Experimental verification of a temperature-induced topological phase transition in TlBiS2 and Tl Bi Se2

Temperature dependence of the band structure, as well as the spin-polarization on the topologically trivial insulator TlBiS2 and nontrivial insulator Tl Bi Se2, have been investigated by spin- and angle-resolved photoelectron spectroscopy. Despite the recent theoretical prediction [Phys. Rev. Lett. 117, 246401 (2016).], topological phase transition (trivial to nontrivial for TlBiS2, nontrivial to trivial for Tl Bi Se2) does not occur in the observed temperature ranges (from 50 to 400 K for TlBiS2, from 50 to 475 K for Tl Bi Se2). Our results indicate that the discrepancy between the theory and the experiment can be understood by considering the overestimation of lattice parameters and the effect of lattice expansion that is neglected in the previous theory. Although some spin-polarized states are observed in TlBiS2, it is probably due to the Rashba-like surface (resonant) states and not the topological surface states. On the other hand, the topological surface states in Tl Bi Se2 is very robust even at 400 K and the spin-polarized states can be used for the real spintronic devices in the large temperature range.