Low spin to high spin transition at the onset of superconductivity in LaSrCuO and high spin state in NdCeCuO compound

Electronic structures of LaSrCuO and NdCeCuO compounds are investigated by first principles cluster calculations. In the hole-doped CuO₆ cluster embedded in LaSrCuO, the ground state changes from low spin to high spin state near the doping concentration of the onset of superconductivity, owing to the effect of the apical oxygens. If we take account of the effect of the dopant divalent ions by virtual crystal approximation, the high spin state becomes more stable. While in the electron-doped CuO₄ cluster in NdCeCuO, the high spin ground state always appears. Using these results, the observed phase diagrams for hole and electron-doped superconductors are explained.