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 Bi2Se3 termination of Bi4Se3(111), where an 85 meV gap along Γ¯K¯ closes to zero toward the mirror-invariant Γ¯M¯ azimuth. The gap opening is attributed to an interband spin-orbit interaction that mixes states of opposite spin helicity.