Nearly free electron model

The model of quasi-free electrons ( engl. nearly- free electron model, often with NFE - model for short) also called, after its author Felix Bloch Bloch theory is a theoretical model in solid state physics to describe the conduction and valence electrons in solids. It extends the model of free electrons by a weak periodic electrostatic potential for the approximate description of the positively charged atomic cores in the crystal lattice. It is initially the periodicity of the energy bands in reciprocal space to having in consideration of a small lattice potential at the zone boundaries a band gap. The presentation takes place in the expanded, reduced or periodic zone scheme.

Even this rather simple improvement allows a theoretical prediction of the band gap and thus a classification of solids according to electrical conductors, semiconductors and insulators. The model of quasi-free electrons is the basis of the band model.

As in the free-electron model, the independent electron approximation will continue to exist, that is, the electrical interactions between the electrons are not considered. As related phenomena such as superconductivity can not be described with this model.