• Y Hariharan

      Articles written in Pramana – Journal of Physics

    • Electrical and thermal conductivity of soft solder at low temperatures

      Y Hariharan M P Janawadkar T S Radhakrishnan

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      The electrical resistivity of soft solder (Pb0.28Sn0.72) has been measured in the temperature range 4.2 K to 300 K. The ‘alloy’ becomes electrically superconducting at a temperature of 6.9 K. Above this, in the entire temperature range, the resistivity could be described, apart from the residual resistivity, by the weighted average of the resistivities of the individual constituents which are derived from the Bloch-Grüneisen relation. The results are in accordance with the phase diagram, which shows a co-existence of two phases in almost the entire range of concentration of the Pb-Sn binary system. It has been shown that the thermal conductivity data on soft solder as well as on Pb0.7Sn0.3, both taken from literature, could be interpreted on the same basis, below and above the ‘superconducting transition temperature’. Recent results on other Pb-Sn systems are discussed in the light of this interpretation.

    • Structure property correlations in superconducting Ti-Nb alloys

      Y Hariharan M P Janawadkar T S Radhakrishnan A L E Terrance G A Dixit V S Raghunathan

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      Titanium-rich transition metal alloys are metastable in their quenched boc β phase. The instability is relieved by low temperature structural transformations. We have investigated this in a series of Ti-Nb alloys, through the measurements of electrical resistivity (ρ), superconducting transition temperature and upper critical field. Supporting structural evidence has been obtained from transmission electron microscopy (tem) and x-ray studies. It is shown that both ρ and dρ/dT can be used as useful indices of this instability. The enhanced value of resistivity on account of the instability results in the enhancement of upper critical field as shown from dHc2/dT measurements.

    • Effect of iron doping and oxygen stoichiometry on infrared absorption in Y1Ba2Cu3O7−δ

      A K Sood K Sankaran Y Hariharan S Vijayalakshmi V Sankara Sastry S Kalavathi J Janaki

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      We report infrared absorption of Y1Ba2Cu3O7−δ as a function of oxygen stoichiometry (0<δ<1) and copper substitution by iron in the spectral range of 450–700 cm−1. The strong bands associated with Cu-O vibrations undergo significant changes in their frequencies and intensities asδ is varied across the orthorhombic to tetragonal phase. These changes coupled with those arising as a result of doping with iron has helped in identifying the nature of the vibrational modes.

    • Structure and vibrational properties of carbon tubules

      N Chandrabhas A K Sood D Sundararaman S Raju V S Raghunathan GVN Rao V S Sastry T S Radhakrishnan Y Hariharan A Bharathi C S Sundar

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      The structure of multilayered carbon tubules has been investigated by electron microscopy and X-ray diffraction. The structure of tubules is characterized by disorder in the stacking of cylindrical graphene sheets. Raman scattering measurements have been carried out in tubules and compared with graphite. The observed features in the Raman spectra in tubules can be understood in terms of the influence of disorder. The additional Raman modes predicted for single layer carbon tubules have not been observed.

    • Electrical transport and magnetic ordering in R2Ti3Ge4 (R=Dy, Ho and Er) compounds

      R Nirmala V Sankaranarayanan K Sethupathi AV Morozkin T Geethakumary Y Hariharan

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      New R2Ti3Ge4 (R=Dy, Ho and Er) intermetallic compounds have been synthesized and characterized by X-ray diffraction and low temperature ac magnetic susceptibility, electrical resistivity and thermoelectric power measurements were carried out. The compounds crystallize in the parent, Sm5Ge4-type orthorhombic structure (space group Pnma) and lanthanide contraction is observed as one moves along the rare-earth series. The changeover from paramagnetic to antiferromagnetic phase happens at low temperatures and the ordering temperature scales with the de Gennes factor. The electrical resistivity is metallic with a negative curvature above 100 K. Thermopower displays a weak maximum at temperatures less than 50 K signifying the possible phonon and magnon drag effects.

    • SQUID-based measuring systems

      MP Janawadkar R Baskaran R Nagendran K Gireesan N Harishkumar Rita Saha L S Vaidhyanathan J Jayapandian Y Hariharan TS Radhakrishnan

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      A program has been developed and initiated at the Indira Gandhi Centre for Atomic Research (IGCAR) for the utilization of SQUID sensors in various application areas. DC SQUID sensors based on Nb-AlOx-Nb Josephson junctions have been designed and developed inhouse along with associated flux-locked loop (FLL) electronics. A compact low field SQUID magnetometer insertible in a liquid helium storage dewar has also been developed inhouse and is in use. Efforts to build a high field SQUID magnetometer, SQUID-DAC system, are in progress. A planar gradiometric DC SQUID sensor for non-destructive evaluation (NDE) application to be used in relatively unshielded environment has been designed and developed. An easily portable NDE cryostat with a small lift-off distance, to be used in external locations has been designed and tested. The magnetic field produced by a given two-dimensional current density distribution is inverted using the Fourier transform technique.

    • Metal-insulator transition in Ni-doped Na0.75CoO2: Insights from infrared studies

      M Premila A Bharathi N Gayathri P Yasodha Y Hariharan C S Sundar

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      Nickel substitution at the cobalt site in Na0.75CoO2 induces an upturn in the resistivity on lowering the temperature, with the metal-to-insulator transition temperature (TMIT) increasing with the Ni content. Low temperature far infrared measurements on polycrystalline samples of Na0.75CoO2 and Na0.75Co0.95Ni0.05O2, the latter having TMIT ∼ 175 K, have been carried out. Dramatic changes in the Na mode frequencies, and relative intensities of the out-of-plane modes corresponding to the two Na sites are observed, coincident with the MIT in Na0.75Co0.95Ni0.05O2. It is argued that these changes are associated with a charge ordering of the CoO2 layer, associated with the metal-insulator transition.

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