title: Gapped Surface States in a Strong-Topological-Insulator Material.
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 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 Γ̅K̅ closes to zero toward the mirror-invariant Γ̅M̅ azimuth. The gap opening is attributed to an interband spin-orbit interaction that mixes states of opposite spin helicity.
authors.affiliation: Department of Physics and Astronomy, University of Missouri-Kansas City, Kansas City, Missouri 64110, USA.
authors.forename: A P
authors.initials: AP
authors.lastname: Weber
authors.affiliation: Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: Q D
authors.initials: QD
authors.lastname: Gibson
authors.affiliation: Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: Huiwen
authors.initials: H
authors.lastname: Ji
authors.affiliation: Department of Physics and Astronomy, University of Missouri-Kansas City, Kansas City, Missouri 64110, USA.
authors.forename: A N
authors.initials: AN
authors.lastname: Caruso
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: A V
authors.initials: AV
authors.lastname: Fedorov
authors.affiliation: Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: R J
authors.initials: RJ
authors.lastname: Cava
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA.
authors.forename: T
authors.initials: T
authors.lastname: Valla