Ultra-Stable Ferrimagnetic Second-Order Topological Insulator in 2D Metal-Organic Framework

TL;DR

This paper predicts a 2D ferrimagnetic second-order topological insulator in a metal-organic framework, demonstrating ultra-stable corner states resilient to various perturbations, advancing potential spintronic applications.

Contribution

It introduces the first 2D ferrimagnetic SOTI in a MOF, with highly robust corner states, expanding the understanding of topological phases in magnetic materials.

Findings

Cr(pyz)2 exhibits a nontrivial real Chern number in the spin-up channel.

Corner states remain stable under defects, strain, electric fields, and ligand rotation.

The material provides a promising platform for spintronic device development.

Abstract

Two-dimensional (2D) magnetic second-order topological insulators (SOTIs) exhibit distinct topological phases characterized by spin-polarized zero-dimensional (0D) corner states, which have garnered significant interest. However, 2D ferrimagnetic (FiM) SOTIs, particularly those that simultaneously exhibit ultra-stable corner states, are still lacking. Here, based on first-principles calculations and theoretical analysis, we reveal such SOTI state in a 2D metal-organic framework (MOF) material, Cr(pyz)2 (pyz = pyrazine). This material exhibits FiM ground state with an easy axis aligned along [001] direction. It hosts a nontrivial real Chern number in the spin-up channel, enabled by PT symmetry, with 0D corner states observable in disk. In contrast, the spin-down channel exhibits a trivial gapped bulk state. Notably, the topological corner states in monolayer Cr(pyz)2 show high robustness, even if the symmetries are broken by introducing defects, the corner states persist. We also considered other external perturbations, including uniaxial/biaxial strain, ligand rotation, and electric fields, the corner states still remain stable. Even more, the energy positions of the corner states are also nearly unchanged. This work is the first to identify ultra-stable FiM SOTI state in the MOF system, and provide an ideal platform for future experimental investigations and applications in spintronic devices.