• Volume 49, Issue 5

      November 1997,   pages  455-561

    • Application of density-functional perturbation theory to calculate nonlinear polarizabilities of helium-like systems

      Arup Banerjee Manoj K Harbola

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      Density-based perturbation theory within the Hohenberg-Kohn (HK) formalism of density functional theory (DFT), developed recently by us, is employed to calculate hyperpolarizabilities of helium-like ions from their ground-state densities obtained from their respective Hylleraas wavefunctions. The only approximation made is that of the local density (LDA) for exchange and correlation. Use of densities — instead of wavefunctions — in density-based perturbation theory together with simple approximate energy functionals makes our calculations much simpler than those based on wavefunctions. They lead, however, to accurate results.

    • Polarization and dichroism in atomic Auger spectroscopy following absorption of circularly polarized light

      S Sen

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      Spin resolved Auger current is shown to be different for the absorption of left and right circularly polarized light. This current is produced in non-radiative spontaneous decay of photoexcited atoms or of excited atomic photoions. Circular dichroism in such polarized Auger electrons exists even in the absence of spin-orbit interaction. Physical and geometrical conditions, necessary for the occurrence of dichroic effects in atomic Auger spectroscopy, have been obtained. Calculations for Ba and Xe show that the effect is substantially large and can be readily observed.

    • Two Coulomb waves theory for direct excitation

      Sima Chakrabarti Durga P Dewangan

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      A theoretical model of Dewangan, in which the total scattering wave function is approximated by a distorted wave containing two Coulomb wave functions, is discussed and its relation with the Brauner-Briggs-Klar model for ionization is examined. An important feature of the theory is that it includes a second Born amplitude naturally and in addition, contains, albeit approximately, both real and imaginary parts of all higher order Born terms. The theory is applied to study the 1s→2s excitation of hydrogen by electrons in the energy range 54.4 to 400eV. The differential and integral cross sections predicted by the theory are compared with the results of other theories and experimental data at 54.4eV and a good agreement is found.

    • On the water-induced critical double point in a polymer solution: the role of isotopic substitution

      B M Jaffar Ali S Venkatachalam A Kumar

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      This paper examines the effect of substitution of water by heavy water in a polymer solution of polystyrene (molecular weight=13000) and acetone. A critical double point (CDP), at which the upper and the lower partially-miscible regions merge, occurs at nearly the same coordinates as for the system [polystyrene+acetone+water]. The shape of the critical line for [polystyrene+acetone+heavy water] is highly asymmetric. An explanation for the occurrence of the water-induced CDP in [polystyrene+acetone] is advanced in terms of the interplay between contact energy dissimilarity and free-volume disparity of the polymer and the solvent. The question of the possible existence of a one-phasehole in an hourglass phase diagram is addressed in [polystyrene+acetone+water]. Our data exclude such a possibility.

    • Self-consistent numerical study of pure and impurity doped three-band Hubbard model

      Prasenjit Sen Avinash Singh

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      The three-band Hubbard model — both pure and with static non-magnetic impurities — has been studied within a self-consistent numerical Hartree-Fock (HF) scheme. The system shows nesting properties only in the absence of direct O-O hopping. Spin excitations in the system are gapless with the existence of a Goldstone mode in the broken-symmetry state. The variation of spinwave velocity with Cu-site Coulomb repulsion shows a (1/(2Ud)+(1/Δ)) dependence in the strong-coupling limit. Each non-magnetic impurity in the system gives rise to two gap states for a particular spin and the local moment produced is robust even at finite concentration of mobile hole doping. The gapless Goldstone mode is preserved even in case of unequal concentration of impurities on the two sublattices.

