Figure 1

The angle resolved photoemission spectroscopy (ARPES) images of Bi₂Te₂Se and Sb₂TeSe₂ single crystals: (a) The ARPES image of a Bi₂Te₂Se single crystal measured with 22 eV photon energy. (b) The ARPES image of a Sb₂TeSe₂ single crystal measured with 24 eV photon energy. All single crystals were the same pieces as those used in ultrafast experiments for the consistency of all measurements. The single crystals were in-situ cleaved under a base pressure 5.1 × 10⁻¹¹ torr at 85 K just before measurements. ARPES experiment was conducted National Synchrotron Radiation Research Center in Taiwan using BL21B1 beamline. The photoemission spectra were recorded with a Scienta R4000 hemispherical analyzer. The polarization vector was always in the angular dispersion plane. The overall energy resolution is about 12 meV. The green dash lines represent as the TSS of crystals, and the blue dash lines show the bulk-conduction-band (BCB) and bulk-valance-band (BVB). The Dirac point in Sb₂TeSe₂ was estimated at 189 meV above the Fermi level (see S1 in Supplementary information). A notable difference of band structure exists between Bi₂Te₂Se and Sb₂TeSe₂, the Dirac point of Bi₂Te₂Se is embedded in the BVB. In contrast to Bi₂Te₂Se, Sb₂TeSe₂ has an isolated Dirac cone and surface carriers cannot be scattered easily by bulk carriers. This difference in their band structure makes a significant difference in optical measurement results.