Large insulating gap in topological insulators induced by negative spin-orbit splitting

In a cubic topological insulator (TI), there is a band inversion whereby the s-like Gamma(6c) conduction band is below the p-like Gamma(7v) + Gamma(8v) valence bands by the inversion energy Delta(i) < 0. In TIs based on the zinc-blende structure such as HgTe, the Fermi energy intersects the degenerate Gamma(8v) state so the insulating gap E-g between occupied and unoccupied bands vanishes. To achieve an insulating gap E-g > 0 critical for TI applications, one often needs to resort to structural manipulations such as structural symmetry lowering (e. g., Bi2Se3), strain, or quantum confinement. However, these methods have thus far opened an insulating gap of only <0.1 eV. Here we point out that there is an electronic rather than structural way to affect an insulating gap in a TI: if one can invert the spin-orbit levels and place Gamma(8v) below Gamma(7v) (negative spin-orbit splitting), one can realize band inversion (Delta(i) < 0) with a large insulating gap (E-g up to 0.5 eV). We outline design principles to create negative spin-orbit splitting: hybridization of d orbitals into p-like states. This general principle is illustrated in the filled tetrahedral structures (FTS) demonstrating via GW and density functional theory (DFT) calculations E-g > 0 with Delta(i) < 0, albeit in a metastable form of FTS.