• S Kumar

      Articles written in Pramana – Journal of Physics

    • Three-body analysis of11Li and itsβ-decay to deuteron channel and to halo analog state11Be* (18.3 MeV)

      S Kumar V S Bhasin

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      The ground state wave function of11Li obtained in a three-body model proposed earlier (S Kumar and V S Bhasin,Phys. Rev.C65, 034007 (2002)) has been employed to study the probability distributions, momentum distributions and n−n correlation. Complex scaling method has been used to find the energy positions and widths of the three resonant states of11Li above the breakup threshold. The formalism is extended further to study the β-decay of11Li to two channels. One is the β-transition of11Li into a high lying excited state of11Be at 18.3 MeV, i.e.,11Be* and the second is the decay to deuteron +9Li channel. The11Be* state has been considered as a halo analog state identified as a bound three-body (9Li + n + p) system. The n-9Li interaction incorporates both the virtual state and the p-wave resonance observed experimentally. For p-9Li interaction, a Coulomb corrected separable interaction is constructed using charge indepedendence for strong interaction part. The n-p interaction is operative only in3S1 state corresponding to the isotopic spin Th =0. As a result the11Be* state has the same isotopic spin as that of9Li core, i.e.,T = 3/2. Using these realistic parameters as input and without invoking any other free parameter, the model has been used to predict the strength of the Gamow-Teller β-decay of11Li to11Be*, i.e.,Bgt = 1.5 and the value of the branching ratio to9Li + deuteron channel to be 1.3 × 10-4. These results are found to be in rather good agreement with the recent experimental findings.

    • Bianchi Type-II inflationary models with constant deceleration parameter in general relativity

      C P Singh S Kumar

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      Einstein's field equations are considered for a locally rotationally symmetric Bianchi Type-II space–time in the presence of a massless scalar field with a scalar potential. Exact solutions of scale factors and other physical parameters are obtained by using a special law of variation for Hubble's parameter that yields a constant value of deceleration parameter. To get inflationary solutions, a flat region is considered in which the scalar potential is constant. Power-law and exponential cases are studied and in both solutions there is an anisotropic expansion of the cosmic fluid, but the fluid has vanishing vorticity. A detailed study of geometrical and kinematical properties of solutions has been carried out.

    • Composition dependence of density of states in a-Se$_{100−x}$Sn$_{x}$ thin films

      N Sharma S P Singh S Kumar

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      The present paper reports the DC conductivity measurements at high electric fields in vacuum evaporated amorphous thin films of a-Se$_{100−x}$Sn$_{x}$ ($x = 0, 2, 4, 6$) glassy alloys. Current–voltage ($I–V$) characteristics have been measured at various fixed temperatures. In these samples, at low electric fields, ohmic behaviour is observed. However, at high electric fields ($E \sim 10^{4}$ V/cm), non-ohmic behaviour is observed. An analysis of the experimental data confirms the presence of space charge limited conduction (SCLC) in the glassy materials studied in the present case. From the fitting of the data to the theory of SCLC, the density of defect states (DOS) near Fermi level is calculated. Increase in DOS with increase in Sn concentration has been found which could be correlated with the electronegativity difference between the two elements used here in making the glassy alloys. The peculiar role of the element Sn as an impurity in the pure Se glassy alloy is also discussed.

    • Conformational properties of polymers

      A R Singh D Giri S Kumar

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      We discuss exact enumeration technique and its application to polymers and biopolymers. Using this method one can obtain phase diagram in thermodynamic limit. The method works quite well in describing the outcomes of single molecule force spectroscopy results where finite size effects play a crucial role.

    • 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.

    • Optical characterization of a-Se$_{85−x}$Te15Zn$_x$ thin films

      S Shukla S Kumar

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      Thin films of Se$_{85−x}$Te15Zn$_x$ ($x = 0$, 2, 4, 6 and 10) glassy alloys have been deposited onto a chemically cleaned glass substrate by thermal evaporation technique under vacuum. The analysis of transmission spectra, measured at normal incidence, in the spectral range of 400–2500 nm helped us in the optical characterization of thin films under study. From the analysis of transmission spectra, the optical parameters such as refractive index (𝑛), extinction coefficient (𝑘), absorption coefficient (𝛼), real and imaginary dielectric constants ($\epsilon'$ and $\epsilon"$) have been calculated. It is observed that the parameters 𝑛, 𝑘, $\epsilon'$, $\epsilon"$ and 𝛼 decrease with increase in wavelength (𝜆) and increase with Zn content. Optical band gap ($E_g$) has also been calculated and found to decrease with Zn content in Se$_{85−x}$Te15Zn$_x$ glassy system which could be correlated with increase in the density of defect states.

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