Doping and Temperature Dependence of Upper Critical Field in Y_(0.8)Ca_(0.2)Ba_2Cu_3O_(7-δ) from Specific Heat Measurement

摘要

The temperature dependence of upper critical field (H_(C2)) is still being understood for high T_C cuprates because of strong thermal fluctuations around Tc and extremely high H_(C2) value at lower temperatures. Using a high-resolution differential technique, we determined the electronic specific heat of Y_(0.8)Ca_(0.2)Ba_2Cu_3O_(7-δ) from liquid helium temperature to room temperature in applied magnetic field up to 13T. From the temperature and field dependence of the superconducting free energy density, obtained by integrating the electronic entropy, the magnitude and temperature dependence of H_(C2) is deduced using the London model. The variation of these quantities with oxygen content is discussed.