• Ashok Rao

      Articles written in Pramana – Journal of Physics

    • An apparatus for the measurement of thermal expansion of solids at low temperatures

      Om Prakash Ashok Rao P N Dheer

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      A dilatometer, using the three terminal capacitance technique, suitable for measurement of linear thermal expansion of solids in the temperature range 1.3–300 K is described. The dialtometer is designed such that the mounting system for the specimen does not undergo any significant changes in dimensions when the specimen is heated. The apparatus, therefore, yields in principle absolute values of α, the coefficient of linear thermal expansion. The performance of the apparatus has been checked by measurements on copper in the temperature range of 77–300 K. Some preliminary results on the behaviour of α for Y1Ba2Cu3O6.9 compound in the vicinity of superconducting transition temperature,Tc are also described. The system can detect relative changes in length Δl/l0 of about 10−8. Attempts are being made to improve the sensitivity.

    • Thermal expansion of orthorhombic and tetragonal phases of the highTc superconductors RE1Ba2Cu3Oy (RE=Y, Gd, Dy)

      O M Prakash Ashok Rao O Prakash P N Dheer

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      Measurements of the coefficient of linear thermal expansion α have been carried out for the orthorhombic and tetragonal phases of RE1Ba2Cu3Oy(RE=Y, Gd, Dy) compounds using a high resolution capacitance dilatometer in the temperature range 77–300 K. All the superconducting samples exhibit a jump Δα at their respective transition temperatures,Tc. Evidences of, sample-to-sample variation in α values and dependence of Δα on the sample preparation conditions, have been obtained. The non-superconducting samples, in general, exhibit lower values of α possibly because of lowering of oxygen content.

    • Effect of Fe substitution on the thermal expansion of the highTc superconductor Y1Ba2Cu3Oy

      Ashok Rao Om Prakash S N Das P N Dheer

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      Thermal expansion measurements have been carried out on Fe substituted superconducting compounds Y1Ba2(Cu1−xFex)3Oy (0<x<4%) using a high resolution dilatometer employing the three terminal capacitance technique. The experimental set up is sensitive enough to detect changes in α of less than 10−8K−1. Results show that the jump Δα in the coefficient of linear thermal expansion at the superconducting transition temperature,Tc, increases almost linearly with Fe concentration. The normal state thermal expansion coefficient α first decreases, attains a minimum value aroundx=1% and then increases for higher Fe concentrations. The oxygen content per unit formula is almost constant up tox=1% and then increases rapidly withx. It has also been observed that the anomalous behaviour of α around 260 K observed by Meingastet al [22] for Fe concentrationx=5% is due to inadequate annealing of the sample.

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