We have obtained an important evidence for a long-standing debate about the metallic surface electronic structure of samarium hexaboride (SmB₆). SmB₆ is known to be an insulator only at low temperature due to the strong electron correlation effect (Kondo effect). The debate is whether the insulating phase of SmB₆ is a topological insulator hosting metallic surface electronic state or not. The surface electronic structure of SmB₆ obtained so far is complex and difficult to interpret. In this work, we observed the surface states from the different crystal orientation and succeeded to simplify the surface states significantly. New concert electronic phenomena driven by electron correlation and "twist (topology)" effect is expected.

Abstract

The peculiar metallic electronic states observed in the Kondo insulator, samarium hexaboride (SmB₆), has stimulated considerable attention among those studying non-trivial electronic phenomena. However, experimental studies of these states have led to controversial conclusions mainly to the difficulty and inhomogeneity of the SmB₆ crystal surface. In this work, the detailed electronic structure of SmB₆ is revealed by angle-resolved photoelectron spectroscopy measurements of the three-fold (111) surface which is prepared for the first time and has a preferable periodic character to pursue the topological order. The metallic two-dimensional state was dispersed across the bulk Kondo gap and its helical in-plane spin polarization around the surface TRIM strongly suggests that SmB₆ is topologically non-trivial. Based on these results, we propose a simple picture of the controversial topological classification of SmB₆. We expect that this discovery could provide a new, fertile playground for the following researches about the concert effect between electron correlation and topology. It would also be applicable for future spintronic technologies.