Fig. 1. Yu-Shiba-Rusinov (YSR) state in the vicinity of a QPT.
a Schematic of the tunnel junction incorporating a magnetic impurity at the tip apex. b Differential conductance spectrum at zero field showing the impurity-induced YSR states at eV = ±(ε + Δs). The spectrum was recorded at τ = 1.54 × 10−3 with the feedback opened at 4 mV. c The YSR excitation energy ε vs. magnetic exchange coupling J. At the crossing of the YSR energies, the system undergoes a quantum phase transition (QPT) from a free spin doublet into a screened spin singlet state (see inset). d Normalized differential conductance spectra as function of junction transmission τ. The QPT occurs, when the YSR peaks are closest to zero. The YSR peak crossing is not directly visible because both tip and sample are superconducting shifting the YSR peaks by the sample gap ±Δs. b, d The coherence peaks are visible at the sum of the tip and sample gap eV = ±(Δt + Δs).