S Kumar
Articles written in Pramana – Journal of Physics
Volume 63 Issue 3 September 2004 pp 509-528
The ground state wave function of^{11}Li obtained in a three-body model proposed earlier (S Kumar and V S Bhasin,
Volume 68 Issue 5 May 2007 pp 707-720 Research Articles
Bianchi Type-II inﬂationary models with constant deceleration parameter in general relativity
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.
Volume 69 Issue 4 October 2007 pp 681-686b Brief Reports
Composition dependence of density of states in a-Se$_{100−x}$Sn$_{x}$ thin films
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.
Volume 71 Issue 2 August 2008 pp 283-295
Conformational properties of polymers
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.
Volume 74 Issue 3 March 2010 pp 411-420 Research Articles
Dielectric relaxation in glassy Se_{75}In$_{25−x}$Pb$_{x}$ alloys
In this paper we report the effect of Pb incorporation in the dielectric properties of a-Se_{75}In_{25} glassy alloy. The temperature and frequency dependence of the dielectric constants and the dielectric losses in glassy Se_{75}In$_{25−x}$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.
Volume 78 Issue 2 February 2012 pp 309-318 Research Articles
Optical characterization of a-Se$_{85−x}$Te_{15}Zn$_x$ thin ﬁlms
Thin ﬁlms of Se$_{85−x}$Te_{15}Zn$_x$ ($x = 0$, 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$_{85−x}$Te_{15}Zn$_x$ glassy system which could be correlated with increase in the density of defect states.
Volume 96, 2022
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