FIGURE 3. Typical series of dI/dV spectra illustrating how the two distinct types of regions are apparent in the raw data.

a, These unprocessed spectra were extracted from the same spectral survey used to create Fig. 2, along the white line shown in Fig. 2a and b. The spectra are separated by 1.1 Å and span a total distance of 140 Å as represented by their vertical offset. The -domain spectra (red) have low gap magnitudes and sharp gap-edge peaks whose amplitude is low at the edges of the domain and rises to a maximum in the centre. The -region spectra have high gap magnitude and very broad gap-edge peaks whose amplitude is relatively low and constant. Additional features seen in this figure are the spectra that lie on the borders between neighbouring distinct regions. These spectra can show four peaks, where each pair of peaks corresponds to the gap value of the neighbouring regions. For the maps shown in Figs 1 and 2, the peak with the overall maximum value of G(eV) is used to determine and G(). In Fig. 2c and d, if four clear peaks can be identified in the dI/dV spectrum, then both values of and G() at that location are plotted. b, Surface topography with the trajectory along which both the spectra in a, and the traces of Fig. 2c and d, were measured, shown as a white line. The atomic resolution associated with the spectral surveys is clearly evident.