The Kondo effect: a magnetic impurity in a metal is screened by conduction electrons below T_K.
Anderson model: impurity at ε_d with Coulomb U hybridized to band (width Γ).
Kondo temperature: T_K ∝ exp(π ε_d(ε_d+U) / (2ΓU)). Spectral function shows three features:
two Hubbard peaks at ε_d and ε_d+U, plus a sharp Abrikosov-Suhl resonance pinned at E_F.
The Kondo screening cloud extends to ξ_K = ℏv_F/(k_B T_K) — potentially mesoscopic in scale.