Emergence of a hidden topological insulator phase in hybrid halide perovskites

Abstract

We report the emergence of a hidden topological insulator phase in hybrid halide perovskites under the influence of a high tunneling electric field generated by bringing a scanning tunneling microscope tip to the close proximity of the sample. As witnessed from scanning tunneling spectroscopy, the low-temperature orthorhombic phase of the compounds in its equilibrium behaves like a normal insulator with a bulk bandgap when the material was probed with a tip placed far away. As the set-current vis-à-vis the tunneling electric field was increased with an approach of the tip, a Stark-effect-induced shift of the bands toward the Fermi level (EF) was observed with a closure of the transport gap. At a (high) critical set-current, the material turned into a 3D topological insulator with gapless surface states and a Dirac point below the EF. The separation of the Dirac point (ED) from the Fermi level (|ED–EF|) could interestingly be tuned through halide modification in the metal halide perovskites, that is, the spin–orbit coupling parameter in the compound.