Experiments in the high-temperature superconductor ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ have revealed both high $T_c$ and strange metal behaviors over a wide pressure range. While first-principles density functional theory has predicted weakly correlated bands that cannot explain these key observations, we report here strongly correlated electronic band structure calculations that reveal the dual nature of Ni-$d$ electrons with almost localized $d_{z^2}$ orbitals due to on-site Coulomb repulsion and flat bands of Ni-$d_{x^2-y^2}$ and Ni-$d_{z^2}$ quasiparticles near the Fermi energy. We find that the quasiparticle effective masses are greatly enhanced by Hund's rule coupling and their lifetimes are inversely proportional to temperature, which explains the experimentally observed strange metal behavior in the normal state. Our calculations also reveal strong antiferromagnetic spin correlations of Ni-$d$ electrons, which may provide the pairing force of quasiparticles for high-temperature superconductivity. The presence of flat bands and the interplay of orbital-selective Mott, Hund, and Kondo physics lay the basis for a two-component theory of its pairing mechanism and make ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ a unique platform for exploring rich emergent quantum many-body phenomena in the future.

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• Electronic structure
• Strongly correlated systems
• Superconductors
• DFT+DMFT
• First-principles calculations