• Issue front cover thumbnail

      Volume 76, Issue 1

      January 2011,   pages  1-188

    • Results from PAMELA, ATIC and FERMI: Pulsars or dark matter?

      Debtosh Chowdhury Sudhir K Vempati Chanda J Jog

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      It is well known that dark matter dominates the dynamics of galaxies and clusters of galaxies. Its constituents remain a mystery despite an assiduous search for them over the past three decades. Recent results from the satellite-based PAMELA experiment show an excess in the positron fraction at energies between 10 and 100 GeV in the secondary cosmic ray spectrum. Other experiments, namely ATIC, HESS and FERMI, show an excess in the total electron $(e^+ + e^−)$ spectrum for energies greater than 100 GeV. These excesses in the positron fraction as well as the electron spectrum can arise in local astrophysical processes like pulsars, or can be attributed to the annihilation of the dark matter particles. The latter possibility gives clues to the possible candidates for the dark matter in galaxies and other astrophysical systems. In this article, we give a report of these exciting developments.

    • An analysis of the nonlinear equation $u_t = f(x, u)u_{xx} + g(x, u)u^2_x + h(x, u)u_x + p(x, u)$

      R M Edelstein K S Govinder

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      We use the method of preliminary group classification to analyse a particular form of the nonlinear diffusion equation in which the inhomogeneity is quadratic in $u_x$. The method yields an optimal system of one-dimensional subalgebras. As a result we obtain those explicit forms of the unknown functions 𝑓, 𝑔, ℎ and 𝑝 for which the equation admits additional point symmetries.

    • Application of Lie transform perturbation method for multidimensional non-Hermitian systems

      Asiri Nanayakkara

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      Three-dimensional non-Hermitian systems are investigated using classical perturbation theory based on Lie transformations. Analytic expressions for total energy in terms of action variables are derived. Both real and complex semiclassical eigenvalues are obtained by quantizing the action variables. It was found that semiclassical energy eigenvalues calculated with the classical perturbation theory are in very good agreement with exact energies and for certain non-Hermitian systems second-order classical perturbation theory performed better than the secondorder Rayleigh–Schroedinger perturbation theory.

    • Iterative approach for the eigenvalue problems

      J Datta P K Bera

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      An approximation method based on the iterative technique is developed within the framework of linear delta expansion (LDE) technique for the eigenvalues and eigenfunctions of the one-dimensional and three-dimensional realistic physical problems. This technique allows us to obtain the coefficient in the perturbation series for the eigenfunctions and the eigenvalues directly by knowing the eigenfunctions and the eigenvalues of the unperturbed problems in quantum mechanics. Examples are presented to support this. Hence, the LDE technique can be used for non-perturbative as well as perturbative systems to find approximate solutions of eigenvalue problems.

    • Nonlinear quantum mechanics, the superposition principle, and the quantum measurement problem

      Kinjalk Lochan T P Singh

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      There are four reasons why our present knowledge and understanding of quantum mechanics can be regarded as incomplete. (1) The principle of linear superposition has not been experimentally tested for position eigenstates of objects having more than about a thousand atoms. (2) There is no universally agreed upon explanation for the process of quantum measurement. (3) There is no universally agreed upon explanation for the observed fact that macroscopic objects are not found in superposition of position eigenstates. (4) Most importantly, the concept of time is classical and hence external to quantum mechanics: there should exist an equivalent reformulation of the theory which does not refer to an external classical time. In this paper we argue that such a reformulation is the limiting case of a nonlinear quantum theory, with the nonlinearity becoming important at the Planck mass scale. Such a nonlinearity can provide insights into the aforesaid problems. We use a physically motivated model for a nonlinear Schr ¨odinger equation to show that nonlinearity can help in understanding quantum measurement. We also show that while the principle of linear superposition holds to a very high accuracy for atomic systems, the lifetime of a quantum superposition becomes progressively smaller, as one goes from microscopic to macroscopic objects. This can explain the observed absence of position superpositions in macroscopic objects (lifetime is too small). It also suggests that ongoing laboratory experiments may be able to detect the finite superposition lifetime for mesoscopic objects in the near future.

