Asymmetric mass acquisition in LaBi: Topological semimetal candidate

We use our high resolution He-lamp-based, tunable laser-based angle-resolved photoemission spectroscopy measurements and density functional theory calculations to study the electronic properties of LaBi, a binary system that was proposed to be a member of a new family of topological semimetals. Both bulk and surface bands are present in the spectra. The dispersion of the surface state is highly unusual. It resembles a Dirac cone, but upon closer inspection we can clearly detect an energy gap. The bottom band follows roughly a parabolic dispersion. The dispersion of the top band remains very linear, ``V'' -shape like, with the tip approaching very closely to the extrapolated location of Dirac point. Such asymmetric mass acquisition is highly unusual and opens a possibility of a new topological phenomenon that has yet to be understood.