Extended Data Fig. 6: Periodic modulation in the areas with high impurity concentration.

A large area (50 × 50 nm²) contains regions with low and high impurity concentrations. a, Atom-resolved STM topography. Some impurities can be identified in the upper region, resulting in a high zero-bias conductance (b) and lower coherence peaks (c). In contrast to the interstitial iron impurities on the bulk single crystals without annealing, the impurities observed here are mobile and can be removed by constant-current-mode scanning (Supplementary Fig. 30). b,c, Differential conductance map at zero bias (0 meV) and the energy of the SC gap coherence peak (+2 meV). d–i, Three dI/dV linecuts measured in this area. Their locations are marked in a–c. They go gradually deeper inside the region with higher impurity concentration from #1 to #3. d,f,h, False-colour plots of the dI/dV linecuts. e,g,i, The corresponding negative curvature plots (see Supplementary information section 6). The periodic gap modulation exists throughout. Note that sharp bound states appear in the dI/dV spectrum when measured near the impurities. These states exhibit the same modulation periodicity as the PDM state. j, Selected spectra from f. The positions of the curves are indicated in f by the colour bar. k, Selected spectra from h feature the spatial modulation of the bound states. The positions of these curves are indicated in h by the colour bar.