Isovalent cation substitutions to control the intrinsic H_irr characteristics of superconductive copper oxides

The crucial role of the overall hole-doping level in controlling the magnetic irreversibility field (H_irr) values has been well documented for a number of copper-oxide superconductors. Here we show that not only the high overall hole-doping level but also the homogeneity of the hole distribution over the layered superconductive M_mA₂Q_n-1Cu_nO_{m+2+2n± δ} or M-m^(A)2^(Q)(n-1)n crystal contributes to enhance the intrinsic H_irr characteristics. To control the homogeneity of the hole distribution, i.e. the distribution of holes (i) between the superconductive CuO₂-(Q-CuO₂)_n-1 block and the non-superconductive AO-M_mO_m±δ-AO blocking block and (ii) within the superconductive CuO₂-(Q-CuO₂)_n-1 block, isovalent cation substitutions are found effective. To demonstrate this, we summarize our studies on the effects of isovalent substitutions at Q and A sites on the H_irr characteristics of polycrystalline Cu(Ba,Sr)₂RECu₂O_7-δ [Cu-1^(Ba,Sr)2^(RE)12] samples containing various "heavy rare earths" up to RE=Yb_0.6Lu_0.4 and Sr at the Ba site up to 40%, and also for single-crystal (Hg,Pb)(Ba,Sr)₂Ca₂Cu₃O_8+δ [(Hg,Pb)-1^(Ba,Sr)2^(Ca)23] samples with a Sr-for-Ba substitution level up to 75%.