Two-orbital model for possible superconductivity pairing mechanism in nickelates

The newly synthesized strontium doped $R{\mathrm{NiO}}_{2}$ ($R$=Nd, Pr, and La) superconductors have stimulated extensive interests in understanding their pairing mechanism nature. Here we study the this family from a two-orbital model comprising Ni- $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ $3{d}_{xy}$ orbitals, equipped with extended Hubbard interactions induced low-energy effective superexchange interactions. We then symmetry system by using large scale variational Monte Carlo approach. Our results yield intraorbital ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave singlet as leading nickelates, which is analogous to cuprates. However, there exist two important differences between physical properties of families due fact that at low Sr-doping regime, while Ni-$3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals remain half-filled singlyins occupied form Mott-insulating background, Ni-$3{d}_{xy}$ accommodate nearly all extra holes, move freely on background. first difference lies single-particle aspect: degree freedom remains Mott insulating spectra weight pinned down zero dopings, one behaves Fermi liquid near 1. second huge intra-$3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-orbital gap actually pseudogap has nothing do SC, small intra-$3{d}_{xy}$-orbital serves true superconducting gap, related ${T}_{c}$ via BCS relation. Both can be verified angle-resolved photoemission spectrum.