A three-dimensional strong-topological-insulator or -semimetal hosts topological surface states which are often said to be gapless so long as time-reversal symmetry is preserved. This narrative can be mistaken when surface state degeneracies occur away from time-reversal-invariant momenta. The mirror-invariance of the system then becomes essential in protecting the existence of a surface Fermi surface. Here we show that such a case exists in the strong-topological-semimetal Bi4Se3. Angle-resolved photoemission spectroscopy and \textit{ab initio} calculations reveal partial gapping of surface bands on the Bi2Se3-termination of Bi4Se3(111), where an 85 meV gap along ΓˉKˉ closes to zero toward the mirror-invariant ΓˉMˉ azimuth. The gap opening is attributed to an interband spin-orbit interaction that mixes states of opposite spin-helicity.
@article{arxiv.1506.06870,
title = {Gapped Surface States in a Strong-Topological-Semimetal},
author = {A. P. Weber and Q. D. Gibson and Huiwen Ji and A. N. Caruso and A. V. Fedorov and R. J. Cava and T. Valla},
journal= {arXiv preprint arXiv:1506.06870},
year = {2015}
}