1. “Positron surface state as a spectroscopic probe for characterizing surfaces of topological insulator materials”.  Callewaert V, Shastry K, Saniz R, Makkonen I, Barbiellini B, Assaf BA, Heiman D, Moodera JS, Partoens B, Bansil A, Weiss AH;, Physical review B 94, 115411 (2016).   http://doi.org/10.1103/PHYSREVB.94.115411