Figure 2: Spectrum and Shiba maps of Cr atoms.

Differential conductance spectra measured with a superconducting (Pb-covered^7,24) tip (a) on the bare Pb(111) surface and (b) on the Cr adatom, where intra-gap states are easily recognized. (c) DOS of the Cr impurity obtained by deconvoluting the tip contribution in a from spectrum b, as described in Supplementary Note 5. Five positive- and five negative-bias states are observed inside the superconducting gap, at opposed bias-polarity values, in agreement with five pairs of Shiba excitations with different intensities. Most of the differences in peak intensity are due to their inhomogeneous spatial distribution. (d) dI/dV maps over the Cr impurity at the energy of the intra-gap states in the spectrum (b) (expanded here for clarity). The maps represent the amplitude of each spectral peak obtained from a 52 × 52 matrix of dI/dV spectra acquired in an area of 2.2 nm × 2.2 nm over the chromium atom (all with set point V=4 mV and I=0.35 nA of the corresponding STM image shown in the inset). Intra-gap peaks are labelled according to their position and hole(h)/particle(p) character. Peaks / correspond, respectively, to injection of holes/electrons into states /, which are thermally populated by particles/holes due to their proximity to the Fermi level. Therefore, states and are imaged twice. Scale bar, 0.5 nm.