# Is there a superconducting phase in the half-filled ionic Hubbard model ?

## TL;DR

This study examines the ionic Hubbard model at half-filling under strong correlations, finding no stable superconducting phase in the ground state, and highlighting a transition from antiferromagnetic order to a band insulator.

## Contribution

It provides a detailed analysis of the phase diagram of the ionic Hubbard model in the strong correlation limit, demonstrating the absence of a stable superconducting phase.

## Key findings

- Superconducting phases are absent in the ground state at strong correlations.
- Antiferromagnetic order transitions directly to a band insulator without superconductivity.
- The paramagnetic superconducting phase is metastable and higher in energy than the AF Mott insulator.

## Abstract

We investigate the ionic Hubbard model (IHM) at half-filling in the limit of strong correlations and large ionic potential. The low energy effective Hamiltonian in this limit, obtained by a similarity transformation, is a modified $t-J$ model with effective second neighbour hopping terms. We explore the possibilities of d-wave pairing and extended s-wave pairing superconducting (SC) phases on a two dimensional square lattice at zero temperature within a Gutzwiller projected renormalized mean field theory. In the sector of solutions that forbid spin ordering, the system shows a finite non-zero d-wave as well as extended s-wave pairing amplitude for $\Delta \sim U \gg t$. The width of the superconducting phase in $U-\Delta$ regime shrinks with increase in $U$ and $\Delta$, though the extended s-wave pairing phase is higher in energy than the d-wave pairing superconducting phase. But in a spin resolved renormalized mean field calculation, which allows for an antiferromagnetic (AF) order along with the d-wave or extended s-wave pairing, the SC phase is no longer viable and the system shows a direct transition from an AF ordered phase to a paramagnetic band insulator. Except for a thin sliver of a half-metallic AF phase close to the AF transition point, most of the AF ordered phase is a Mott insulator. We benchmarked the AF Mott insulator to band insulator transition within the Gutzwiller projected renormalized mean field theory against the dynamical mean field theory (DMFT) solved using continuous time quantum Monte-Carlo (CTQMC). Our work suggests that the ground state phase diagram of the IHM at half-filling in the limit of extreme correlations does not have any SC phase. The SC phase seen in the paramagnetic sector is a metastable phase, being higher in energy than the AF Mott insulator phase.