Imaging the Tunable Topological Surface State in GeBi_2-x Sb_xTe₄

3D Topological insulators (TI) host robust, spin-momentum locked, linearly dispersing surface states which have lead to the successful observation of quantum anomalous hall effect and chiral Majorana modes and also promise interesting applications. Isolation of these topological surface states is often hindered by large contributions by bulk bands, which can be mediated by a Dirac point in the band gap near the fermi energy. Here we investigate a new TI system, GeBi_2-x Sb_xTe₄ , using low temperature scanning tunneling microscopy. We track the surface state band structure with quasiparticle interference as Bi is substituted with Sb. Interestingly, we find Sb substitution effectively p dopes the sample, shifting the Dirac point from approximately -280 meV below the Fermi energy for GeBi₂Te₄ to close to the Fermi energy for intermediate substitutions. As the Dirac point is located within the band gap, tuning with substitution could provide enhanced isolation of topological surface states.