• Volume 30, Issue 6

      June 1988,   pages  1-595

    • Orthorhombic structure: a necessity in superconducting 1-2-3 compounds

      R Prakash O Prakash N S Tavare

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      X-ray and resistivity measurements on YBa2Cu3O7−δ(1-2-3) samples show that for the same but low oxygen concentration,δ⋍0·55, no superconducting transition down to 4·2 K is observed for the tetragonal phase samples while the orthorhombic phase shows aTc ∼ 31 K. The effect of oxygen concentration onTc is isolated.Tc=91±1 K has, however, been observed continuously for the normal oxygen annealed samples,δ⋍0·07. The experimental results suggest strongly the necessity of the 1-2-3 compound to be in the orthorhombic phase for the superconducting mechanism to be operative.

    • Use of Padé approximants in the evaluation of α and δ for one-dimensional maps

      G Ambika K M Valsamma

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      Recently an analytic algorithm for evaluating the Feigenbaum indices of one-dimensional maps was developed using a perturbative expansion. We find that the use of Padé approximants in the resulting asymptotic series, significantly improves the technique.

    • Spin and mass content of linearized Poincaré gauge theories

      Eric A Lord K P Sinha

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      A unified gauge theory of massless and massive spin-2 fields is of considerable current interest. The Poincaré gauge theories with quadratic Lagrangian are linearized, and the conditions on the parameters are found which will lead to viable linear theories with massive gauge particles. As well as the 2+ massless gravitons coming from the translational gauge potential, the rotational gauge potentials, in the linearized limit, give rise to 2+ and 2 particles of equal mass, as well as a massive pseudoscalar.

    • Post-collision interaction effects on the triple-differential cross-section for the ionization of helium by fast positrons

      Sadhana Sharma M K Srivastava

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      Triple differential cross-sections for the ionization of helium by fast positrons are calculated in a ‘correlated’ first Born approximation supplemented by the inclusion of post-collision interaction effects. The results are analysed with respect to electron-helium experimental data of Jung and coworkers in coplanar asymmetric geometry.

    • Non-thermal rotational and vibrational excitation of CN produced in the flash photolysis of thiazole

      R Venkatasubramanian S L N G Krishnamachari

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      In the flash photolysis of thiazole at low pressure without any diluent, the 0–0, 1–1 and 0–1 bands of the CN violet system were observed in absorption; the 0–0 band at 3883·4 Å showed a high rotational excitation corresponding to a temperature of ⋍2000 K. The addition of argon makes the NCS bands appear with good intensity and at the same time relaxes the CN rotationally and vibrationally. These observations suggest that highly excited NCS is initially formed in the photodecomposition of thiazole which acted as a precursor to the rotationally and vibrationally excited CN radical. This paper deals with studies on the effect of argon on the relative intensities of CN and NCS and on the non-thermal rotational and vibrational intensity distribution of the CN violet system. The mechanism of formation of rotationally unrelaxed CN in the flash photolysis of thiazole has been proposed.

    • Double layers in a field-aligned current driven plasma

      S Das Sutradhar S Bujarbarua

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      The solutions of small amplitude ion acoustic double layers in current-carrying plasma in the presence of the magnetic field have been presented. The electron beam along the magnetic field has only been considered and the drift velocity assumed to be less than the electron thermal velocity. The velocity, the width and the potential of such DLs are calculated.

    • Frequency tunability and temporal characteristics of a DAMC distributed feedback dye laser

      P K Palanisamy A Ramalingam V Masilamani B M Sivaram

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      The frequency tunablity characteristics of a simple prism configuration distributed feedback dye laser (DFDL) pumped by a low pressure nitrogen gas laser are described. Tunability is studied as a function of the refractive index of the dye solution and also as a function of the angle of the interfering beams of the pump laser. The tunability range for the dye studied is from 440 to 480 nm with a spectral width of 0·1 Å and the time duration of the DFDL pulses was 50 ps.

    • A simple technique for laser-induced ablation pressure measurement

      T S Shirsat N S Shikharkhane H C Pant

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      A simple method for measuring laser-induced ablation pressure is described. The technique utilizes the well-known double foil concept. In the present experiment the impact times were estimated by monitoring the reflectivity of the impact foil rear. The measurements were performed using a glass laser (1·06 µm wavelength) in the 1011−1013 W/cm2 irradiance range. Experimental results showed good agreement with those obtained using other techniques as also those with the self-regulating ablation model prediction.

    • On multiple small-angle scattering

      S Mazumder A Sequeira

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      An analytical approach has been taken for analyzing the multiple scattering effects in small angle scattering (SAS) from both monodisperse and polydisperse systems. Two limiting regions, viz the Guinier region and the Porod region have been studied. A modified form of Guinier law has been deduced for the scattered intensity distribution in the region of small wave vector transfer, q. In the regionq(=|q|)→∞, it is shown that the effect of multiple scattering does not alter the Porod (q−4) law. In the case of polydisperse systems, a correlation has been established between the size distribution of the inhomogeneities and the experimentally extractable parameters. The validity of the formalism has been examined by reinterpretation of the multiple SANS data (Hardman-Rhyne and Berk 1985) on polydisperse Al2O3 samples. This formalism is useful in characterizing the inhomogeneities from SAS measurements, particularly when thick samples are used.

    • Measurement and understanding of magnetization in AC and DC fields and the determination of intragrainHc1 in highTc RBa2Cu3O7 superconductors

      A K Grover C Radhakrishnamurty P Chaddah G Ravi Kumar G V Subba Rao

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      We present DC and low frequency AC magnetization measurements on various RBa2Cu3O7 superconductors. We identify features intrinsic to these compounds, and establish the features originating from intergranular links in sintered pellets. The isothermal magnetization curves, and the temperature dependence of magnetization in field-cooled and zero field-cooled states are shown to be consistent with the calculations done following a recent extension of Bean’s model. Low field anomalies predicted within this model are observed, and yieldHc1 values of a few Oe. These values are shown to be consistent with the temperature variation of magnetization. A comparison is made with the other existing data and it is demonstrated that earlier quoted values ofHc1 are gross overestimates.

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