Robustness and nature of the experimentally realized second-order topological quantum magnet

Determine the robustness and characterize the physical nature of the experimentally realized second-order topological quantum magnet in the breathing square-lattice J1–J2 antiferromagnetic Heisenberg model (as realized with Ti atoms on MgO), including how its defining features persist or change under parameter variations and perturbations.

Background

Topological quantum magnets, particularly higher-order symmetry-protected phases, are a frontier in condensed matter physics. A second-order topological quantum magnet with protected corner excitations was recently realized experimentally, raising fundamental questions about its stability and defining characteristics.

This work maps the phase diagram of the breathing frustrated square-lattice J1–J2 Heisenberg model and relates many-body results to an auxiliary fermion (spinon) description, providing groundwork to address the unresolved issue of robustness and the precise nature of the second-order topological phase.