Observation of a non-Kondo-like topological insulator state in the correlated rare-earth hexaboride YbB$_6$

We present angle-resolved photoemission studies on the rare-earth hexaboride YbB$_{6}$, which has recently been predicted to be a topological Kondo insulator. Our data do not agree with the prediction and instead show that YbB$_6$ exhibits a novel topological insulator state in the absence of a Kondo mechanism. We find that the Fermi level electronic structure of YbB$_6$ has three 2D Dirac cone surface states enclosing the Kramers' points, while the $f-$orbital which would be relevant for the Kondo mechanism is about 1 eV below the Fermi level. Our first-principles calculation shows that the topological state which we observe in YbB$_6$ is due to an inversion between Yb $d$ and B $p$ bands. I will also present some of our recent results on other member of hexaborides. These experimental and theoretical results provide a new approach for realizing novel correlated topological insulator states in rare-earth materials.