Type-II t-J model in superconducting nickelate Nd1-xSrxNiO2

Abstract

The recent observation of superconductivity at relatively high temperatures in hole-doped NdNiO2 has generated considerable interest, particularly due to its similarity with the infinite layer cuprates. Building on the observation that the Ni2+ ions resulting from hole doping are commonly found in the spin-triplet state, we introduce and study a variant of the t-J model in which the holes carry S=1. We name this model the type-II t-J model. We find two distinct mechanisms for d-wave superconductivity. In both scenarios the pairing is driven by the spin coupling J. However, coherence is gained in distinct ways in these two scenarios. In the first case, the spin-one holes condense, leading to a d-wave superconductor along with spin-symmetry breaking. Different orders, including spin-nematic orders, are possible. This scenario is captured by a spin-one slave boson theory. In the second scenario, a coherent and symmetric d-wave superconductor is achieved from "Kondo resonance": Spin-one holes contribute two electrons to form a large Fermi surface together with the spin-1/2 singly occupied sites. The large Fermi surface then undergoes d-wave pairing because of spin coupling J, similar to heavy fermion superconductors. We propose a three-fermion parton theory to treat these two different scenarios in one unified framework and calculate its doping phase diagram within a self-consistent mean-field approximation. Our study shows that a combination of "cuprate physics" and "heavy fermion physics" may emerge in the type-II t-J model.