Fig. 4: Crossing the quantum phase transition during tip approach.
a Differential conductance spectrum of a YSR state (blue). The fit (red) considers two channels, one through the YSR state and one through an empty BCS gap. b YSR state energy positions as function of tip-sample distance extracted from a data set with high point density along the distance direction. The two branches cross zero energy indicating that they move across the quantum phase transition. c Scaled impurity-substrate coupling calculated from the YSR state energies in b. d The reduced transmission \({\tau }_{\text{exp}}/{\tilde{\Gamma }}_{\text{t}}\) emphasizes the deviation of the experimental data τexp compared with the model with constant coupling τ0. We find good agreement with the branch τs, where the impurity-substrate coupling decreases during the tip approach.
