The Debye model reference article from the English Wikipedia on 24-Jul-2004
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Debye model

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In thermodynamics and solid state physics, the Debye model is a method of calculating the phonon contribution to the specific heat in a solid. It improves upon the Einstein model, which assumes a single phonon frequency, by approximating the phonon density of states as a constant up to a cutoff frequency, called the Debye frequency. This model correctly predicts the low temperature dependence of the heat capacity, which is proportional to T3, and it also recovers the Dulong-Petit law at high temperatures.

The Debye model was developed by Peter Debye in 1913.