Antiferromagnetic second-order topology in two-dimensional NiRuCl₆

In recent years, 2D second-order topological insulators (SOTIs) have garnered considerable interest because of their unique properties. However, only the FeSe monolayer with four corner states (two occupied and two unoccupied states) near the Fermi level has been reported to be a candidate for 2D intrinsic antiferromagnetic SOTIs in theory. The limited amount of antiferromagnetic SOTIs has hindered future research, and corner states should be at the Fermi level in order to manifest interesting physics. Herein, we propose NiRuCl6 as a candidate for 2D antiferromagnetic SOTIs with corner states strictly at the Fermi level. Without spin-orbit coupling (SOC), NiRuCl6 is an antiferromagnetic half-metal with a compensating magnetic moment and decoupled spin bands. In the spin-up channel, NiRuCl6 hosts a nontrivial gap of 1.11 eV, where zero-dimensional corner states appear. In the spin-down channels, NiRuCl6 hosts metallically behaved bands, where a spin-polarized quadratic Weyl point emerges. With SOC, two spin bands are coupled, and NiRuCl6 becomes an antiferromagnetic SOTI with three degenerate corner states at the Fermi level inside the SOC-induced gap with a value of 0.11 eV. Remarkably, the corner states in NiRuCl6 are resistant to changes in SOC strength and magnetization orientation. We also reveal that the phononic second-order topology and corner vibrational modes appear in the phonon dispersion curves of NiRuCl6. The presented results improve the general understanding of antiferromagnetic SOTIs and contribute to the prediction of materials with ideal corner states at the Fermi level, thereby advancing the field of topological antiferromagnetic spintronics.