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δ-Bonding and Spin-Orbit Coupling Make SrAg4Sb2 a Topological Insulator
H W T Morgan1, W T Laderer1, A N Alexandrova1
1University of California, Los Angeles, Department of Chemistry and Biochemistry, 607 Charles E Young Drive East, Los Angeles, CA, 90034, USA.
Abstract:
Bonding interactions and spin-orbit coupling in the topological insulator SrAg4Sb2 are investigated using DFT with orbital projection analysis. Ag-Ag delta bonding is a key ingredient in the topological insulating state because the delta antibonding band forms a band inversion with the 5 s sigma bonding band. Spin-orbit coupling is required to lift d orbital degeneracies and lower the antibonding band enough to create the band inversion. These bonding effects are enabled by a longer-than-covalent Ag-Ag distance in the crystal lattice, which might be a structural characteristic of other transition metal based topological insulators. A simplified model of the topological bands is constructed to capture the essence of the topological insulating state in a way that may be engineered in other materials.
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