The discovery of high-Tc superconductivity in Ruddelsden–Popper bilayer nickelates under applied pressure and/or compressive strain has opened a promising avenue to explore the interplay of multiorbital intra- and interlayer Cooper pairing in correlated oxide systems. In particular, dominant interlayer pairing may naturally give rise to a bonding–antibonding s± gap structure that directly reflects the bilayer geometry, leading to experimental signatures distinct from conventional d-wave pairing. In the first part of my talk I will explore possible superconducting gap structures as observed in STM and propose experimentally accessible observables to distinguish an interlayer-driven s± state from competing pairing symmetries.
Beyond superconductivity, nickelates comprise two main material families: low-valence Ni(3d9−δ) compounds and Ruddelsden–Popper systems with Ni(3d8±δ) alence. While both host NiO2 square planes, they differ crucially in the presence or absence of apical oxygen atoms. A potential bridge between these families is provided by controlled reduction of the bilayer compound La3Ni2O7, yielding La3Ni2O6.5 and La3Ni2O6. These reduced materials remain largely unexplored experimentally but exhibit intriguing correlation effects in theory. Using advanced first-principles many-body calculations, we analyze their electronic structure and uncover distinct mechanisms of Mott criticality, as well as unconventional low-temperature behavior. Together, these results establish a broader framework connecting superconductivity, electronic correlations, and structural tuning in nickelate systems.