Conductivity, thermal

Mobile electrons and phonons are carriers of heat in solids. The amplitude and the temperature dependence of thermal conductivity can be understood by quantifying the concentration of such carriers, their velocity, and the distance they travel between two successive collisions. In insulators, itinerant electrons are absent, and heat propagates thanks to phonons. The latter are scattered off each other at high temperature and by defects at lower temperatures. In metals, heat can be carried by electrons, which often dominate heat conductivity thanks to their larger velocity. In superconductors, electron carriers of heat vanish to a condensate and heat is carried by phonons and thermally excited quasi-particles, which may persist down to zero temperature when the superconducting gap has nodes. Heat can also be carried by magnetic excitations, such as magnons. Analysis of thermal transport is often complicated due to the fact that there are multiple carriers and various groups of scatterers and a quantitative account requires a disentanglement of different carriers and different scattering mechanisms.