• V Vijayashree

      Articles written in Bulletin of Materials Science

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

      V Vijayashree C K Subramaniam R Srinivasan

      More Details Abstract Fulltext PDF

      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.

    • 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

      More Details Abstract Fulltext PDF

      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.

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