Structural and Electronic Evolution of Bilayer Nickelates Under Biaxial Strain

The discovery of superconductivity above 40 K in strained bilayer nickelates provides a tunable platform for exploring unconventional superconductivity under ambient pressure. However, the microscopic mechanisms driving this strain-induced state and how they contrast with high-pressure effects remain poorly understood. Here we show, using first-principles calculations across the rare-earth series RE3Ni2O7, that in-plane compressive strain systematically alters atomic structures and electronic hybridization. Wannier downfolding reveals that 2.5% compression enhances interlayer and in-plane orbital hoppings while significantly enlarging the crystal field splitting, which shifts the Ni d_z² bands entirely below the Fermi level. These findings elucidate the fundamental differences between strain-tuned and high-pressure electronic phases, offering critical guidance for optimizing superconducting properties in lanthanide-based thin films. Understanding how high-temperature superconductivity emerges beyond high pressure is a central goal for newly discovered nickelate families. Using first-principles calculations, the authors show that applying in-plane compressive strain to bilayer nickelates alters their atomic and electronic structures, uncovering the fundamental physics of strain-induced superconductivity.