• Volume 60, Issue 3

      March 2003,   pages  415-573

    • Maximum stellar iron core mass

      F W Giacobbe

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      An analytical method of estimating the mass of a stellar iron core, just prior to core collapse, is described in this paper. The method employed depends, in part, upon an estimate of the true relativistic mass increase experienced by electrons within a highly compressed iron core, just prior to core collapse, and is significantly different from a more typical Chandrasekhar mass limit approach. This technique produced a maximum stellar iron core mass value of 269 × 1030 kg (1.35 solar masses). This mass value is very near to the typical mass values found for neutron stars in a recent survey of actual neutron star masses. Although slightly lower and higher neutron star masses may also be found, lower mass neutron stars are believed to be formed as a result of enhanced iron core compression due to the weight of non-ferrous matter overlying the iron cores within large stars. And, higher mass neutron stars are likely to be formed as a result of fallback or accretion of additional matter after an initial collapse event involving an iron core having a mass no greater than 2.69 × 1030 kg

    • Gravitational collapse in higher-dimensional charged-Vaidya space-time

      Kishor D Patil

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      We analyze here the gravitational collapse of higher-dimensional charged-Vaidya spacetime. We show that singularities arising in a charged null fluid in higher dimension are always naked violating at least strong cosmic censorship hypothesis (CCH), though not necessarily weak CCH. We show that earlier conclusions on the occurrence of naked singularities in four-dimensional case can be extended essentially in the same manner in 5D case also

    • Bianchi type I string cosmologies

      D N Pant Sanjay Oli

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      By making use of Letelier’s form of energy—momentum tensor for a cloud of stringdust we present some classes of solutions of general relativistic field equations which describe cosmological string-dust models in Bianchi type I space-time. Some of the classes of models obey Takabayashi’s equation of state whereas a class of models exhibits inflation in the initial stage. Two of the classes presented here have Kasner’s space-time as past asymptote

    • Cosmic ray air showers in the knee energy region

      S Sanyal

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      The cosmic ray extensive air showers in the knee energy region have been studied by the North Bengal University array. The differential size spectra at different atmospheric depths show a systematic shift of the knee towards smaller shower size with the increase in atmospheric depth. The measured values of spectral indices at below and above the knee are −2.45 ±0.03 and −2.91 ±0.05 respectively. Measurements at different atmospheric depths correspond to the same values within the error limits both for below and above the knee. The present experimental results have been compared with similar such experiments

    • Higher-dimensional string theory in Lyra geometry

      F Rahaman S Chakraborty S Das M Hossain J Bera

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      In this paper, a study on string theory has been done in five-dimensional space-time based on Lyra geometry. Also a polynomial relation between the two scale factors is assumed. The equations of state for strings have been used for different solutions

    • Stability of extended scalar diquark stars vis-à-vis soliton stars

      R S Kaushal

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      Motivated by relatively large values of the maximum mass (M = 8.92M⊙) and radius (R = 507 km) obtained for an extended scalar diquark star within the framework of an effective φ4-theory (S K Karnet al [1]) some interesting observations are made with regard to the stability of stellar objects describable in general in terms of the polynomial field theories

    • Neutron spallation source and the Dubna Cascade Code

      V Kumar H Kumawat Uttam Goel V S Barashenkov

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      Neutron multiplicity per incident proton,n/p, in collision of high energy proton beam with voluminous Pb and W targets has been estimated from the Dubna Cascade Code and compared with the available experimental data for the purpose of benchmarking of the code. Contributions of various atomic and nuclear processes for heat production and isotopic yield of secondary nuclei are also estimated to assess the heat and radioactivity conditions of the targets. Results obtained from the code show excellent agreement with the experimental data at beam energy,E < 12 GeV and differ maximum up to 25% at higher energy

