Among the family of topological superconductors derived from ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, ${\mathrm{Cu}}_x{\left(\mathrm{PbSe}\right)}_5{\left({\mathrm{Bi}}_2{\mathrm{Se}}_3\right)}_6$ is unique in its surface termination of a single quintuple layer (QL) of the topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ on an ordinary insulator PbSe. Here, we report a combined scanning tunneling microscopy (STM) and density functional theory (DFT) characterization of the cleaved surface of the parent compound ${\left(\mathrm{PbSe}\right)}_5{\left({\mathrm{Bi}}_2{\mathrm{Se}}_3\right)}_6$ (PSBS). Interestingly, the potential disorder due to the random distribution of native defects is only $\mathrm{\Gamma }\sim 4\mathrm{meV}$, comparable to the smallest reported for TIs. Performing high-resolution quasiparticle interference imaging (QPI) near the Fermi energy ($E-E_{\mathrm{F}}=-1$ to $0.6\mathrm{eV}$) 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 due to 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 $\overline{\mathrm{\Gamma }K}$ direction. We show that this effect is not due to the forbidden backscattering in the spin-momentum locked surface state in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ but reflects the threefold symmetry of the scattering potential.

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• Fermi surface
• Local density of states
• Quantum interference effects
• Spin-orbit coupling
• Surface states
• Topological materials
• Topological superconductors
• Density functional theory
• Scanning tunneling microscopy
• Scanning tunneling spectroscopy