Unusual superexchange pathways in a Ni triangular lattice of NiGa$_2$S$_4$ with negative charge-transfer energy

TL;DR

This study investigates the electronic structure of NiGa$_2$S$_4$, revealing unusual superexchange pathways facilitated by a negative charge-transfer energy and a dominant S 3$p$ hole character in the ground state.

Contribution

It uncovers the role of negative charge-transfer energy and S 3$p$ holes in enabling unconventional superexchange interactions in a Ni triangular lattice.

Findings

Charge-transfer energy is approximately -1 eV.

Ground state dominated by Ni 3$d^9L$ configuration.

Enhanced superexchange between third nearest neighbors.

Abstract

We have studied the electronic structure of the Ni triangular lattice in NiGa$_2$S$_4$ using photoemission spectroscopy and subsequent model calculations. The cluster-model analysis of the Ni 2$p$ core-level spectrum shows that the S 3$p$ to Ni 3$d$ charge-transfer energy is $\sim$ -1 eV and the ground state is dominated by the $d^9L$ configuration ($L$ is a S 3$p$ hole). Cell perturbation analysis for the NiS$_2$ triangular lattice indicates that the strong S 3$p$ hole character of the ground state provides the enhanced superexchange interaction between the third nearest neighbor sites.