Motivated by the report of superconductivity in bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ at high pressure, we examine the interacting electrons in this system. First-principles many-body theory is utilized to study the normal-state electronic properties. Below 100 K, a multiorbital non-Fermi-liquid state resulting from a loss of Ni-ligand coherence within a flat-band-dominated low-energy landscape is uncovered. The incoherent low-temperature Fermi surface displays strong mixing between $\mathrm{Ni}\text{−}{d}_{z^2}$ and $\mathrm{Ni}\text{−}{d}_{x^2-y^2}$ orbital character. In a model Hamiltonian picture, spin fluctuations originating mostly from the $\mathrm{Ni}\text{−}{d}_{z^2}$ orbital give rise to strong tendencies towards a superconducting instability with a $B_{1g}$ or $B_{2g}$ order parameter. The dramatic enhancement of $T_{\mathrm{c}}$ in pressurized ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ is due to stronger $\mathrm{Ni}\text{−}{d}_{z^2}$ correlations compared to those in the infinite-layer nickelates.

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• Superconductivity
• Strongly correlated systems
• Many-body techniques