Hasan Research Group

Laboratory for Topological Quantum Matter & Advanced Spectroscopy

We present high-resolution angle-resolved photoemission spectroscopy studies of trivalent CeB₆ and divalent BaB₆ rare-earth hexaborides. We find that the Fermi surface electronic structure of CeB₆ consists of large oval-shape pockets around the X points of the Brillouin zone, while the states around the zone centre G point are strongly renormalized. Our first-principles calculations agree with data around the X points, but not at the G points, indicating areas of strong renormalization located around G. The Ce quasi-particle states participate in formation of hotspots at the Fermi surface, while the incoherent f-states hybridize and lead to the emergence of dispersive features absent in non-f counterpart BaB₆. These experimental and theoretical results provide a new understanding of rare-earth hexaboride materials.

We present angle-resolved photoemission studies on the rare-earth- hexaboride YbB₆, which has recently been predicted to be a topological Kondo insulator. Our data do not agree with the prediction and instead show that YbB₆ exhibits a novel topological insulator state in the absence of a Kondo mechanism. We find that the Fermi level electronic structure of YbB₆ has three 2D Dirac cone surface states enclosing the Kramers' points, while the f-orbital that would be relevant for the Kondo mechanism is ∼1 eV below the Fermi level. Our first-principles calculation shows that the topological state that we observe in YbB₆ is due to an inversion between Yb d and B p bands. These experimental and theoretical results provide a new approach for realizing novel correlated topological insulator states in rare-earth materials