Superconductivity and density wave transition in Ruddlesden‒Popper phase nickelates

Recently, the discovery of superconductivity in Ruddlesden‒Popper (RP) phase nickelates La₃Ni₂O₇and La₄Ni₃O₁₀under high pressure has stimulated intensive research interest in the community. In both La₃Ni₂O₇and La₄Ni₃O₁₀, previous studies presented a similar pressure-dependent phase diagram between superconductivity and density wave (DW) transition, in which the DW transition is suppressed with increasing pressure before the entrance of superconductivity. Adopting the molten salt method and post high-pressure oxygen annealing, we have synthesized the crystal of tetragonal La₃Ni₂O₇and tetragonal La₄Ni₃O₁₀ without DW transition at ambient pressure. The robust metallic ground state can survive up to ~70 GPa for tetragonal La₃Ni₂O₇and 160 GPa for tetragonal La₄Ni₃O₁₀, where both the DW and superconducting transitions are absent. Furthermore, we also observed superconductivity with the onset T_c of 64 K in a hybrid nickelate superconductor of La₅Ni₃O₁₁, which is formed by alternative stacking of La₃Ni₂O₇ with and La₂NiO₄ along the c axis. With increasing pressure, the phase diagram of La₅Ni₃O₁₁ is mapped out, where the DW transition shows a different evolution from the previous La₃Ni₂O₇ and La₄Ni₃O₁₀. The DW transition temperature continuously increases from 170 K at ambient pressure to approximately 210 K at 12 GPa, which abruptly fades out in a first-order manner. These results shed new light on the superconducting mechanism, especially for the relationship between density wave transition and superconductivity in the RP-phase nickelates.