Phonon-phonon interaction and phonon drag in potassium at low temperatures

The coupled Boltzmann equations for phonons and electrons in the low-temperature limit are solved through a perturbative method in which the phonon-phonon interaction is treated as a small perturbation. In this way it is possible to define the extension of the temperature range where phonon drag is effective, that is the range where the above perturbation is negligible; the onset of the transition to Bloch regime is found to occur at about 5 K, in agreement with a result empirically obtained by Kaveh.