Supplementary Information for the Manuscript

Supplementary Information for the manuscript

Atomic and electronic structure of an alloyed

topological insulator, Bi1.5Sb0.5Te1.7Se1.3

Wonhee Ko1, Insu Jeon1, Hyo Won Kim1, Hyeokshin Kwon1, Se-Jong Kahng1,2, Joonbum

Park3, Jun Sung Kim3†, Sung Woo Hwang1, and Hwansoo Suh1*

1Frontier Research Lab., Samsung Advanced Institute of Technology, Yongin 446-712, Korea

2Department of Physics, Korea University, Seoul 136-713, Korea

3Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea

Correspondence and requests for materials should be addressed to Jun Sung Kim†

([email protected]) or Hwansoo Suh* ([email protected]).

1. Atomic structure of the Bi1.5Sb0.5Te1.7Se1.3 surface

In Fig. S1, we present a large scale topograph of BSTS that shows a well-ordered hexagonal lattice over the 46 nm × 46 nm area. By taking cross sections along two lines that include large corrugations, 1 and 2, we confirm that at each atomic lattice point, there is always a small peak that corresponds to the atom. This observation supports the hypothesis that corrugations are not defects but a distortion coming from alloying. Actual defects are revealed in Fig. S2, where three defects are present in the bottom of the topograph. Two of these defects are due to atomic size protrusions, whilst one is a triangular shaped feature. Figure S1 | Large scale topograph of the atomic lattice. a. Topograph with a well-ordered hexagonal lattice that spans over the whole region without disruption (except for a few pieces of dumps at the top right corner). The image size was 46 nm × 46 nm, VB = -0.4 V, and I = 400 pA. b. Cross sections along the lines marked 1 and 2 in a. Peaks that correspond to the atomic lattice points are marked with arrows.

Figure S2 | Topograph with defects. At the bottom of the image, three defects are marked by dashed white circles; two defects are due to atomic size protrusions and the third is a triangular shaped feature. The image size was 70 nm × 70 nm, VB = 0.6 V, and I = 50 pA. 2. Various scaling methods of the Fourier-transform scanning tunnelling spectroscopy (FT-STS)

The Fourier-transform (FT) of the conductance map, or FT-STS in other words, is a powerful method to visualize electron scattering and quasi-particle interference (QPI). However, plotting all the FT maps on the same scale can mask important information, because the fluctuation in conductance maps may vary significantly for different bands. Various types of normalization methods have been employed to overcome this problem.

In the main text we plotted all the FT maps in real units (nS), as it visualizes the dispersion of topological surface states very well (Fig. 3 and Fig. 4). The consequence of this, however, is that the detailed structure of the conductance and valence band has been obscured. To emphasize this detailed structure, it is better to normalize each FT map so that saturation of the colormap can be avoided. In Fig S3, we replotted all the FT maps included in the main text using both the real unit scale (Fig. S3a, c) and the normalized unit scale (Fig. S3b, d). We chose the normalization method so that the maximum of each FT map becomes 1.0. The normalization process prevented the saturation of FT maps in the energy range of the valence band (see maps in Fig S3b with energies -300 and -140 mV), and revealed the appearance of the new band, the conductance band, at 400 mV (Fig S3d). Figure S3 | FT maps in real and normalized unit scales. The FT map at each energy is plotted in the real unit scale in a, and the normalized unit scale in b. The cross sections of the FT maps stacked in the vertical direction in the real unit scale and the normalized unit scale are also shown in c and d.