Figure 4: Scanning tunnelling spectroscopy (STS) measurements.

(a) dI/dV/(I/V) spectra of the B-2✓3Sn, p-2✓3Sn and n-2✓3Sn surfaces obtained at 77 K. The spectra for p-2✓3Sn and B-2✓3Sn are shifted vertically for clarity. The Fermi level is located at 0 V. By shifting the p-2✓3Sn spectrum 0.36 eV toward lower energy (dashed line), and superposing it with n-2✓3Sn spectrum, it is seen that the low-energy spectral features line-up almost perfectly. Triangles below each curve mark the highest occupied surface state (left) and lowest unoccupied surface state (right), while a solid bar indicates the estimated location of the VBM (left) and CBM (right) of the bulk Si bands near the surface with the bar length representing the error range (see Supplementary Note 1). (b) dI/dV/(I/V) spectra of the B-2✓3Sn and B-4✓3Sn interfaces obtained at 4.4 K, showing a slightly larger band gap for the B-4✓3Sn phase. All STS spectra were measured on top of the bright dimers (STS at other locations within a unit cell are showing similar features). Each dI/dV/(I/V) spectrum is obtained by dividing the dI/dV curve by its smoothed I(V) curve³⁴.