The crucial role of the overall hole-doping level in controlling the magnetic irreversibility field (Hirr) 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 MmA2Qn-1CunOm+2+2n± δ or M-m(A)2(Q)(n-1)n crystal contributes to enhance the intrinsic Hirr 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-MmOm±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 Hirr characteristics of polycrystalline Cu(Ba,Sr)2RECu2O7-δ [Cu-1(Ba,Sr)2(RE)12] samples containing various "heavy rare earths" up to RE=Yb0.6Lu0.4 and Sr at the Ba site up to 40%, and also for single-crystal (Hg,Pb)(Ba,Sr)2Ca2Cu3O8+δ [(Hg,Pb)-1(Ba,Sr)2(Ca)23] samples with a Sr-for-Ba substitution level up to 75%.
- Chemical pressure
- Isovalent cation substitution
- Magnetic irreversibility field