Quantum Theory of the Generalized Nonlocal Linear Response
When a system of charged particles is subjected to a prescribed external electromagnetic field an induced current density distribution will in general be generated in the particle system. If the external field is sufficiently weak the induced current density distribution will be related in a linear fashion to the external field, as described from different points of views, and in general terms in Chap. 10. In the present section, we shall see how a generalized linear constitutive equation of the form given in (10.79) appears from quantum mechanical considerations.