Figure 2: Tunneling spectra at and near the Cu impurity.

(a) The topographic image near a Cu impurity, the bright pattern with a dumbbell shape is corresponding to a Cu impurity. One can also see several Co impurities with lighter brightness. (a) Scale bar, 1 nm. (b) The STS measured at the Cu impurity (red line) and that measured 2 nm away (dark line). The dashed lines are the spectra measured at 17 K, and they can be used as the normal state background. (c) The STS measured at the Cu impurity (red line) and those measured with steps of every half Na–Na lattice constant on the surface. (d) The subtracted results of the STS with that measured at 2 nm away from the Cu site, the colours of the lines are the same in (c). The STS in (c,d) are off-set for clarity. One can easily see the emerging in-gap states when approaching the Cu impurity.