Quasiparticle interference on the surface of Bi 2 Se 3 -terminated (PbSe) 5 ( Bi 2 Se 3 ) 6

Among the family of topological superconductors derived from Bi 2 Se 3 , Cux (PbSe) 5 (Bi 2 Se 3 ) 6 is unique in its surface termination of a single quintuple layer (QL) of the topological insulator (TI) Bi 2 Se 3 on an ordinaryinsulator PbSe. Here, we report a combined scanning tunneling microscopy (STM) and density functional theory (DFT) characterization of the cleaved surface of the parent compound (PbSe)5 (Bi 2 Se 3 ) 6 (PSBS). Interestingly,the potential disorder due to the random distribution of native defects is only ∼ 4 meV, comparable to the smallest reported for TIs. Performing high-resolution quasiparticle interference imaging (QPI) near the Fermienergy (E − EF = −1 to 0.6 eV) we reconstruct the dispersion relation of the dominant spectral feature and our ab initio calculations show that this surface feature originates from two bands with Rashba-like splitting dueto strong spin-orbit coupling and inversion symmetry breaking. Moreover, only a small hexagonal distortion of the calculated Fermi surface is seen in the full momentum space distribution of the measured scattering data.Nevertheless, the scattering pattern at lower energies transforms into a flowerlike shape with suppressed intensity along the K direction. We show that this effect is not due to the forbidden backscattering in the spin-momentumlocked surface state in Bi 2 Se 3 but reflects the threefold symmetry of the scattering potential.