The doping and magnetic field dependence of the heat capacity of yttrium barium copper oxide superconductors
This is a report of a series of measurements made on the doping effects in reduced oxygen content YBa2Cu3O7-x. The two most remarkable findings are: 1) there is a rapid decrease of the amplitude of the heat capacity anomaly with the reduction of the oxygen content and 2) the magnitude of the fluctuation effects near the superconducting transition temperature is also sharply decreased upon the removal of oxygen and, hence, the transition temperature. This is in stark contrast to what is observed in conventional superconductors when impurities are introduced. In order to make quantitative comparisons with various models and experiments, the heat capacity has been mathematically fit taking into account the effects of fluctuations and finite-size scaling. The method of fitting was motivated by measurements of the field dependence of the heat capacity upon the removal of oxygen, which indicate that the fluctuations are best approximated as being in the critical regime for all oxygen contents. Fits using these approximations give an accurate representation of the data for all temperatures near the transition. The results of the fits have been quantitatively compared to the universal trend observed by T. Schneider and H. Keller in extreme type-II superconductors and the calculations of P. Monthoux and D. Pines. There is a remarkable agreement with these results, indicating that spin-fluctuation-induced superconductivity is a possible scenario for the origin of the superconductivity in these compounds. While the data support the Monthoux and Pines model other possible explanations have also been qualitatively and quantitatively examined.
superconductors, yttrium barium copper oxide
University of Illinois - Urbana-Champaign
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