• C K Subramaniam

      Articles written in Bulletin of Materials Science

    • Magnetic scattering and superconductivity in Nd1·86Ce0·14CuO4−y

      V Vijayashree C K Subramaniam R Srinivasan

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      Nd1·86Ce0·14CuO4−y is superconducting below about 20 K and electrons are considered to be responsible for superconductivity in these materials as in the case of Ln2−xCexCuO4−y and Ln2−xThxCuO4−y. Structurally these materials are not very different from thep-type superconductors La2−xSrxCuO4−y. In both these types of superconductors, the parent compounds are antiferromagnetically-ordered insulators. The induction of holes or electrons by substitution destroys magnetic interactions and brings about superconductivity. Peng and coworkers have studied the resistivity variation of both superconducting and nonsuperconducting Nd1·85Ce0·15CuO4−y and have found a decrease in resistivity with temperature, obeying a lnT dependence in the superconducting samples. Such a variation was not seen by them in the nonsuperconducting samples. They ascribe the lnT variation seen in their superconducting samples to arise from magnetic scattering of electrons. To study whether such an effect exists, Nd1·86Ce0·14CuO4−y was subjected to various annealing conditions and the resistivity behaviour is presented here. Our results differ from those of Penget al.

    • Fluctuation-induced excess conductivity in the compounds CaREBaCu3O7−y (RE=La and Sm)

      H Krishnan R Srinivasan V Sankaranarayanan C K Subramaniam G V Subba Rao

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      The compounds CaREBaCu3O7−y (RE=La and Sm) are tetragonal at room temperature withTc between 60 and 70 K. The single-phase compounds were prepared by solid-state reaction. The resistivity was measured by a four-probe technique in a continuous flow cryostat with the temperature being controlled to an accuracy of 10 mK. The resistivity vs temperature showed a break in slope around 180 K in CaLaBaCu3O7−y and around 220 K in CaSmBaCu3O7−y. The results were analysed for fluctuation conductivity from 180 K downwards. A plot of dρ/dT vsT showed a sharp peak atTm=69·69 K for La compound and 66·00 K for the Sm compound. Detailed analysis of the resistivity in the regionTon to 180 K was carried out using the procedure due to Veira and Vidal. The results are discussed in this paper.

    • Thermoelectric power of single-phase samples of Tl2CaBa2Cu2Oy and Ba2CaSr2Cu2Oy

      V Vijayashree C K Subramaniam R Srinivasan I K Gopalakrishnan PVPSS Sastry J V Yakhmi R M Iyer

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      Single-phase 2122 samples of thallium and bismuth superconductors were made by the precursor matrix method. The thermopower of these samples was measured in the temperature range 250 K-Tc. The thermopower was positive and decreased linearly with increasing temperature aboveTc (onset). The exponential enhancement of thermopower seen in the undoped and doped YBCO was not observed in these samples. The linear variation of thermopower can be explained on the basis of either a two-band model or a narrow band model.

    • Thermopower of non-superconducting and superconducing Nd1·85Ce0·15CuO4−y samples

      V Vijayashree C K Subramaniam R Srinivasan

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      The thermopower of single-phase samples of Nd1·85Ce0·15CuO4−y was measured from 250 K down to 10 K. The as-prepared sample was not superconducting. It had a negative thermopower at 250 K, whose magnitude increased as temperature was decreased to 95 K. A further reduction in temperature caused a decrease in magnitude of thermopower. The sign of the thermopower changes to positive at 12 K. The superconducting sample also showed the same behaviour but the change of sign now occurred at 40 K. Below 40 K, the thermopower showed a positive peak and reduced to zero at the superconducting transition. These results were compared with previous studies.

    • DC magnetization studies of (PrxY1−x)Ba2Cu3O7−y

      L S Vaidhyanathan C K Subramaniam G Rangarajan

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      Magnetization was measured on Pr-doped YBa2Cu3O7−y for fields up to 5·5T. The paramagnetic behaviour is correlated to the free ion values of Pr3+ and Pr4+. Hysteresis experiments were done for superconducting composition for fields up to 1100 Gauss. The intragrain critical current density is calculated using Bena’s formula.

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