Interfacing 2D and 3D Topological Insulators: Bi(111) Bilayer on ${\mathrm{Bi}}_2{\mathrm{Te}}_3$

We report the formation of a bilayer Bi(111) ultrathin film, which is theoretically predicted to be in a two-dimensional quantum spin Hall state, on a ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ substrate. From angle-resolved photoemission spectroscopy measurements and ab initio calculations, the electronic structure of the system can be understood as an overlap of the band dispersions of bilayer Bi and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Our results show that the Dirac cone is actually robust against nonmagnetic perturbations and imply a unique situation where the topologically protected one- and two-dimensional edge states are coexisting at the surface.

Figure 1

(a),(b) Crystal structure of Bi(111) (a) and ${\mathrm{Bi}}_2{\mathrm{Te}}_3(111)$ (b), respectively. The top and side views are shown explicitly with the unit cells. (c)–(e) RHEED patterns of a Bi(111) surface (c), a single-bilayer Bi(111) grown on ${\mathrm{Bi}}_2{\mathrm{Te}}_3(111)$ (d), and ${\mathrm{Bi}}_2{\mathrm{Te}}_3(111)$ (e), respectively. The electrons are incident along the [$11\overline{2}$] direction of the underlying Si substrate, and the energy is 15 keV. (f) RHEED spot intensity of the (00) spot during Bi and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ film growth showing clear oscillation and the periodicity. (g) A STM image of the single-bilayer Bi(111) film ($V_{\mathrm{tip}}=-3\mathrm{V}$, $I=0.35\mathrm{nA}$). 3.1 Å corresponds to the step height of Si(111).

Figure 2

(a)–(d) The band dispersion image along the $\overline{\Gamma }\mathrm{\text{−}}\overline{\mathrm{K}}$ direction taken at $h\nu =21\mathrm{eV}$ for a 18 QL thick ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ (a) and its momentum distribution curves (b), respectively. The same for a single-bilayer Bi(111) terminated ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ [(c) and (d)]. (e),(f) The band dispersion image along the $\overline{\Gamma }\mathrm{\text{−}}\overline{\mathrm{K}}$ direction taken at $h\nu =30\mathrm{eV}$ for a 18 QL thick ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ (e) and single-bilayer Bi(111) terminated ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ (f), respectively.

Figure 3

(a)–(c) Calculated band structure along the $\overline{\Gamma }\mathrm{\text{−}}\overline{\mathrm{K}}$ direction for a freestanding Bi bilayer (a), 6 QL thick ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ (b), and single-bilayer Bi(111) terminated ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ (c), respectively. (b) and (c) also show the spin polarization. (d) The calculated band structure of (c) overlapped on the experimentally obtained band dispersion of Fig. 2c. (e) The spin-polarized band structure of single-bilayer Bi(111) terminated ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ along the $\overline{\Gamma }\mathrm{\text{−}}\overline{\mathrm{M}}$ direction. (f) Spin-polarized band dispersion of (c) shown in a larger energy range. The states marked $a$, $b$, and $c$ are the counterparts of the freestanding Bi bilayer shown in (a). The symbols $i$, $ii$, and $iii$ show the Dirac points. The spin component shown is oriented in-plane perpendicular to $k$. (g),(h) Spatial distribution of the charge density of the surface state at the Dirac point on the ${\mathrm{Bi}}_2{\mathrm{Te}}_3(111)$ surface (g), and the single-bilayer Bi(111) terminated ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ surface (h), respectively.