Spin density wave rather than tetragonal structure is prerequisite for superconductivity in La3Ni2O7-δ

The pressure-induced high-temperature (Tc) superconductivity in nickelates La3Ni2O7-δ has sparked significant interest to explore its superconductivity at ambient pressure. Whether the pressure-stabilized tetragonal structure is a prerequisite for achieving nickelate superconductivity at ambient pressure is under hot debate. Here, by post-annealing in high oxygen pressure environment, tetragonal La3Ni2O6.92 single crystals are successfully obtained at ambient pressure, which exhibits a metallic behavior without a SDW transition. Moreover, superconductivity is also absent at high pressures up to ~ 70 GPa. Furthermore, by utilizing Helium as the pressure medium, we found that the superconductivity in pressurized orthorhombic La3Ni2O6.85 is achieved in orthorhombic structure rather than tetragonal structure claimed previously. All these findings demonstrate that tetragonal structure is not prerequisite for achieving superconductivity in La3Ni2O7-δ. Finally, our present work suggests a deep correlation between SDW order and superconductivity, which imposes stringent constraints on the underlying mechanism for pressure-induced superconductivity in nickelates. The authors obtain tetragonal La3Ni2O6.92 single crystals at ambient pressure by post-annealing in a high-oxygen-pressure environment. Experimental results on these samples suggest that the tetragonal structure is not a prerequisite for achieving superconductivity in La3Ni2O7.