Microstructure and the superconductivity in La_2-x-ySrxNa_yCuO₄ systems

The mechanism of high-temperature superconductivity is still an open question. Although the relationship between microstructure and superconductivity has been noticed in the early days, there is still lack of a definite explanation. In this paper we study the microstructure and the superconductivity of La_2-yNa_yCuO₄ samples and La_1.85-ySr_0.15Na_yCuO₄ samples. It is surprising that the doping of Na⁺ does not introduce itinerant hole carriers into the CuO₂ plane. All La_2-yNa_yCuO₄ samples show insulatorlike behavior just like La₂CuO₄. For La_1.85-ySr_0.15Na_yCuO₄ samples, they exhibit superconductivity even at Na doping content y=0.30. We suggest that the microstructure directly dominates the carrier concentration in the CuO₂ plane, so that it directly determines the occurrence of the superconductivity.