T C Decrease Under Copper Substitution by Metallic Ions in LSCO and YBCO Cuprates

Abstract

Experimental evidence reported by various authors concerning the dependence of TC on the y percentage of metal (M) ions in (La1.85Sr0.15Cu1−yMyO4) (LSCMO) and (YBa2(Cu1−yMy)3O7−δ) (YBCMO, where δ = 0.07 ± 0.03) cuprates under Cu2+ substitution by M ions is scrutinized. The collected data represent a preliminary report on the selective Cu2+ substitutions by various metal ions of ionic radii close to that of Cu2+ in optimally doped La2−xSrxCuO4 (LSCO) and YBa2Cu3Oy (YBCO) cuprates. Each TC(yi) set of values for each yi concentration of M ion was replaced by a unique value TC(yi) obtained as the arithmetic average of the data reported in the literature at various cooper substitution levels yi by a specific M ion in a given cuprate. The data analysis of the uniquely resulting TC(yi) distributions for different M ions was performed by least squares fit (LSQF) analysis using the Hamming termination criterion which assumes Hamming (1986) that the optimal degree of the fitting polynomial is reached when the distribution of the residuals gets stochastic (for implementation details, see Adam et al., Rom. Journ. Phys. 53:665–677, 2008). The accumulated experimental evidence probing the sharp decrease of the transition temperature TC under gradual substitutions of the in-plane Cu2+ ions by M ions is strong arguments in favor of the effective two-dimensional two-band Hubbard model (see Plakida and Oudovenko, Europ. Phys. J. B. 86:115–130, 2013) and references therein), which starts from the fundamental assumption that the high-temperature superconductivity in cuprates originates in the interactions occurring inside the individual CuO2 planes of the existing layered perovskite structures. The results substantiate the key role of the CuO2 planes as a source of the high TC superconductivity in cuprates.