As a result of a joint intensive work of several groups from five different countries, including Basque Country, a new wide class of topological insulators - materials that are insulators in the bulk but conductors at the surface - with technologically very promising properties has been discovered.

Topological insulators, first observed in 2007, are materials that, while being an insulator inside or in the bulk, behave as metals at the surface.

Their unique properties can be used for new applications in spintronics and quantum computation as well. Physicists from Donostia International Physics Center (DIPC) and Centro de Fisica de Materiales (CFM) - joint center CSIC-UPV/EHU - have participated in this research project. The results have been just published in the prestigious journal Nature Communications.

The surface metallic character of these only-bulk-insulators is due to a special electronic state confined at the surface. However, to take advantage of this singular phenomenon, a tuning of this conducting state is often required.

The international research team has demonstrated that most of the ordered ternary compounds (Germanium, Tin, Lead, Bismuth, Antimony, Tellurium and Selenium complexes) are actual three dimensional topological insulators showing properties distinct from those found in other binary compounds.

In particular, their metallic state is buried 1-2 nm in depth, making it more stable and more protected against any surface modification, while its spin characteristics allow magnetic modifications of the material.

The existence of these exotic topological insulators was first theoretically predicted by scientists from Tomsk (Russia), Halle (Germany) and Donostia. Following this prediction, chemists from Baku (Azerbajan) grew a single crystal sample of one of those compounds. The sample was then studied by experimentalists in Zurich (Switzerland) and Hamburg (Germany), confirming all theoretical predictions.

The finding provides a promising pathway to tune both electronic and spin (that is, magnetic) properties by using different compounds and confirms the possibility to grow topological insulators with deep-laying, self-protecting and, thus, technologically relevant conducting states.