    • Momentum densities and Compton profiles of alkali-metal atoms

      Pranab Sarkar Anupam Sarkar S N Roy B Talukdar

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      It is assumed that the dynamics of valence electrons of alkali-metal atoms can be well accounted for by a quantum-defect theoretic model while the core electrons may be supposed to move in a self-consistent field. This model is used to study the momentum properties of atoms from3Li to37Rb. The numerical results obtained for the momentum density, moments of momentum density and Compton profile are found to be in good agreement with the results of more detailed configuration-interaction calculations for the atom3Li. Similar results for11Na,19K and37Rb are compared with the corresponding Hartree-Fock-Roothaan values only, for want of data from other realistic calculations

    • Thermal condensation mode in a dusty plasma

      B P Pandey J Vranješ S Parhi

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      In the present work, the radiative condensation instability is investigated in the presence of dust charge fluctuations. We find that the charge variability of the grain reduces the growth rate of radiative mode only for fluctuation wavelength smaller or of the order of the Debye length and this reduction is not very large. Far from the Debye sphere, radiative mode can damp due to thermal conduction of electrons and ions

    • Correlation of thickness and magnetization in LCMO film

      Sanghamitra Khatua P K Mishra J John V C Sahni

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      High quality thin films of La0.67Ca0.33MnO3 (LCMO) of different thickness were grown on LAO substrates by pulsed laser deposition (KrF, λ = 278 nm). The AFM images suggest a twodimensional step-growth. DC magnetization measurements of the films in a field of 500 Oe show that the magnetic ordering temperature is the same for all the films in both FC and ZFC conditions and is the same as that for the bulk. However, a difference is seen between the FC and ZFC magnetization of the films. There seems to be a systematic in this difference with respect to the thickness of the film, with the difference decreasing with thickness. We suggest that the difference in the magnetization under FC and ZFC conditions may be due to strain-induced anisotropy arising from the lattice mismatch between the substrate and the film or due to the shape anisotropy due to epitaxial growth

    • Is gadolinium a helical antiferromagnet or a collinear ferromagnet?

      S N Kaul

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      Controversial issues concerning the nature of magnetic ordering in gadolinium are briefly reviewed. The recent experimental results are shown to resolve most of such issues in that they rule out the possibility of a helical spin structure in Gd and clearly bring out the role of longrange dipolar interactions in stabilising collinear ferromagnetic order for temperatures between the spin-reorientation temperature and the Curie point

    • Effect of site disorder on the magnetic properties of weak itinerant ferromagnet Ni75Al25

      Anita Semwal S N Kaul

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      Detailed study of Ni75Al25 samples with varying degree of site disorder reveals that site disorder promotes magnetic excitations such as spin waves and local spin-density fluctuations and thereby reduces both spin-wave stiffness and Curie temperature. Irreversibility lines in theT-H phase diagram of the weak itinerant ferromagnet Ni75Al25 have been determined for the first time and the effect of site disorder on them has been ascertained

    • Site occupancy of Fe in ternary Ni75−xFex+yAl25−yalloys

      B Annie D'santhoshini S N Kaul

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      The results of a detailed structural and magnetic study clearly indicate that regardless of the thermal history of the samples, Fe has a strong preference for the Ni sites in Ni-poor (nonstoichiometric) Ni75Al25 alloys. Fe substitution has a profound effect on the nature of magnetism in Ni25Al25

    • Experimental observation of quantum corrections to electrical resistivity in nanocrystalline soft magnetic alloys

      K Balakrishnan Y Sundarayya M K Naidu S N Kaul

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      X-ray diffraction patterns of nanocrystalline Fe-Cu-Nb-Si-B (FINEMET) alloys reveal that bcc α-Fe/α-FeSi crystallites with the average grain size of 20(5) nm are dispersed in amorphous matrix. Enhanced electron—electron interaction (EEI) and quantum interference (QI) effects as well as electron-magnon (and/or electron-spin fluctuation) scattering turn out to be the main mechanisms that govern the temperature dependence of resistivity. Of all the inelastic scattering processes, inelastic electron-phonon scattering is the most effective mechanism to destroy phase coherence of electron wave functions. The diffusion constant, density of states at the Fermi level and the inelastic scattering time have been estimated, for the first time, for the alloys in question

