Recent progress in clean-surface formation of topological Kondo insulators and topological surface states observed there

The peculiar metallic electronic states proposed and observed in Kondo insulators (KIs), whose bandgap opens at low temperature derived from the Kondo effect between itinerant and localized electrons, have attracted considerable attention in this decade, because it suggests the coexistence of strong electron correlation and non-trivial topological order (TO) in such KIs, namely topological Kondo insulators (TKIs). However, experimental studies of these states have led to controversial conclusions mainly owing to the difficulty and inhomogeneity of the single crystal surfaces of the TKI candidates, samarium hexaboride (SmB6) and ytterbium dodecaboride (YbB12). In this article, we review studies focused on the surface atomic and electronic structures of TKI candidates and recent progress to form homogeneous, well-defined clean surfaces of them. Due to the homogeneous surface formation, the surface electronic states and their non-trivial TO are elucidated well in SmB6 and YbB12, by using spin- and angle-resolved photoelectron spectroscopy.