The Anderson impurity model describes a magnetic impurity (energy ε_d, interaction U) hybridized with a metal (Γ). Below the Kondo temperature T_K = √(ΓU/2)·exp(π ε_d(ε_d+U)/(ΓU)), a sharp resonance appears in the impurity spectral function A(ω) pinned at the Fermi level — the Kondo peak. Its width is ~T_K and it narrows as T decreases.