Electronic structure of Ruddlesden-Popper nickelates: Strain to mimic the effects of pressure

Signatures of superconductivity under pressure have recently been reported in the bilayer ${\mathrm{La}}_{3}{\mathrm{Ni}}_{2}{\mathrm{O}}_{7}$ and trilayer ${\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ Ruddlesden-Popper (RP) nickelates with the general chemical formula ${\mathrm{La}}_{n+1}{\mathrm{Ni}}_{n}{\mathrm{O}}_{3n+1}$ ($n$ = number of perovskite layers along the $c$-axis). The emergence of superconductivity is always concomitant with a structural transition in which the octahedral tilts are suppressed, bringing the apical Ni-O-Ni angle to ${180}^{\ensuremath{\circ}}$ and causing an increase in the out-of-plane ${d}_{{z}^{2}}$ orbital overlap. Due to this strong interlayer coupling, a flat band of pure ${d}_{{z}^{2}}$ character crosses the Fermi level. Here, using first-principles calculations, we explore biaxial strain (both compressive and tensile) as a means to mimic the electronic structure characteristics of RP nickelates (up to $n=5$) under hydrostatic pressure. Our findings highlight that strain enables the decoupling of the structural and electronic structure effects obtained under hydrostatic pressure: While compressive strain brings the apical Ni-O-Ni angle closer to ${180}^{\ensuremath{\circ}}$, it shifts the ${d}_{{z}^{2}}$ flat bands away from the Fermi energy, giving rise to a more cupratelike electronic structure. In contrast, tensile strain reduces the apical Ni-O-Ni angle (to values of $\ensuremath{\sim}{160}^{\ensuremath{\circ}}$), but it recovers the flat ${d}_{{z}^{2}}$ band at the Fermi level appearing in the bilayer and trilayer RP nickelates under pressure. Overall, strain represents a promising way to tune the electronic structure of RP nickelates and could be an alternative route to achieve superconductivity at ambient pressure in this family of materials.