Spin polarization of gapped dirac surface states near the topological phase transition in TlBi(S _1-xSe _x) ₂

We performed systematic spin- and angle-resolved photoemission spectroscopy of TlBi(S _1-xSe _x) ₂ which undergoes a topological phase transition at x∼0.5. In TlBiSe ₂ (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 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.