Phonons are quantized lattice vibrations. They carry heat in insulators. Two types of phonon-phonon scattering govern thermal conductivity:
Umklapp processes flip phonon momentum (the word means "flipping over" in German — Peierls, 1929). They are the dominant mechanism for thermal resistance in crystals at high temperature.
At low T, Umklapp is exponentially suppressed (few high-momentum phonons), so κ rises steeply. This lab simulates a 1D chain with phonon quasi-particles bouncing and scattering.
Hot end (left, red): phonons injected at high amplitude. Cold end (right, blue): phonons absorbed. Umklapp events (marked ×) randomize phonon direction, impeding heat flow.