Angle-resolved Photoemission Spectroscopy Study on the Surface States of the Correlated Topological Insulator YbB6

Topological insulator (TI) is a new class of matter with topologically protected surface states that possess unique electronic and spin properties. Recently, how the topological order interplays with the electronic correlation has attracted a lot of theoretical considerations. As materials with significant correlations, the rare-earth borides are interesting with a variety of correlated phenomena. Especially, samarium hexaboride (SmB6) has recently been predicted to be a topological Kondo insulator, which fueled intense effort in search for topological orders in correlated systems. YbB6 is a related rare-earth hexaborides which shares the same structure with SmB6, and is predicted to be a moderately correlated TI with a larger band gap than that of SmB6, YbB6 might be another promising candidate for applications based on topological surface states. However, despite the previous successes of density functional theory in predicting TIs, the correlations in rare-earth compounds pose challenges to an accurate calculation. Therefore, it is crucial to experimentally determine the electronic structure and topological nature of YbB6.
With angle-resolved photoemission spectroscopy, we studied the electronic structure of YbB
6 single crystals. Linearly dispersive bands were observed in its insulating gap around Γ and Χ with negligible kz dependences, indicative of their surface origins. The Circular dichroism (CD) of these in-gap states at various photon energies show patterns consistent with the locked spin- momentum texture of TIs. The CD pattern of the α band around Γ shows obvious two-fold symmetry, while the β band around Χ presents anti-symmetric pattern about the Γ-Χ axis in the surface Brillouin zone (BZ). Moreover, we found that the chemical potential varies up to about 500 meV, from one cleaved surface to another, or sometimes even across the same surface. The bulk bands often coexist with the topological surface states, however, we also identified a bulk insulating gap of about 100 meV, much larger than predicted. It is thus possible to tune the chemical potential into such a gap and realize the surface states dominated transport.

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This work has been published as Xia, M. et al. Sci. Rep. 4, 5999; DOI:10.1038/srep05999 (2014).