¹A_1<i>g</i>to³B_1<i>g</i>Conversion at the Onset of Superconductivity in La_2-<i>x</i>Sr_<i>x</i>CuO₄Due to the Apical Oxygen Effect

Abstract

By the first-principles variational calculations we show that, with the increase of Sr concentration, the ground state of CuO 6 octahedron in La 2- x Sr x CuO 4 changes from 1 A 1 g to 3 B 1 g near the observed onset concentration of superconductivity while the ground state of the CuO 4 cluster in Nd 2- x Ce x CuO 4 is always 3 B 1 g 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 .