    • Microwave dielectric tangent losses in KDP and DKDP crystals

      Trilok Chandra Upadhyay Birendra Singh Semwal

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      By adding cubic and quartic phonon anharmonic interactions in the pseudospin lattice coupled mode (PLCM) model for KDP-type crystals and using double-time temperature dependent Green's function method, expressions for soft mode frequency, dielectric constant and dielectric tangent loss are obtained. Using model parameters given by Ganguliet al [9] the dielectric losses are calculated for KDP and DKDP crystals. In the microwave frequency range an increase in frequency (1–35 GHz) is followed by an increase in dielectric tangent loss (1–35) at 98 K and (1–15) × 10−2 at 333 K for KDP and DKDP crystals respectively. The dielectric tangent loss decreases from 0.052 to 0.042 for KDP crystals with increase in temperature from 130 to 170 K and for DKDP crystals it decreases from 0.0166 to 0.0074 with an increase in temperature from 230–343 K in their paraelectric phases at 10 GHz. This shows Curie-Weiss behavior of the dielectric tangent loss

    • Non-equilibrium and band tailing in organic conductors

      A T Oza P C Vinodkumar R G Patel

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      The concept of band tailing with focal point and width of the tail from IR absorption spectra of different organic conductors is found valid even for thermal and elastic changes. The experimental situations like change of solvents, method of preparation, applied pressure and pressure cycle apart from compositions is analyzed within the framework of tailing of states. Non-equilibrium due to coupling between applied energy and free electrons can be responsible for the exponential relaxation from non-equilibrium to equilibrium

    • Phonon density of states in nanocrystalline57Fe

      Ranber Singh S Prakash R Meyer P Entel

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      The Born-von Karman model is used to calculate phonon density of states (DOS) of nanocrystalline bcc Fe. It is found that there is an anisotropic stiffening in the interatomic force constants and hence there is shrinking in the nearest-neighbour distances in the nanophase. This leads to additional vibrational modes above the bulk phonons near the bottom of the phonon band. It is found that the high energy phonon modes of nanophase Fe are the surface modes. The calculated phonon DOS closely agree with the experimental data except a peak at 37 meV. The calculated phonon dispersion relations are also compared with those of the bulk phonons and anomalous behaviour is discussed in detail. The specific heat in nanophase enhances as compared to bulk phase at low temperatures and the calculated Debye temperature ΘD agrees with the experimental results. It is predicted that the nanocrystalline Fe may consist of about 14 GPa pressure

    • On unitarity relations and their application to meson—antimeson mixing

      G V Dass

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      In view of the recent observation of nonexponential decay features for a certain quantummechanical system, we revisit our earlier study of the small-time behaviour of the meson-antimeson complex wherein the commonly employed Weisskopf—Wigner approximation could be tested, in principle. We find that the experiments for this testing are difficult because of (i) the smallness of the times required for this study, (ii) the high accuracy required for probing the nonleading terms (which distinguish this approximation from the general theory) in the small-time behaviour of the relevant probabilities and (iii) the crude knowledge of the required ‘flavour-tagging’ procedures, as available at present

    • The exponent λ (x, Q2) of the proton structure functionF2(x, Q2) at lowx

      D K Choudhury P K Sahariah

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      The exponent λ of the structure function F2x−λ is calculated using the solution of the DGLAP equation for gluon at lowx reported recently by the present authors. The quantity λ is calculated both as a function ofx at fixedQ2 and as a function ofQ2 at fixedx and compared with the most recent data from H1

    • Compton profile of tantalum

      Thomas Varghese K M Balakrishna K Siddappa

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      The Compton profile of tantalum (Ta) has been measured using IGP type coaxial photon detector. The target atoms were excited by means of 59.54 keV γ-rays from Am-241. The measurements were carried out on a high purity thin elemental foil. The data were recoreded in a 4 K multichannel analyzer. These data duly corrected for various effects are presented and compared with theoretical and measured values. Best agreement with experiment is found for the 5d36s2 electron configuration

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