Oxygen annealing and the related thermodynamics in HgBa2CuO4+δ

Several well-characterized HgBa2CuO4+δ samples were annealed under various conditions. Above 300°C and at various oxygen partial pressures, both reversible oxygen intake/release and irreversible Hg-loss take place with different time constants. By choosing the proper annealing temperature Ta, oxygen pressure P(O2) and annealing time t, the Hg-loss effects were negligible and the bulk superconducting transition temperature Tc was reversibly changed with δ from 0 K on the underdoped side (an insulator) through a peak of 97 K, then down to ∼ 20 K on the overdoped side. The measured 3D variation of Tc vs. (1/Ta, ln P(O2)) appears as a parabolic surface with iso-Tc segments projected on a 2D 1/Ta-ln P(O2) plane as straight lines. The slope [∂ ln P(O2)/∂(1/Ta)]Tc of the projections corresponds to the oxidation enthalpy ΔH, and the derivative (∂ ln P(O2)/∂Tc)Ta which is a measure of the Tc variation with the oxygen partial pressure used in the annealing, is related to the defect-interactions and the Tc−δ correlation. The data for YBa2Cu3O7−δ, Bi2Sr2CaCu2O8+δ, and HgBa2CuO4+δ are compared.