Experimental Realization of a Three-Dimensional Topological Insulator Phase in Ternary Chalcogenide ${\mathrm{TlBiSe}}_2$

We report the first observation of a topological surface state on the (111) surface of the ternary chalcogenide ${\mathrm{TlBiSe}}_2$ by angle-resolved photoemission spectroscopy. By tuning the synchrotron radiation energy we reveal that it features an almost ideal Dirac cone with the Dirac point well isolated from bulk continuum states. This suggests that ${\mathrm{TlBiSe}}_2$ is a promising material for realizing quantum topological transport.

Figure 1

(a) Rhombohedral primitive cell of ${\mathrm{TlBiSe}}_2$. The equivalent hexagonal lattice of ${\mathrm{TlBiSe}}_2$ with atomic layers stacked in the sequence -Tl-Se-Bi-Se-Tl-. (b) A schematic diagram of the bulk Brillouin zone (BZ) of ${\mathrm{TlBiSe}}_2$ and the two-dimensional BZ of the (111) surface. (c) $k_z$ plot as a function of $k_{\parallel }$ (see text). In going from $h\nu =70$ to 98 eV, the whole Brillouin zone is scanned along the $c$ axis.

Figure 2

ARPES on ${\mathrm{TlBiSe}}_2(111)$: (i) ${\mathbf{k}}_{\parallel }$ dispersion and (ii) momentum-space cuts measured with photon energy of 58 eV with $p$- and $s$-polarized light along $\overline{\Gamma }\overline{M}$  (a),(c) and $\overline{\Gamma }\overline{K}$ (b),(d) lines. (e) Intensity maxima of the surface state along $\overline{\Gamma }\overline{K}$ line together with the result for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ (ARPES energy dispersion for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is in inset). The Dirac point is located at $E_B=390$ and 400 meV for ${\mathrm{TlBiSe}}_2$

Figure 3

(a) ARPES energy dispersions for $h\nu =84\mathrm{eV}$ in the wide $k_{\parallel }$ range along $\overline{M}\overline{\Gamma }\overline{M}$ together with the momentum distribution curves at the binding energies of (b) 180, (c) 340, and (d) 620 meV (cut 1–cut 3). (e) The experimental energy dispersion for $h\nu =98\mathrm{eV}$.

Figure 4

(a) Schematic energy diagram of surface and bulk states of ${\mathrm{TlBiSe}}_2$ derived from the experimental results. Theoretical projected bulk band structure of ${\mathrm{TlBiSe}}_2$ obtained with ideal atomic positions (b) and with fully optimized geometry (c). Lines show constant $k_z$ contours.