KONDO EFFECT

Magnetic impurity screened by conduction electrons

J (exchange coupling) 0.3

T / T_K (reduced temp) 1.0

ρ₀ (density of states) 0.5

OBSERVABLES

T_K (Kondo temp)—

Resistivity regime—

Impurity entropy—

Spin susceptibility—

T_K ~ D · exp(-1 / ρ₀J)

Below T_K, conduction electrons form a spin-singlet "Kondo cloud" around the magnetic impurity. The local moment is screened, and resistivity rises logarithmically as T→0 (rather than decreasing). This is the hallmark of asymptotic freedom: the effective coupling flows to strong coupling at low energies (RG fixed point).