Gapped Surface States in a Strong-Topological-Semimetal

TL;DR

This paper reveals that in a strong-topological-semimetal, surface states can be gapped away from time-reversal-invariant points due to mirror symmetry protection, with experimental and theoretical evidence showing partial surface band gapping.

Contribution

It demonstrates the existence of gapped surface states in a strong-topological-semimetal caused by mirror symmetry, challenging the common assumption of gapless surface states.

Findings

Partial gapping of surface bands observed in Bi4Se3

Gap size varies along different azimuths, closing at mirror-invariant points

Interband spin-orbit interaction causes the gap opening

Abstract

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 Bi$_4$Se$_3$. Angle-resolved photoemission spectroscopy and \textit{ab initio} calculations reveal partial gapping of surface bands on the Bi$_2$Se$_3$-termination of Bi$_4$Se$_3$(111), where an 85 meV gap along $\bar{\Gamma}\bar{K}$ closes to zero toward the mirror-invariant $\bar{\Gamma}\bar{M}$ azimuth. The gap opening is attributed to an interband spin-orbit interaction that mixes states of opposite spin-helicity.