    • Next-to-next-leading order correction to 3-jet rate and event-shape distribution

      Mohammad Ebrahim Zomorrodian Alireza Sepehri Tooraj Ghaffary Parvin Eslami

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      The hadronic events from the $e^+ e^−$ annihilation data at the centre-of-mass energies ranging from 60 to 197 GeV were studied. The AMY and OPAL Collaborations offered a unique opportunity to test QCD by measuring the energy dependence of different observables. The coupling constant, $\alpha_s$, was measured by two different methods: first by employing the three-jet observables. Combining all the data, the value of as at next-to-next leading order (NNLO) was determined to be $0.117 \pm 0.004$(hard) ± 0.006(theo). Secondly, from the event-shape distributions, the strong coupling constant, $\alpha_s$, was extracted at NNLO and it’s evaluation was tested with the energy scale. The results were consistent with the running of $\alpha_s$, expected from QCD predictions. Averaging over different observables, $\alpha_s$ was determined to be $0.115 \pm 0.007$(hard) $\pm 0.003$(theo).

    • Use of combined Hartree–Fock–Roothaan theory in evaluation of lowest states of $K[Ar]4s^0 3d^1$ and $Cr^+ [Ar]4s^0 3d^5$ isoelectronic series over noninteger 𝑛-Slater type orbitals

      I I Guseinov M Erturk E Sahin

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      By using noninteger n-Slater type orbitals in combined Hartree–Fock–Roothaan method, self-consistent field calculations of orbital and lowest states energies have been performed for the isoelectronic series of open shell systems $K[Ar]4s^0 3d^1 ({}^2D) (Z = 19–30)$ and $Cr^+[Ar]4s^0 3d^5 ({}^6 S) (Z = 24–30)$. The results of the calculations for the orbital and total energies obtained by using minimal basis-sets of noninteger 𝑛-Slater type orbitals are given in the tables. The results are compared with the extended-basis Hartree–Fock computations. The orbital and total energies are in good agreement with those presented in the literature. The results can be useful in the study of various properties of heavy atomic systems when the combined Hartree–Fock–Roothaan approach is employed.

    • Influence of the laser-diode temperature on crystal absorption and output power in an end-pumped Nd:YVO4 laser

      Ebrahim Safari

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      In this work, we studied the influence of heat loaded into the laser crystal in an endpumped solid-state Nd:YVO4 high power laser. We have shown experimentally that the optimum value of the laser-diode temperature for the maximum pump power absorption by the Nd:YVO4 crystal and the maximum Nd:YVO4 laser output power are approximately similar to that of a system of the low power type, but by increasing the pump power, different values can be obtained.

    • Atomistic simulation of the point defects in TaW ordered alloy

      Zhong-Liang Lin Jian-Min Zhang Yan Zhang Vincent Ji

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      Combining molecular dynamics (MD) simulation with modified analytic embeddedatom method (MAEAM), the formation, migration and activation energies of the point defects for six-kind migration mechanisms in B2-type TaW alloy have been investigated. The results showed that the anti-site defects TaW and W$_{\text{Ta}}$ were easier to form than Ta and W vacancies owing to their lower formation energies. Comparing the migration and activation energies needed for six-kind migration mechanisms of a Ta (or W) vacancy, we found that one nearest-neighbour jump (1NNJ) was the most favourable because of its lowest migration and activation energies, but it would lead to a disorder in the alloy. One next-nearest-neighbour jump (1NNNJ) and one third-nearest-neighbour jump (1TNNJ) could maintain the ordered property of the alloy but required higher migration and activation energies. So the 1NNNJ and 1TNNJ should be replaced by straight [100] six nearestneighbor cyclic jumps (S[100]6NNCJ) (especially) or bent [100] six nearest-neighbour cyclic jumps (B[100]6NNCJ) and [110] six nearest-neighbor cyclic jumps ([110]6NNCJ), respectively.

