Nature Commun. 5, 4566 (2014)

Topological protection ensures that the conductive surface states of topological insulators are robust against defects and impurities — making them prime candidates for spintronic and quantum-computing applications. But three-dimensional topological insulators typically also have conducting states in the bulk, which makes it difficult to study or use the topological properties of the surface states. Nan Xu and colleagues now provide evidence for topological surface states atop a truly insulating bulk crystal.

Kondo insulators are materials with strongly correlated electrons that give rise to a bulk bandgap at low temperatures due to a hybridization of localized and conduction electrons. Using photoemission spectroscopy, Xu et al. show that the surface states of the Kondo insulator samarium hexaboride are spin polarized, with the spin locked to the crystal momentum, fulfilling time-reversal and crystal symmetries.

The reported spin texture is precisely what one would expect of surface states with topological origins, supporting recent theoretical proposals. As topological Kondo insulators are insulating in the bulk they provide an ideal platform for probing the topological behaviour of the surface states.