Strong pairing from doping-induced Feshbach resonance and second Fermi liquid through doping a bilayer spin-one Mott insulator: application to La$_3$Ni$_2$O$_7$

We study the physics of doping a bilayer spin-one Mott insulator with $S=1$ moment formed by $d_{z^2}$ and $d_{x^2-y^2}$ orbitals, motivated by the superconducting La$_3$Ni$_2$O$_7$. When the $d_{z^2}$ orbital is Mott localized, the itinerant $d_{x^2-y^2}$ orbital inherits a large inter-layer antiferromagnetic coupling $J_\perp$, but we show here that a one-orbital bilayer $t-J_{\parallel}-J_\perp$ model for $d_{x^2-y^2}$ orbital only misses the important ingredients of polaron formation between hole in $d_{x^2-y^2}$ orbital and local spin moment from the $d_{z^2}$ orbital. We argue that the minimal model should be the type II t-J model proposed by the two of us. Through DMRG calculation of the type II t-J model in a two-leg ladder configuration ($L_z=2$, $L_y=1$, $L_x\rightarrow \infty$), we find that the pairing gap increases with hole doping $x$ (per site per layer) until $x \approx 0.5$ and then decreases, in contrast to the conventional t-J model.