For typical three-dimensional metals, the temperature-dependence of the electrical resistivity ρ(T) due to the scattering of electrons by acoustic phonons changes from a high-temperature regime in which ρ ∝ T to a low-temperature regime in which ρ ∝ T5 at a characteristic temperature known as the Debye temperature. For low density electron systems, however, the Fermi surface can be substantially smaller than the size of the Brillouin zone, and only a small fraction of acoustic phonons can scatter off electrons. This results in a new characteristic temperature known as the Bloch–Grüneisen temperature that is lower than the Debye temperature. The Bloch–Grüneisen temperature is defined as 2ħvskF/kB, where ħ is the Planck constant, vs is the velocity of sound, ħkF is the Fermi momentum, and kB is the Boltzmann constant.
When the temperature is lower than the Bloch–Grüneisen temperature, the most energetic thermal phonons have a typical momentum of kBT/vs which is smaller than ħkF, the momentum of the conducting electrons at the Fermi surface. This means that the electrons will only scatter in small angles when they absorb or emit a phonon. In contrast when the temperature is higher than the Bloch–Grüneisen temperature, there are thermal phonons of all momenta and in this case electrons will also experience large angle scattering events when they absorb or emit a phonon.
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