Instability of the topological surface state in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ upon deposition of gold

Momentum-resolved photoemission spectroscopy indicates the instability of the Dirac surface state upon deposition of gold on the (0001) surface of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Based on the structure model derived from extended x-ray absorption fine structure experiments showing that gold atoms substitute bismuth atoms, first-principles calculations provide evidence that a gap appears due to hybridization of the surface state with gold $d$ states near the Fermi level. Our findings provide insights into the mechanisms affecting the stability of the surface state.

Figure 1

Normalized fluorescence yield collected for about 0.6 ML of gold deposited on ${\mathrm{Bi}}_2{\mathrm{Se}}_3\left(0001\right)$ with the electric field vector parallel $\left(E_{\parallel }\right)$ and perpendicular $\left(E_{\perp }\right)$ to the sample surface. Curves are shifted for clarity. The inset shows the $k^2$-weighted interference function $\mathrm{[}\chi \left(k\right)]$ and the magnitude of the Fourier transforms. The absence of any polarization dependence of peak 1 indicates a nearly isotropic environment compatible with the substitutional site (see Fig. 2). The reduction of peak 2 in the magnitude of the Fourier transform of $E_{\perp }$ is predicted for the substitutional site.

Figure 2

Schematic view of adsorption sites for gold on ${\mathrm{Bi}}_2{\mathrm{Se}}_3\left(0001\right)$. Bismuth and selenium atoms are shown as large blue and small red spheres, respectively. Possible gold positions are indicated by yellow spheres and labeled by “f”, “h”, “S”, “I”, and “O” for the fcc, hcp, substitutional, interstitial, and octahedral van der Waals gap sites, respectively.

Figure 3

(a), (b) Spin-integrated photoemission spectra of pristine and gold-covered ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ using unpolarized He i radiation $\left(h\nu =21.2\mathrm{eV}\right)$. (c)–(e) Spin-resolved spectra using $s$-polarized laser light $\left(h\nu =6.0\mathrm{eV}\right)$ collected for 0.3, 0.5, and 1.0 ML of gold. Dashed lines indicate the opening of a gap in the surface state. Further details are provided in the Supplemental Material.

Figure 4

Evolution of the TSS in ${\left({\mathrm{Bi}}_{1\text{−}x}{\mathrm{Au}}_x\right)}_2{\mathrm{Se}}_3/{\mathrm{Bi}}_2{\mathrm{Se}}_3\left(0001\right)$: Calculated spectral density for (a) $x=0.0125$, (b) $x=0.0375$, (c) $x=0.0625$, and (d) $x=0.1250$, respectively.