for different electric fields. The photoconduction was found to be ohmic at low fields and Poole–Frenkel type at high fields. Meyer–Neldel rule (MN rule) for the pre-exponential factor $(\sigma_{\text{ph}})_{0}$ and activation energy ($\Delta E_{\text{ph}}$) of photoconduction in thin films of Se70Te$_{30−x}$Cd$_{x}$ is observed in the present study.

    • Chameleon field and the late time acceleration of the Universe

      Narayan Banerjee Sudipta Das Koyel Ganguly

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      In the present work, it is shown that a chameleon scalar field having a non-minimal coupling with dark matter can give rise to a smooth transition from a decelerated to an accelerated phase of expansion for the Universe. It is surprising to note that the coupling with the chameleon scalar field hardly affects the evolution of the dark matter sector, which still redshifts as $a^{−3}$.

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