SFB colloquium Claessen

Topological insulators go elemental

Ralph Claessen

Topological insulators (TIs) are perfect insulators in the bulk, while their boundaries carry topologically protected metallic surface or edge states. These states are spin-polarized and helical in nature, i.e. the spin is intrinsically locked to the electron´s momentum. As a consequence, backscattering is largely impeded, thereby strongly reducing the ohmic losses of surface (or edge) transport in TIs, with obvious application potential in semiconductor electronics. Furthermore, it has been proposed that TI-superconductor interfaces may host Maiorana fermion-like zero-energy quantum states of extraordinary large lifetimes, which could serve as solid-state realization of a qubit, the basic element for quantum computing.

While many TI materials have been discovered since their original prediction about a decade ago, most of them seem unsuitable for actual applications. Either their complex composition makes it difficult to realize a truly insulating defect-free bulk (e.g., Bi₂(Se,Te)₃), or their volume band gap is too small to facilitate room-temperature applications (e.g., HgTe). Against this background the hunt is on for simple, e.g., elemental TIs with large band gaps. Here I will report on our experimental activities in epitaxial thin film growth of such materials and their characterization by photoelectron spectroscopy and scanning tunnelling microscopy. My examples range from a-Sn (aka grey tin) as a 3D topological material to "stanene" and "bismuthene" (graphene-like 2D modifications of Sn and Bi) as candidates for the Quantum Spin Hall effect.