Phase Diagram of Pressure-Induced High Temperature Superconductor La3Ni2O7+δ

We successfully synthesized samples of La3Ni2O7+S (S = - 0.50, - 0.16, 0.00, +0.01, and +0.12) and measured the resistance under extremely high pressures using a diamond anvil cell to establish the electronic phase diagram. A Mott insulating state appears at S = - 0.50, where all Ni ions are divalent. With increasing oxygen content, superconductivity appears at S = 0.00 and higher, above approximately 25 GPa, passing through Anderson localization at S = - 0.16. The superconducting transition temperature (Tc) decreases with increasing pressures for both S = 0.00 and +0.12, with the pressure dependence of T c being much stronger in the latter than in the former.