Anomaly in Thermal Expansion of YB 2 Cu 3 0 7-y Around 250K
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ANOMALY IN THERMAL EXPANSION OF YBa
2 Cu3 O7
.Y AROUND 250K
G.A. RAMADASS, V.RAMCAHDNRAN AND R.SRINIVASAN Department of Physics, Indian Institute of Technology Madras 600036, INDIA ABSTRACT The linear thermal expansion coefficient of YBa 2 Cu 3 O7 as a function of temperature has been studied. An anomaly is observed at 250K. This temperature is found to be dependent on oxygen deficiency. INTRODUCTION A large number of reports on thermal and elastic properties have appeared recently with contrasting results. Many reports point to interesting anomalies in their properties in the temperature range 220-240K. The cause of these anomalies and their relation with the superconducting transition is not clear. We have carried out the study of the thermal expansion of YBa 2 Cu 3O7.. y in both freshly oxygenated and oxygen deficient samples in the temperature range 85-300K using Fizeau's optical interference technique. The results are compared with the other properties reported in the literature. EXPERIMENTAL The samples of YBa2Cu 3 0 7 were prepared by the usual high temperature solid state reacb on using high purity starting materials. The samples were characterised using X-ray diffraction and were found to be in single phase. The Tc was found to be around 90K. Three
pyramid
of
approximately 5 mm height cut
from
a
sintered pellet having a density of 80% of ideal value were used as spacers in the interferometer. The light of wavelength 632.8 nm was used to obtain the interference fringes. The temperature was measured usinq a calibrated CODoer-constantan thermocouple and a voltmeter of liV accuracy. The interfero meter was placed in a bath type cryostat with an optical window. Helium gas at low pressure was maintained in the sample chamber to ensure good thermal equilibrium between sample and thermocouple. The bath was filled with liquid nitrogen and the sample chamber was allowed to cool to lowest possible temperature and warmed at a rate of 0.5 K/min. The passage of interference fringes was recorded as a function of temperature using a light dependent resistor and a chart recorder. The shifting of each fringes corresponds to a change in length of the sample by A/2. Thus the length of the sample was noted as a function of temperature and the linear thermal expansion coefficient, o , was calculated by processing the data as expalined below.
Mat. Res. Soc. Symp. Proc. Vol. 169. @1990 Materials Research Society
1098
RESULTS AND DISCUSSION The reference temperature, To, is taken as room temperature. The change in length as a function of temperature was first fitted to a second order polynomial + C(T-To)
a + b(T-To)
=
8
2
In such a fit it was found that the overall rms deviation
was
nearly 2.5% which is much larger than the experimental error. The deviation was even more than 4% around 240K. Then two
different
polynomials
were
fitted in
the
two
temperature
ranges 85-250K and 250-295K. The fit was found to be less 1% in both the ranges the linear thermal expansion coefficient, -was obtained from the rela
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