Specific heat, definition electronic

The function for an atomic system must, in order that the Pauli principle shall hold good, possess a character of antisymmetry, changing its sign when the coordinates of two electrons are interchanged. This principle, though abstract, is unambiguously related to very definite experimental facts, such as the character of the helium spectrum, and the specific heat relations of hydrogen. 

Effective masses can also be defined (somewhat confusingly) by a variety of other comparisons. A fitting of the free-electron-like part of the optical absorption of a metal (the Drude term) defines an optical mass. The electronic specific heat of a metal gives a thermal mass. The latter is an example of an effective mass which is not related simply to some average or derivative of the band structure (k) as most such masses are. The thermal mass includes important contributions from many-body enhancements. In comparing different effective masses one must therefore consider not just their different definitions, but the different degree to which they are enhanced by such effects (Ref. 9,... 

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