Figure 1: Observation of gapped quasi-2D states before the topological critical point (TCP) of the topological quantum phase transition (TQPT).
From: Unconventional transformation of spin Dirac phase across a topological quantum phase transition
(a–c), ARPES k||-EB maps of BiTl(S1−δSeδ)2 obtained using incident photon energy of 16 eV. The nominal composition values (defined by the mixture weight ratio between the elements before the growth) are noted on the samples. For a, conventional band insulator, a band gap is clearly observed for δ=0.0–0.4; For b, compositions near the topological critical point of the topological quantum phase transition, δ=0.45, 0.50, 0.525 and 0.55; And for c, topological band insulator, the conduction and valence bands are observed to be well separated with the surface states connecting the band gap for δ=0.6–1.0. (d) The energy levels of the first-principles calculated bulk conduction and valence bands of the two end compounds (δ=0.0 and 1.0) are connected by straight lines to denote the evolution of the bulk bands. The compositions selected for detailed experimental studies are marked by red arrows. The + and − signs represent the odd and even parity eigenvalues of the lowest lying conduction and valence bands of BiTl(S1−δSeδ)2. (e) Incident photon energy-dependence spectra for δ=0.45. (f,g) kz versus k|| Fermi surface maps for δ=0.525 and 0.4. The kz range shown for δ=0.4 samples corresponds to the incident photon energy from 14 to 26 eV.
