Distinguishable effects of oxygen and rhenium in HgBa₂CuO_4+δ superconductors

Muon spin relaxation and magnetization experiments on Hg_1-xRe_xBa₂CuO_4+δ ceramics with different oxygen and Re contents allow us to discern the effects of the incorporation of both elements in this superconductor. For all compositions, T_c and σ∼λ^-2 are strongly dependent on δ and display a maximum at optimal doping. The optimal T_c and σ values are nearly independent of x, thus indicating that the Re substitution has only second order effects on the basic superconducting properties of this compound, near optimal doping. Samples with Re have higher oxygen contents, due to the high valence of this cation. However, the shift of the T_c(δ) and σ(δ) curves toward higher δ values as x increases suggests that the oxygen atoms coordinating the Re cations are less effective to doping than the excess oxygen atoms present in Re-free samples.