• Volume 52, Issue 1

      January 1999,   pages  1-120

    • Particle production, back reaction and singularity avoidance

      A Shaw D Biswas B Modak S Biswas

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      We consider particle production in Robertson-Walker spacetime as particle-antiparticle rotation. We thereby obtain a scale factor that guarantees particle production. We then study quantum field effects in spatially flat homogeneous and isotropic spacetime with energy density of created particles and one loop quantum correction as back reaction. In the numerical solution initial values are determined from particle production simulated scale factor and obtain the evolution of the universe both at early and late times having a bounce.

    • Theory of combined AM and high-harmonic FM mode locked laser

      Anand V Ramamurthi K P J Reddy

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      Closed form solutions for a simultaneously AM and high-harmonic FM mode locked laser system is presented. Analytical expressions for the pulsewidth and pulsewidth-bandwidth products are derived in terms of the system parameters. The analysis predicts production of 17 ps duration pulses in a Nd:YAG laser mode locked with AM and FM modulators driven at 80 MHz and 1.76 GHz for 1 W modulator input power. The predicted values of the pulsewidth-bandwidth product lie between the values corresponding to the pure AM and FM mode locking values.

    • Noise reduction in chaotic time series data

      A Bhowal T K Roy

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      With a prescription for an ‘effective noise’ in the given noisy chaotic time series and searching out ‘similar’ strings of data, we give a simple method of ‘cleaning’ the data by an iterative process converging with decreasing length of the string. This has been found efficient even for small amount of data.

    • Modified Kadomtsev-Petviashvili equation in cold collisionless plasma

      S G Bindu V C Kuriakose

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      Nonlinear electromagnetic wave propagation through cold collisionless plasma in (2+1) dimensions is studied using the nonlinear reductive perturbation method. It is shown that to the lowest order of perturbation, the system of equations can be reduced to modified Kadomtsev-Petviashvili equation.

    • An x-ray biplanar photodiode and the x-ray emission from magnetically confined laser produced plasma

      V N Rai M Shukla H C Pant

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      Development of a single and multichannel biplanar vacuum photodiode for x-ray detection is reported, which has been used to study the x-ray emission from laser produced plasma expanding across an externally applied magnetic field. Two to three times enhancement in x-ray emission has been observed which was found correlated with decrease in size of the x-ray emitting plasma plume (expansion velocity of plasma). Experimental observations were found in close agreement with the analytical model based on an increase in plasma density as a result of plasma confinement in magnetic field. Temporal evolution of x-ray emission indicates that recombination radiation seems to be playing an important role in x-ray enhancement.

    • Orientational distribution function in nematic liquid crystals by x-rays: Fourier method

      R Somashekar H Somashekarappa A P Divya D Revannasiddaiah M S Madhava

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      The existing methods for the determination of the orientational distribution functionf(β) in the nematic liquid crystals using X-rays have been reviewed. A simple Fourier method which givesf(β) in terms of the measured intensity is analysed. Using this distribution function, the accuracy with which the order parameters could be evaluated is discussed and the results show the elegance of the Fourier method used here.

    • Thermal effect and equation of state in solids

      Piyush Kuchhal Narsingh Dass

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      In the present paper, an attempt has been made to include the thermal effect into an isothermal equation of state using the Debye approximation representing volume as a function of pressure. The calculations are done in case of NaCl, CsCl, Mo, and W. The present calculations are in good agreement with the reported results.

    • Bondlength distortion of atomic substitutions in semiconductors

      D Sasireka E Palaniyandi K Iyakutti

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      The distortion in bondlength is calculated for a wide range of impurities, both isovalent and heterovalent, in all I-VII, II-VI and HI-V compound semiconductors and in elemental semiconductors, silicon and germanium. Universal parameter tight binding (UPTB) theory of Harrison combined with the spring constant model is used. The results agree well with earlier theoretical estimates and also with the available extended X-ray absorption fine structure (EXAFS) data. A systematic behaviour of the bondlength distortion of impurities in semiconductors with respect to (i) the difference in covalent energy of the host-impurity bond, (ii) the difference in electronegativity between the host and that of the impurity atom and (iii) the change in covalency of the bond when the impurity replaces the host is observed.

    • Plasmons and their damping in a doped semiconductor superlattice

      P Tripathi A C Sharma

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      The complex zeroes of dielectric response function of a doped GaAs superlattice are computed to study the frequencies and damping rates of oscillations in coupled electron-hole plasma. The real part of a complex zero describes the plasma frequency, whereas imaginary part of it yields the damping rate. Strong scattering of charge carriers from random impurity potentials in a doped GaAs superlattice gives rise to a large value of damping rate which causes over-damping of plasma oscillations of coupled electron-hole gas below qc, a critical value of wave vector component (q) along the plane of a layer of electrons (holes). The plasma oscillations which correspond to electrons gas enter into over-damped regime for the case of weak coupling between layers. Whereas, plasma oscillations which belong to hole gas go to over-damped regime of oscillations for both strong as well as weak coupling between layers. The damping rate shows strongq-dependence forq < qc, whereas it weakly depends onq forqqc. The damping rate exhibits a sudden change atq =qc, indicating a transition from non-diffusive regime (where collective excitation can be excited) to diffusive regime (over-damped oscillations).

    • Magnetic structure of (Fe0.97Cr0.03)2P

      Sudhish Kumar S K Paranjpe Bipin K Srivastava Anjali Krishnamurthy V C Sahni

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      Magnetic behaviour of di-metal iron phosphide with a small substitution of iron by chromium, (Fe0.97Cr0.03)2P, has been studied using SQUID magnetometry and powder neutron diffraction. It is paramagnetic at temperatures above ∼180 K with persisting short range ferromagnetic (FM) order. At lower temperatures three different regions of magnetic behaviour are identified. FM order evolves in the region 180 K-120 K but much more slowly and with much less magnetic moments than in Fe2P. In the region 120 K-50 K negative exchange interactions gain some importance leading to a loss of FM order. Below 50 K FM interactions again dominate. Pinning centres influence the behaviour at low temperature up to ∼100 K.

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