Spin Polarization of Gapped Dirac Surface States Near the Topological Phase Transition in TlBi(S1-xSex)2

TL;DR

This study investigates the spin polarization of surface states in TlBi(S1-xSex)2 across a topological phase transition, revealing persistent in-plane spin polarization in gapped states and confirming their topological origin.

Contribution

It provides direct experimental evidence of spin textures in gapped Dirac surface states near the topological phase transition, clarifying their topological nature.

Findings

Helical spin texture observed in TlBiSe2 surface states.

Spin polarization persists in gapped states beyond the transition.

No out-of-plane spin polarization detected.

Abstract

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.