• V Sankaranarayanan

      Articles written in Pramana – Journal of Physics

    • Electron transport in Chevrel phase superconductors, Cu1.8Mo6S8−ySey, 0⩽y⩽8 and Cu1.8Mo6S8−yTey, 0⩽y⩽4—Fit to Cote-Meisel theory

      V Sankaranarayanan G Rangarajan R Srinivasan

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      Normal state electrical resistivity of the Chevrel phase compounds of the type Cu1.8Mo6S8−ySey, 0⩽y⩽8 and Cu1.8Mo6S8−yTey, 0⩽y⩽4 is analysed on the basis of the generalized diffraction model which incorporates a postulate on electron-phonon interaction,viz phonons with wavelength exceeding the electron mean-free path are ineffective electron scatterers. Fit obtained by this model was found to be superior to other models based on the interbands-d scattering of electrons.

    • Enhancement of thermopower in the highTc superconductor YBa2Cu3O7 and related compounds

      R Srinivasan V Sankaranarayanan N P Raju S Natarajan U V Varadaraju G V Subba Rao

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      The absolute thermopower of single phase YBa2Cu3O7 and Y0.8Er0.2Ba2Cu3O7 has been measured in the range 250 K to the superconducting transition temperature. It is found that these compounds show a large enhancement of thermopower in the range 150 K down toTc. This enhancement shows a steep exponential drop as the temperature increases from the transition temperature. The temperature variation of the enhancement is too steep to be accounted for by electron-phonon or electron-local structural excitation mechanisms.

    • Specific heat of YBa2Cu3O7−x from 4.2 to 60 K

      N Sankar V Sankaranarayanan R Srinivasan G Rangarajan G V Subba Rao

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      The specific heat of superconducting oxide compound, YBa2Cu3O7 −x, is studied using a quasi-adiabatic calorimeter from 4.2 to 60 K. The analysis of the specific heat data below 15 K gives a value of 17 mJ/mole K2 for the electronic heat capacity coefficient. The value ofθD(0) is determined to be 397±8 K. The variation ofθD with temperature was calculated in the temperature range 4.2 to 60 K.

    • Magnetization studies in niobium and YBa2Cu3O7 samples

      B V B Sarkissian A K Grover G Balakrishnan Ravi Kumar P L Paulose R Vijayaraghavan V Sankaranarayanan C K Subramanian

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      The results of experimental studies on hysteresis in magnetization, thermomagnetic history effects, anomalous variations in magnetic hysteresis curves and the decay rates of magnetization obtained under different thermomagnetic histories in specimens of conventional and high temperature superconductors are presented. The Bean’s critical state model is considered adequate to explain magnetic behaviour in conventional hard superconductors. The similarity in the general features of the results of different experiments on specimens of the two families of superconductors underscores the efficacy of the said model to understand some aspects of the macroscopic magnetic response of high temperature superconductors as well. For instance, the isothermal magnetization hysteresis loop which comprises of magnetization curves along forward (−Hmax to +Hmax) and reverse (+Hmax to −Hmax) paths define an envelop within which all isothermal magnetization data along different thermomagnetic histories lie. There exist inequality relationship between various field values identified asHpeak,HI,HII etc. in isothermal magnetization hysteresis as well as magnetic relaxation data. The entire field span of an isothermal magnetization hysteresis data set can be considered to comprise of three parts corresponding to (Mrem(H)−MFC(H)+MZFC(H)) being equal to, less than or greater than zero, whereMrem(H) are the remanent magnetization values obtained on reducing field to zero after having the specimen in different applied field (H) values. There are, however some situations amongst thermomagnetic history effects in specimens which show incomplete flux trapping on field cooling, where the critical state model has been found inadequate.

    • Thermal conductivity of YBa2Cu3O7 from 7 K to 260 K

      T R Ravindran V Sankaranarayanan R Srinivasan

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      Thermal conductivity of YBa2Cu3O7 has been measured on samples of a few mm thickness. AboveTc thermal conductivity is found to decrease with increase in temperature, pointing towards a contribution to thermal resistivity from three-phonon Umklapp processes. BelowTc thermal conductivity increases rapidly before reaching a maximum at about 50 K and then falls towards zero at lower temperatures. The experimental set up is described and results discussed.

    • 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.

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