Competing topological phases in a non-Hermitian time-reversal symmetry broken Bernevig-Hughes-Zhang model

The Bernevig-Hughes-Zhang (BHZ) model, which serves as a cornerstone in the study of the quantum spin Hall insulators, showcases robust spin-filtered helical edge states in a nanoribbon geometry. In the presence of an in-plane magnetic field, these (first-order) helical states gap out to be replaced by second-order corner states under suitable open boundary conditions. Here, we show that the inclusion of a spin-dependent non-Hermitian balanced gain/loss potential induces competition between these first- and second-order topological phases. Surprisingly, the previously dormant first-order helical edge states resurface as the non-Hermitian effect intensifies, effectively neutralizing the role played by the magnetic field. By employing the projected spin spectra and the spin Chern number, we conclusively explain the resurgence of the first-order topological properties in the time-reversal symmetry-broken BHZ model in the presence of nonhermiticity. Finally, the biorthogonal spin-resolved Berry phase, exhibiting a nontrivial winding, definitively establishes the topological nature of these revived edge states, emphasizing the dominance of nonhermiticity over the magnetic field.