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 similar to 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 similar to 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 nontopological 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.