• Vijay Kumar

      Articles written in Pramana – Journal of Physics

    • Anisotropic ferromagnet with biquadratic exchange

      Vijay Kumar K C Sharma

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      A formulation, using the double time Green’s function method, is presented for the anisotropic spin-one ferromagnetic system in the presence of Ising part of the biquadratic exchange and an expression is given for the critical temperatureTc in terms of two parameters. One is associated with the uniaxial anisotropy (treated exactly) and the other one corresponds to the biquadratic interaction (treated in RPA). The bilinear Heisenberg exchange term is treated in Callen’s approximation emphasizing the role of intersite correlation for small parameters. The prediction thatTc increases with the biquadratic exchange is found to be consistent with the recent calculation based on high temperature series expansion.

    • Diffraction from a quasi-crystalline chain

      M C Valsakumar Vijay Kumar

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      We present a general formalism for diffraction from a one-dimensional quasicrystal with arbitrary length scales and sequences. The notion of sub-quasi-lattices is introduced and the effect of different basis on different sites is studied. The relevance of this work for the study of vibrational and electronic spectra of the chain is discussed.

    • Proton and α-particle impactM-shell ionization of atoms in binary encounter approximation

      Vijay Kumar S K Shrivastava B N Roy

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      M-shell ionization cross sections for atoms due to the impact of proton and α-particles have been calculated in the binary encounter approximation. The effects of Coulomb deflection of the incident projectile and increase in binding of the target electron have been investigated. Roothan-Hartree-Fock velocity distribution for the target electrons has been used in the present work. The calculated cross-sections have been compared with experimental results and other theoretical calculations wherever available. The present calculations give a good account of experimental observations.

    • Fluorescence studies of CS2 and SO2 at 121.6, 73.6–74.4 and 58.4 nm

      I A Prajapati S M Ahmed Vijay Kumar

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      The fluorescence spectra of CS2 and SO2 have been studied at three incident photon wavelengths of 121.6, 73.6–74.4 and 58.4 nm and relative production cross sections for different product states have been measured. The CS(A1Π→X1Σ+) system between 240 and 290nm has been obtained when CS2 is photoexcited at 121.6nm whereas CS2+(B2Σu+X2Πg) and CS2+(A2ΠuX2Πg) systems have been produced between 276 and 295 and 437 and 555nm respectively when excited by both the incident photon wavelengths of 73.6–74.4 and 58.4nm. The fluorescence spectra of SO2 obtained at 121.6 and 73.6–74.4nm include the vibrational bands of SO(A3Π→X3Σ) and SO(B2ΠX3Σ) systems from 240 to 268 and 268 to 442nm respectively whereas the emission spectrum at 58.4nm, has contributions from the two SO systems and SO+(A2Π→X2Π) system. In all these emission spectra, the fluorescence bands of different systems have been analyzed and their relative production cross sections have been measured. The results obtained in the present investigations have been compared with a few recent reliable measurements reported in literature.

    • A multifractal study of wave functions in 1-D quasicrystals

      G Ananthakrishna Vijay Kumar

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      Using multifractal analysis we study extended, self-similar and non-self-similar type of wave functions in the Fibonacci model. Extended states arising due to commutation of transfer matrices for certain blocks of atoms in quasiperiodic systems are shown to have the same signature as the Bloch states in terms of the singularity spectrum withf(α)=α=1. Numerically, however, the extended states show a typical multifractal behaviour for finite chain lengths. Finite size scaling corrections yield results consistent with that obtained analytically. The self-similar states at the band edges show a multifractal behaviour and they are energy dependent in the case of blocks of atoms arranged in a Fibonacci sequence. For non-self-similar states we obtain a non-monotonic behaviour off(α) as a function of the chain length. We also show that in cases where extended states exist, the cross-over from extended to non-self-similar states in gradual.

    • Measurement of photoabsorption and fluorescence cross-sections for CS2 at 188.2–213 and 287.5–339.5 nm

      S M Ahmed Vijay Kumar

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      The photoabsorption and fluorescence cross-sections for carbon disulphide have been measured in the 188.2–213 and 287.5–339.5 nm spectral regions using an argon mini-arc light source. The absorption cross-sections have been measured with an accuracy of ±4.2% whereas the most probable error estimated in the case of fluorescence cross-sections is ±5.1%. The fluorescence quantum yields for CS2 have also been obtained in the two spectral regions.

    • Multiple scattering of gamma rays in water, concrete and sand

      G S Bhandal Vijay Kumar Rama Rani K A Waldeep K Singh

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      The transmitted photon spectra of133Ba,22Na,137Cs,54Mn and60Co point sources are measured through different thicknesses of water, concrete and sand. The multiple-scatter peaks observed in these materials at 60, 90 and 100 keV energies respectively are found to be independent of incident photon energy and thickness of the medium.

    • Total electron scattering cross-sections for nitrous oxide at low electron energies

      P Rawat K P Subramanian Vijay Kumar

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      Absolute total electron scattering cross sections for nitrous oxide have been measured at low electron energies using a photoelectron source. The measurements have been carried out at 19 electron energies varying from 0.73–9.14 eV with an accuracy of ±3%. The cross sections obtained in the present experiment have been compared with other measurements.

    • Total electron scattering cross sections for carbon dioxide at low electron energies

      P Rawat K P Subramanian Vijay Kumar

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      Absolute total electron scattering cross sections for carbon dioxide have been measured at low electron energies using a photoelectron source. The measurements have been carried out at 27 electron energies varying from 0.91–9.14 eV with an accuracy of ±3%. The cross sections obtained in the present experiment have been compared with other measurements and theoretical computations.

    • Total electron scattering cross-sections for carbon monoxide at low electron energies

      P Rawat K P Subramanian Vijay Kumar

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      Absolute total electron scattering cross-sections for carbon monoxide have been measured at low electron energies using a photoelectron source. The measurements have been carried out at 22 electron energies varying from 0.73 to 9.14 eV with an accuracy of ±3%. The cross-sections obtained in the present experiment have been compared with other measurements and theoretical computations.

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