Experimental Evidence of Hidden Topological Surface States in ${\mathrm{PbBi}}_4{\mathrm{Te}}_7$

A topological surface state that is protected physically under the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$-like five-layer block has been revealed on the Pb-based topological insulator (TI) ${\mathrm{PbBi}}_4{\mathrm{Te}}_7$ by bulk sensitive angle-resolved photoelectron spectroscopy (ARPES). Furthermore, conservation of the spin polarization of the hidden topological surface states is directly confirmed by bulk-sensitive spin ARPES observation. This finding paves the way to realize the real spintronics devices by TIs that are operable in the real environment.

Figure 1

(a) Crystal structure of ${\mathrm{PbBi}}_4{\mathrm{Te}}_7$. (b) Calculated band structure of ${\mathrm{PbBi}}_4{\mathrm{Te}}_7(0001)$ with seven- and five-layer block termination (black solid and green dashed curves, respectively); circles represent weights of the states in the two outermost blocks multiplied by value of in-plane spin components (red (filled circles) and blue (open circles) denote positive and negative values of spin, respectively). Shaded area is the bulk-projected bands, and the bulk band gap is about 100 meV. (c) Depth profile of the charge density of the topological surface state for 5LB- (green dashed line) and 7LB-terminated (black solid line) surface.

Figure 2

Band dispersion of ${\mathrm{PbBi}}_4{\mathrm{Te}}_7$ along $\overline{\Gamma }\mathrm{\text{−}}\overline{\mathrm{M}}$ and $\overline{\Gamma }\mathrm{\text{−}}\overline{\mathrm{K}}$ directions observed with (a) surface-sensitive ARPES measurement with SR excitation ($h\nu =22\mathrm{eV}$) and (b) bulk-sensitive ARPES measurement with Xe excitation ($h\nu =8.4\mathrm{eV}$). The second derivative images of both (a) and (b) are also presented in (c) and (d). (e) Iso-energy contour (IEC) plots of the band dispersion of ${\mathrm{PbBi}}_4{\mathrm{Te}}_7$ taken with Xe excitation. The patterns are second derivative of the raw data with threefold symmetrization. Energies indicated on each IEC are binding energy (left), estimated energy from the DP of DC1 (middle), and DC2 (right), respectively.

Figure 3

(a) Surface-sensitive spin ARPES spectra and polarization map taken with SR excitation ($h\nu =22\mathrm{eV}$). (b) Bulk-sensitive spin ARPES spectra and polarization spectra obtained with Xe excitation at $k_x=\pm 0.14{\AA }^{-1}$. The measurement positions of the data are indicated with the rectangles in Fig. 2d. The width of the rectangles indicates the angular resolution of the measurement.

Figure 4

(a) STM topographic image and (b) $dI/dV$ image of ${\mathrm{PbBi}}_4{\mathrm{Te}}_7$ taken at $\mathrm{Vs}=-0.6\mathrm{V}$. (c) The height profile along the line in (a). Two kinds of steps with different step height that correspond to 7 LB and 5 LB are clearly seen. Atomic resolution $dI/dV$ images are obtained at $\mathrm{Vs}=-0.1\mathrm{V}$ on the terrace of (d) 5LB and (e) 7LB termination, respectively. Image of ${\mathrm{PbBi}}_2{\mathrm{Te}}_4$ in the same scale is also indicated in the inset of (e). (f) STS spectra taken at the different terraces. On the 7LB termination, DP is observed at around $-0.39\mathrm{V}$. While on the 5LB termination, no clear evidence of DC is observed.