We clarify the pairing mechanism of high-$T_c$ superconductivity in bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under high pressure by employing the static auxiliary field Monte Carlo approach to simulate a minimal effective model that contains local $d_{z^2}$ interlayer spin singlets and metallic $d_{x^2-y^2}$ bands. Superconductivity is induced when the local spin singlet pairs are mobilized and attain long-distance phase coherence by hybridization with the metallic bands. When projected onto realistic Fermi surfaces, it yields a nodeless $s$-wave gap on the $\gamma$ Fermi surface, and extended $s$-wave gaps of the same (opposite) sign on the $\alpha$ ($\beta$) Fermi surface due to its bonding (antibonding) character, with nodes or gap minima along the diagonal direction of the two-dimensional Brillouin zone. We find a dual role of the hybridization that not only induces global phase coherence but also competes with the spin singlet formation. This leads to a tentative phase diagram where $T_c$ varies nonmonotonically with the hybridization, in good correspondence with experimental observations. A roughly linear relation is obtained for realistic hopping and hybridization parameters: $T_c\approx 0.04-0.05J$, where $J$ is the interlayer superexchange interaction. We emphasize the peculiar tunability of the bilayer structure and propose that $T_c$ may be further enhanced by hole doping or applying uniaxial pressure along the $c$ axis on superconducting ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$. Our work provides reliable numerical evidence for the pairing mechanism of high-$T_c$ superconductivity in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ and points out a potential route to achieve even higher $T_c$.

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• Superconductivity
• Nickelates
• Unconventional superconductors
• Monte Carlo methods