split by a combination of spin-orbit coupling and a loss of inversion symmetry at the crystal surface. The spin polarized surface states have an asymmetric Dirac like dispersion that gives rise to its k -splitting between spin up and spin down bands at E$_{F}$ . The large splitting could be due to the nature of spin-polarized photoemission and detection methods and may or may not be intrinsic in nature however, this does not affect our conclusions re- garding the topological band aspects of the system. The topologically non-trivial surface band structure makes Sb(111) an especially appealing candidate for an unusual 2D Dirac protected free fermion system that exhibits an- tilocalization [9]. k -point, it is meaningful to assume some perturbation, such as alloying with Bi [27] that does not significantly alter the spin splitting [Fig. 5], that pushes the bulk va- lence H and conduction L bands completely below and above E$_{F}$ respectively without changing its Z$_{2}$ class. Un- der such an operation, it is clear that the spin polarized surface bands must traverse E$_{F}$ an odd number of times between ¯ Γ and ¯ M, consistent with the 1;(111) topological classification of Sb. This conclusion can also be reached by noticing that the spin-split pair of surface bands that emerge from ¯ Γ do not recombine at ¯ M, indicative of a “partner switching” [9] characteristic of topological insu- lators. In conclusion, we have mapped the spin structure of the surface bands of Sb(111) and shown that the purely surface bands located in the projected bulk gap are spin We thank F. Meier, H. Dil, J. Osterwalder for technical assistance and C.L. Kane for theoretical discussion. [27] B. Lenoir et al ., Fifteenth International Conference on Thermoelectrics (Pasadena, California) 113 (IEEE, New York, 1996). [26] E. Kneedler, K.E. Smith, D. Skelton and S.D. Kevan, Phys. Rev. B 44 , 8233 (1991). [25] Y. Liu and E. Allen, Phys. Rev. B 52 , 1566 (1995). [24] F. Meier et al ., Phys. Rev. B 77 , 165431 (2008). [23] L.M. Falicov and P.J. Lin, Phys. Rev. 141 , 562 (1965). [22] X. Gonze et al ., Phys. Rev. B 44 , 11023 (1991). [21] M. Hoesch et al ., Phys. Rev. B 69 , 241401(R) (2004). [20] T. Hirahara et al ., Phys. Rev. B 76 , 153305 (2007). [19] J. Teo, L. Fu and C.L. Kane, arXiv:0804.2664v1 [cond- mat.mes-hall] (2008). [18] M. Hochstrasser et al ., Phys. Rev. Lett. 89 , 216802 (2002). [17] Yu. M. Koroteev et al ., Phys. Rev. Lett. 93 , 046403 (2004). [16] S. Bengi´ o et al ., Surface Science 601 , 2908 (2007). [15] M. Hoesch et al ., J. Electron Spectrosc. Relat. Phenom. 124 , 263 (2002). [14] H. H¨ ochst and C.R. Ast, J. Electron Spectrosc. Relat. Phenom. 137 , 441 (2004). [13] K. Sugawara et al ., Phys. Rev. Lett. 96 , 046411 (2006). [12] D. Hsieh et al ., Nature 452 , 970 (2008). [11] R. Roy, arXiv:cond-mat/0604211 (2006). [10] S. Murakami, New J. Phys. 9 , 356 (2007). [9] L. Fu et al ., Phys. Rev. Lett. 98 , 106803 (2007). [8] C. Wu et al ., Phys. Rev. Lett. 96 , 106401 (2006). [7] F.D.M. Haldane, Phys. Rev. Lett. 61 , 2015 (1988). [6] M. K¨ onig et al ., Science 318 , 766 (2007). [5] B.A. Bernevig et al ., Science 314 , 1757 (2006). [4] C.L. Kane & E.J. Mele, Phys. Rev. Lett. 95 , 226801 (2005). [3] J.E. Moore and L. Balents, Phys. Rev. B 75 , 121306(R) (2007). [2] L. Fu and C.L. Kane, Phys. Rev. B 76 , 045302 (2007). [1] C.L. Kane & E.J. Mele, Phys. Rev. Lett. 95 , 146802 (2005). 5