  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.
