Probing Edge State Conductance in Ultra-Thin Topological Insulator Films

Quantum spin Hall (QSH) insulators have unique electronic properties, comprising a band gap in their 2D interior and 1D spin-polarized edge states in which current flows ballistically. In scanning tunneling microscopy (STM), the edge states manifest themselves as an enhanced local density of states (LDOS). However, there is a significant research gap between the observation of edge states in nanoscale spectroscopy, and the detection of ballistic transport in edge channels which typically relies on transport experiments with microscale lithographic contacts. Here, few-layer films of the 3D topological insulator (Bi(x)Sb1-x$_{1-x}$)(2)Te-3 are studied, for which a topological transition to a 2D topological QSH insulator phase has been proposed. Indeed, an edge state in the LDOS is observed within the band gap. Yet, in nanoscale transport experiments with a four-tip STM, two-quintuple-layer films do not exhibit a ballistic conductance in the edge channels and thus no QSH edge states. This demonstrates that the detection of edge states in spectroscopy can be misleading with regard to the identification of a QSH phase. In contrast, nanoscale multi-tip transport experiments are a robust method for effectively pinpointing ballistic edge channels, as opposed to trivial edge states, in quantum materials.