Quantum phase transition driven by competing intralayer and interlayer hopping of Ni-$d_{3z^2-r^2}$ orbitals in bilayer nickelates

Abstract: Bilayer nickelates exhibit high-temperature superconductivity under proper hydrostatic pressure or epitaxial strain, signifying the emergence of quantum phase transitions whose physical mechanisms remain unclear. Using a minimal bilayer Hubbard model incorporating only the essential Ni-$d_{3z^2-r^2}$ orbitals, we demonstrate that a phase transition naturally arises from tuning the ratio of intralayer to interlayer hopping amplitudes. The transition point separates regimes with a rich interplay between superconducting and density-wave orders. In the regime with weaker intralayer hopping, quasi-long-range spin-density-wave and pair-density-wave orders coexist, with the former being dominant. Across the transition, the spin-density-wave order becomes short-ranged, accompanied by the emergence of the quasi-long-range charge-density-wave order. Most significantly, superconductivity is dramatically enhanced in this regime, though it no longer exhibits the pair-density-wave signature. This quantum phase transition, driven by the competition between intralayer and interlayer hopping, provides a plausible microscopic explanation for the experimentally observed correlation between the superconducting transition temperature and ratio of out-of-plane to in-plane lattice constants. Our findings may assist future efforts in optimizing experimental conditions to further enhance superconductivity in bilayer nickelates.