¹A_1g to ³B_1g Conversion at the Onset of Superconductivity in La_2-xSr_xCuO₄ Due to the Apical Oxygen Effect

By the first-principles variational calculations we show that, with the increase of Sr concentration, the ground state of CuO₆ octahedron in La_2-xSr_xCuO₄ changes from ¹A_1g to ³B_1g near the observed onset concentration of superconductivity while the ground state of the CuO₄ cluster in Nd_2-xCe_xCuO₄ is always ³B_1g in the presence of dopant electrons. Based on this result, two kinds of phase diagrams are predicted for hole- and electron-doped superconductors. These results support the spin polaron mechanism proposed by Kamimura et al.