A condensed matter system in its most basic form can be viewed in terms of two components. One corresponds to electrons which, as discussed in Chapter 5, arrange themselves into energy levels as a consequence of Fermi statistics, with each level containing no more than two electrons with opposite spin orientations. These are the Fermi levels and the energy of the electrons in the topmost level is called the Fermi energy. Thus, in metals, where electrons can move about freely, even near absolute zero temperatures the electrons possess kinetic energy.

The second component corresponds to ions, or atomic lattices, the vibrational modes of which are described in terms of simple harmonic ...

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