    • Measurement of fast neutron induced fission cross section of thorium using Lexan plastic track detector

      R K Jain H S Virk J Rama Rao S K Bose

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      Fission-track registration characteristics of Lexan solid state nuclear track detectors have been used to measure the fast neutron induced fission cross section of232Th. The fast neutrons (≅14.2MeV) were produced with the help of an AN-400 model Van-de-Graaff accelerator at Banaras Hindu University laboratory using3H(2H,n)4He reaction and were used to irradiate the fissile target deposited on the plastic detector. The track densityT, registered on the plastic detector is related to the fission cross sectionσf, through the relationT=knσføt wheren is the number of fissile atoms per cm2 in the deposit, ø is the neutron flux,k is fission track registration efficiency andt is the time of irradiation. The fission cross sectionσf of232Th, relative to the well measured fission cross section of238U, was found to be 0.36±0.04 barn.

    • SCORPION: A system for coincidences between recoil and projectile ions at NSC, New Delhi

      M J Singh S K Goel R Shanker D O Kataria N Madhavan P Sugathan J J Das D K Avasthi A K Sinha

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      An on-line facility to measure coincidences between the recoil ions and the scattered projectiles (SCORPION) has been designed, fabricated and commissioned at Nuclear Science Centre (NSC), New Delhi. The facility consists of a four jaw slit assembly, a time of flight (TOF) spectrometer, a parallel plate electrostatic charge analyser and a one dimensional position sensitive parallel plate avalanche counter (PPAC). Details of the design and working principles of various components and the test results obtained for the Siq+-Ar collision system are presented to highlight the performance of the system. A multiple loss of up to four electrons has been observed for 60 MeV Si4+ ions colliding with argon atoms in a single collision condition. Spectra of recoil ions detected in coincidence with a particular charge state of the scattered projectile show a bell shaped distribution as a function of the recoil charge state (r) for the electron loss events. However, the yield of recoil ions drops asr increases for the direct ionization channel. Also for electron loss, the peak of the recoil ion distribution is seen to shift to a higher recoil charge state as the number of lost electrons from the projectile increases.

    • TDPAC measurements in textured hcp samples

      R Govindaraj K P Gopinathan

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      Time differential perturbed angular correlation measurements done on the 482 keV level of181Ta probe nuclei in well annealed reference foil samples ofα-ZrHf,α-TiHf andα-Hf indicate that all probe nuclei are defect free, substitutional and experience quadrupole frequencies characteristic of these matrices. But mostly the ratio of the intensities of the harmonic components of the quadrupole frequency is never in 3:2:1, which is understood to be due to the selective orientation of microcrystallites brought out by the pre thermo-mechanical treatments on the sample. This paper illustrates as to how texturing in general could cause ambiguities in the measurements of the hyperfine interaction parameters of probe nuclei particularly in hcp foil samples with or without the association of any defects and the ways to delineate the effect of texturing.

    • Phonon dispersion in aluminium arsenide and antimonide

      Prafulla K Jha Sujata Rath Sankar P Sanyal

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      The phonon dispersion curves for aluminium arsenide and antimonide have been investigated by using a deformation bond approximation model. The results obtained from this model are compared with the experimental values wherever it is available. Since there is no complete experimental phonon dispersion curves for AlAs, we could not compare our calculated results, but the results of AlSb have been compared with the inelastic neutron scattering measurements at 15 K. However, we compare the phonon frequencies of AlAs and AlSb at critical points of the Brillouin zone obtained by our calculations and Raman spectroscopy measurements. This model predicts the phonon modes satisfactorily in all the symmetry directions of the Brillouin zone (BZ). The spectrum has similar features as observed in other III–V compound semiconductors.

    • Slowness surfaces, ray velocity surfaces and phonon focussing in highTc superconductor crystals

      J Philip M S Kala

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      This paper reports the results of the study of anisotropy in elastic wave propagation in single crystal superconducting BSCCO. The inverse and group velocities of elastic waves propagating in different directions have been computed and the corresponding slowness and ray velocity surfaces plotted, taking elastic constant data from literature. In addition, the phenomenon of phonon focussing has been investigated in this material by computing the phonon enhancement factor along different directions in spherical polar coordinates. The abnormally high values in phonon enhancement factor exhibited in certain directions for the phonon modes are interpreted as due to caustics occurring in the geometrical acoustics approximation adopted in the computational analysis. The results in LSCO and YBCO are found to be similar to those in BSCCO.

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