Type II $t-J$ model in superconducting nickelate Nd$_{1-x}$Sr$_x$NiO$_2$

TL;DR

This paper introduces the Type II $t-J$ model for superconducting nickelates, revealing two distinct mechanisms for $d$ wave superconductivity driven by spin coupling, combining features of cuprate and heavy fermion physics.

Contribution

The study proposes a new Type II $t-J$ model with spin-one holes, unifies two superconductivity mechanisms, and develops a three-fermion parton theory to analyze doping phases.

Findings

Two mechanisms for $d$ wave superconductivity identified.

Spin-one holes can condense or form large Fermi surfaces.

Unified framework shows interplay of cuprate and heavy fermion physics.

Abstract

The recent observation of superconductivity at relatively high temperatures in hole doped NdNiO$_2$ has generated considerable interest, particularly due to its similarity with the infinite layer cuprates. Building on the observation that the Ni$^{2+}$ 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 new 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 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 superconductor. 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.