Tailoring the topological surface state in ultrathin α -Sn(111) films by Rogalev, VA, Reis, F, Adler, F, Bauernfeind, M, Erhardt, J, Kowalewski, A, Scholz, MR, Dudy, L, Duffy, LB, Hesjedal, T, Hoesch, M, Bihlmayer, G, Schäfer, J