Resonant tunneling via Dirac electron states in a topological-insulator / semiconductor junction

A defining characteristic of the topological classification of solids is the existence of gapless modes at the interface of materials with unequal topological invariants. In the context of the $Z_{2}$ topological invariant, this has been verified by the spectroscopic observation of spin-polarized Dirac electron states at the interface of three-dimensional topological insulators (3D TIs) and the vacuum. By performing tunneling spectroscopy in heterojunction devices based on the TI (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ and band insulator InP, we report the observation of such states at the interface between a 3D TI and a topologically trivial solid. In an applied magnetic field, the tunneling conductance through these heterojunctions resonates due to the formation of Landau levels at the interface; the observed energy and angular dependence indicates these carriers are two-dimensional surface electrons obeying a Dirac-like energy dispersion. Furthermore, the composition $x$ dependence of the deduced Fermi velocity and Dirac point energy agree with previous photoemission observations for the surface states of (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ with a vacuum interface. This study gives strong evidence for the existence of interface topological states in solid heterojunction, which will provide new functional devices based on TI.