Sample Preparation and Specific Heat Measurements of Hg _x Ba₂CuO_4+δ with Different Transition Temperatures

Abstract

Various ceramic samples of Hg _x Ba₂CuO_4+δ have been synthesised by the quartz ampoule method with previously prepared Ba₂CuO_3+y as a precursor. By tuning the mercury content x < 1 and controlling the synthesis of Ba₂CuO_3+y, we have prepared samples which show sharp superconducting transitions with different T_c-values for different samples. The temperature of the onset of diamagnetism varies between 80 K and 96 K. For two samples, each with T_c = 95 K, the specific heat c_p(T) has been measured between 60 and 110 K with a continuous heating method. An O₂-annealed sample A reveals a broad magnetic superconducting transition, whereas the transition of the as-synthesised sample B is narrow. The specific heat anomaly of the superconducting transition can clearly be identified for sample B and is broadened in applied magnetic fields. It cannot be resolved in the O₂-annealed sample A, most likely because of a broad distribution of T_c-values as indicated by the broad transition.