Gapped Surface States in a Strong-Topological-Insulator Material

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 ab initio calculations reveal partial gapping of surface bands on the Bi4Se3 termination of Bi4Se3(111), where an 85 meV gap along (Gamma) over bar (K) over bar closes to zero toward the mirror-invariant (Gamma) over bar (K) over bar azimuth. The gap opening is attributed to an interband spin-orbit interaction that mixes states of opposite spin helicity.