• R Srinivasan

      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.

    • Magnetic shielding using high-temperature superconductors

      V Ganesan R Srinivasan S Aswathy K D Chandrasekaran B Srinivas U V Varada Raju G V Subba Rao R Gopalan T Rajasekaran

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      Magnetic shields of various high-temperature superconductors, YBa2Cu3O7−x (YBCO), YBa2Cu3O7−x-Ag composites (random inclusions as well as non-random coatings) and Bi2Sr2Ca2Cu3Ox (BSCCO) were prepared by uniaxial as well as isostatic compression with various dimensions. The shielding properties were measured at 77 K for dc and ac magnetic fields in the range of frequencies from 100 Hz to 10 kHz. The critical penetration field (CPF), defined as the value of the applied magnetic field at which a detectable field was observed inside the cylinder, varied from cylinder to cylinder and also with the ageing of the cylinders in the case of YBCO shields. The highest value of CPF was 16 G at 77 K for YBCO shield prepared by isostatic compression. Even though the stability of BSCCO shields with respect to ageing is good, the CPF values are very low compared to those for YBCO. Detailed studies were performed in the case of YBCO shields. The CPF decreased as a function of time over a period of 90 days. The CPF decreased as the frequency of the applied field was increased. The wave form of the field inside the pot for a sinusoidal applied field was highly distorted and showed the presence of higher harmonics with appreciable amplitude. The wave form was Fourier-analysed to yield the field inside the shield along with the harmonics. The shields with Ag addition seem to give better performance at high fields.

    • Critical exponent of the electrical conductivity in the paracoherence region of a thin film of YBa2Cu3O7−x

      Manmeet K Marhas K Balakrishnan V Ganesan R Srinivasan D Kanjilal G K Mehta G K Muralidhar G Mohan Rao Senthil Nathan S Mohan

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      Critical exponent of the electrical conductivity in the paracoherence region (γ) of the high temperature superconductor YBa2Cu3O7−x (YBCO) has been estimated for high quality thin film on ZrO2 substrate prepared by high pressure oxygen sputtering. High energy ion irradiation was carried out using 100 MeV16O7+ ions at liquid nitrogen temperature to see the effects of disorder on the value of the exponent. The critical exponent (γ) changes from a value of about 2 to 1·62 upon irradiation. Studies were also carried out on this film to see the effect of ageing and annealing.

    • History effects in low-field magnetoresistance of BPSSCO polycrystals

      A Das A Banerjee R Srinivasan

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      Low-field (H<40 G) magnetoresistance measurements have been made on Bi1·6Pb0·4Sr2Ca2Cu3O10 polycrystals at several temperatures between 80 and 105 K. Considerable hysteresis in ρ(H) is found in a zero-field-cooled sample when the applied field is increased from 0 to a maximum value and then lowered back to 0 at all temperatures. The observation of hysteresis is taken as an evidence for field trapping in the grains. We show that the hysteresis in ρ(H) occurs for applied fields much lower than that at whichdρ(H)/dH exhibits a discontinuity. In addition, we find that when the applied magnetic field (Ha) is lowered from a maximum field, the effective intergranular field,Heff, becomes zero forHc>0, which gives rise to a minimum in ρ(H).

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