Decoupling Structural and Electronic Drivers of Superconductivity in Tetragonal La3Ni2O7+δ

Abstract

The recent discovery of high-temperature superconductivity observed in pressurized La3Ni2O7-δ has stimulated intense interest in the superconducting mechanism of Ruddlesden–Popper nickelates. La3Ni2O7 exhibits a maximum superconducting transition temperature (Tc) near 80 K accompanied by a structural phase transition. This raises an important question of whether the structural transition is a prerequisite for superconductivity and whether the tetragonal phase of La3Ni2O7 can host superconductivity at ambient pressure. Here, we report high-pressure studies on polycrystalline samples of tetragonal La3Ni2O7+δ and Co-doped La3Ni2-xCoxO7+δ (x = 0.05, 0.10), prepared at ambient pressure via high oxygen pressure annealing. Combining high-pressure electrical transport and low-temperature synchrotron X-ray diffraction measurements, we reveal that tetragonal La3Ni2O7+δ exhibits superconductivity above 7 GPa, with a maximum Tc of ∼75 K near 16 GPa, while maintaining its high-symmetry tetragonal structure throughout the entire pressure range with no structural phase transition occurs in the superconducting regime. Instead, theoretical calculations reveal a pressure-induced electronic reconstruction, characterized by the metallization of the Ni-dz2 band, that is closely associated with the emergence of superconductivity. Co-doping induces anisotropic lattice distortions analogous to the strain effects in thin films yet markedly suppresses the Tc under high pressure. Our results demonstrate that the tetragonal symmetry alone is insufficient to induce superconductivity and highlight the significance of pressure-induced electronic reconstruction in the superconducting state. These findings clarify the contribution of pressure-induced electronic reconstruction to superconductivity in Ruddlesden–Popper nickelates, challenging the prevailing structural-transition-driven scenario and providing a pathway toward the rational design of nickelate superconductors.