    • Role of correlated hopping in mixed valence phenomena

      N K Ghosh S K Bhowmick N S Mondal

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      Role of correlated hopping is studied using extended Falicov–Kimball model in a small cluster. A discontinuous insulator-to-metal transition is observed at a critical 𝑓-level energy. Transition is sharper for larger correlated hopping. In the specific heat curves a two-peak structure consisting of a sharp peak followed by a Schottky-type broad peak is exhibited. In a limited parameter region, some heavy-fermion like characteristics have been observed

    • Optical spectra and spin-Hamiltonian parameters of trivalent ytterbium in lead tungstate

      W-L Feng X-M Li

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      By using crystal-field theory, the optical spectra and spin-Hamiltonian parameters (abbr. SH parameters, i.e. the anisotropic 𝑔 factors $g_{\|} g_{\perp}$, and hyperfine structure constants $A_{\|}, A_{\perp}$) of 171Yb3+ and 173Yb3+ isotopes in the tetragonal PbWO4 are calculated. The theoretical results agree well with the experimental values. The crystal-field parameters and the signs of the hyperfine structure constants for both 171Yb3+ and 173Yb3+ isotopes are determined. The validities of the theoretical results are discussed.

    • Structural, electrical and optical studies on spray-deposited aluminium-doped ZnO thin film

      S Tewari A Bhattacharjee

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      Thin films of zinc oxide (ZnO) were deposited on cleaned glass substrates by chemical spray pyrolysis technique using Zn(CH3COO)2 as precursor solution. Also, aluminium-doped thin films of ZnO were prepared by using AlCl3 as doping solution for aluminium. The dopant concentration [Al/Zn atomic percentage (at%)] was varied from 0 to 1.5 at% in thin films of ZnO prepared in different depositions. Structural characterization of the deposited films was performed with X-ray diffraction (XRD) studies. It confirmed that all the films were of zinc oxide having polycrystalline nature and possessing typical hexagonal wurtzite structure with crystallite size varying between 100.7 and 268.6 nm. The films exhibited changes in relative intensities and crystallite size with changes in the doping concentration of Al. The electrical studies established that 1 at% of Aldoping was the optimum for enhancing electrical conduction in ZnO thin films and beyond that the distortion caused in the lattice lowered the conductivity. The films also exhibited distinct changes in their optical properties at different doping concentrations, including a blue shift and slight widening of bandgap with increasing Al dopant concentration.

    • Variation of interface trap level charge density within the bandgap of 4H-SiC with varying oxide thickness

      Sanjeev K Gupta A Azam J Akhtar

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      Interfacial characteristics of metal oxide-silicon carbide (MOSiC) structure with different thickness of SiO2, thermally grown in steam ambient on Si-face of 4H-SiC (0 0 0 1) substrate were investigated. Variations in interface trapped level density ($D_{\text{it}}$) was systematically studied employing high-low (H-L) frequency $C–V$ method. It was found that the distribution of $D_{\text{it}}$ within the bandgap of 4H-SiC varied with oxide thickness. The calculated $D_{\text{it}}$ value near the midgap of 4H-SiC remained almost stable for all oxide thicknesses in the range of $10^9 –10^{10}$ cm-2 eV-1. The $D_{\text{it}}$ near the conduction band edge had been found to be of the order of 1011 cm-2 eV-1 for thicker oxides and for thinner oxides $D_{\text{it}}$ was found to be the range of 1010 cm-2 eV-1. The process had direct relevance in the fabrication of MOS-based device structures.

    • Accretion, primordial black holes and standard cosmology

      B Nayak P Singh

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      Primordial black holes evaporate due to Hawking radiation. We find that the evaporation times of primordial black holes increase when accretion of radiation is included. Thus, depending on accretion efficiency, more primordial black holes are existing today, which strengthens the conjecture that the primordial black holes are the proper candidates for dark matter.

    • Thermoelastic properties of minerals at high temperature

      Sanjay Upadhyay Hem Chandra Meenakashi Joshi Deepika P Joshi

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      The knowledge of elasticity of the minerals is useful for interpreting the structure and composition of the lower mantle and also in seismic studies. The purpose of the present study is to discuss a simple and straightforward method for evaluating thermoelastic properties of minerals at high temperatures. We have extended the Kumar’s formulation by taking into the account the concept of anharmonicity in minerals above the Debye temperature ($\theta_D$). In our present study, we have investigated the thermophysical properties of two minerals (pyrope-rich garnet and MgAl2O4) under high temperatures and calculated the second-order elastic constant ($C_{ij}$) and bulk modulus ($K_T$) of the above minerals, in two cases first by taking Anderson–Gruneisen parameter ($\delta_T$) as temperature-independent and then by treating $\delta_T$ as temperature-dependent parameter. The results obtained when $\delta_T$ is temperature-dependent are in close agreement with experimental data.

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