Kondo effect: Each f-electron screens conduction electrons, forming a spin singlet. T_K = D·e^(−1/Jρ) is exponentially small — so J must be strong.
Lattice coherence: At T ≪ T_K, individual Kondo clouds hybridize into coherent "heavy" bands with m* ∼ 100–1000 mₑ. At T ≫ T_K: local moments scatter incoherently (resistivity rises).
Flat band: Near hybridization gap, band flattens → huge effective mass → heavy fermion.
Purple = heavy band, orange = light band