Strong Pairing Originated from an Emergent ℤ_2 Berry Phase in La_3Ni_2O_7

The recent discovery of high-temperature superconductivity in La_3Ni_2O_7 offers a fresh platform for exploring unconventional pairing mechanisms. Starting with the basic argument that the electrons in d_z^2 orbitals nearly form local moments, we examine the effect of the Hubbard interaction U on the binding strength of Cooper pairs based on a single-orbital bilayer model with intralayer hopping t_ and interlayer super-exchange J_⊥. By extensive density matrix renormalization group calculations, we observe a remarkable enhancement in binding energy as much as 10-20 times larger with U/t_ increasing from 0 to 12 at J_⊥/t_∼ 1. We demonstrate that such a substantial enhancement stems from a kinetic-energy-driven mechanism. Specifically, a ℤ_2 Berry phase will emerge at large U due to the Hilbert space restriction (Mottness), which strongly suppresses the mobility of single particle propagation as compared to U=0. However, the kinetic energy of the electrons (holes) can be greatly restored by forming an interlayer spin-singlet pairing, which naturally results in a superconducting state even for relatively small J_⊥. An effective hard-core bosonic model is further proposed to estimate the superconducting transition temperature at the mean-field level.