Figure 2
From: Perturbation theory of a superconducting 0 − π impurity quantum phase transition
Phase diagrams in the Γ − U (a), U − Φ (b) and 
(c) parameter planes.
We compare the phase boundaries calculated by numerically exact NRG with various analytical approximations: fRG (only in panel (a); data taken graphically from Fig. 2 of Ref. 27), spin-symmetric HF, the second-order PT/dynamical corrections (DC) and generalized atomic limit approximation (GAL) U2/(1 + Γ/Δ)2 = (2ε + U)2 + 4Γ2 cos2(Φ/2).
