Spin Polarization of Gapped Dirac Surface States Near the Topological   Phase Transition in TlBi(S1-xSex)2
  We performed systematic spin- and angle-resolved photoemission spectroscopy of TlBi(S1-xSex)2 which undergoes a topological phase transition at x ~ 0.5. In TlBiSe2 (x = 1.0), we revealed a helical spin texture of Dirac-cone surface states with an intrinsic in-plane spin polarization of ~ 0.8. The spin polarization still survives in the gapped surface states at x > 0.5, although it gradually weakens upon approaching x = 0.5 and vanishes in the non-topological phase. No evidence for the out-of-plane spin polarization was found irrespective of x and momentum. The present results unambiguously indicate the topological origin of the gapped Dirac surface states, and also impose a constraint on models to explain the origin of mass acquisition of Dirac fermions. 
S. Souma, M. Komatsu, M. Nomura, T. Sato, A. Takayama, T. Takahashi,   K. Eto, Kouji Segawa, and Yoichi Ando
cond-mat.mes-hall cond-mat.str-el